mirror of
https://github.com/revng/revng
synced 2026-06-21 14:07:57 +00:00
Andrea Gussoni
cf42e497aa
This commit introduces the Function Isolation Pass. We use the information provided by the Function Boundaries Detection Pass to organize the code that `revamb` places inside the `root` function in different LLVM functions. To do this we obviously need to introduce some changes and tricks to handle the execution of the translated program. The main idea is to have two different realms (one where the isolated functions live, one in which we have basically the old root function). We start the execution from the realm of the *non isolated* functions, and we transfer, as soon as possible, the execution to the *isolated functions* realm. We then have a fallback mechanism to restore the execution in the right place in the *non isolated* functions realm, and so on. The largest change, besides the re-organization of the code in different functions, is the use of the exception handling mechanism provided by the LLVM framework in order to be able to manage the switch between the two realms. We also introduce the `support.h` header file, which contains a couple of definitions used by `support.c` and that need to be shared with some of the components involved in the translation process. We have defined some helper functions, directly in C, that we use both for handling the exception mechanism and for giving extra debug informations when an exception is raised. The `revamb-dump` utility now supports the `-i` option to specify the path were to save the new LLVM module. The `translate` utility now supports the `-i` option that produces a binary in which the function isolation has been applied. We also introduced some tests that apply the function isolation pass to the `Runtime/` tests already present. In this way we can verify that the translation and the following function isolation preserve the behavior of the program. When serializing the new LLVM module we regenerate the metadata used for debug purposes, and for doing this, since we not longer have only the `root` function, we have changed some details in the `DebugHelper` class in order to be able to emit the metadata for all the functions of our interest in a single shot.
647 lines
24 KiB
ReStructuredText
647 lines
24 KiB
ReStructuredText
***********************
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revamb Output Reference
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***********************
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The main goal of revamb is to produce an LLVM module reproducing the behavior of
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the input program. The module should be compilable and work out of the
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box. However, such module contains also a rich set of additional information
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recovered during the analysis process that the user can exploit to develop any
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kind of analysis he wants.
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This document details how to interpret the information present in the generated
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module. Please refer to the `LLVM Language Reference Manual`_ for details on the
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LLVM language itself.
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The various sections of this document will present example to clarify the
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presented concepts. All the examples originate from the translation of a simple
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program compiled for x86-64:
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.. code-block:: c
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int myfunction(void) {
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return 42;
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}
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int _start(void) {
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int a = 42;
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return a + myfunction();
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}
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The program has been compiled as follows:
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.. code-block:: sh
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x86_64-gentoo-linux-musl-gcc -static -nostdlib -O0 -fomit-frame-pointer example.c -o example
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Producing the following assembly:
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.. code-block:: objdump
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00000000004000e8 <myfunction>:
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4000e8: mov eax,0x2a
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4000ed: ret
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00000000004000ee <_start>:
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4000ee: sub rsp,0x10
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4000f2: mov DWORD PTR [rsp+0xc],0x2a
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4000fa: call 4000e8 <myfunction>
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4000ff: mov edx,eax
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400101: mov eax,DWORD PTR [rsp+0xc]
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400105: add eax,edx
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400107: add rsp,0x10
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40010b: ret
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And it has been translated as follows:
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.. code-block:: sh
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./revamb --no-link --functions-boundaries --use-sections --debug-info ll example example.ll
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Global variables
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================
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The CPU State Variables
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-----------------------
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The CPU State Variables (or CSV) are global variables that represent a part of
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the CPU. They vary from architecture to architecture and they are created
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on-demand, which means that not all modules will have all of them.
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Some CSV variables have a name, in particular registers (e.g., `rsp`), some
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others are instead identified by their position within the QEMU data structure
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that contains them (e.g., ``state_0x123``). For example:
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.. code-block:: llvm
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@pc = global i64 0
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@rsp = global i64 0
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@rax = global i64 0
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@rdx = global i64 0
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@cc_src = global i64 0
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@cc_dst = global i64 0
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@cc_op = global i32 0
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`@pc` represents the program counter (aka `rip`), `@rsp` the stack pointer
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register, while `@rax` and `@rdx` are general purpose registers. The `@cc_*` are
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helper variables used to compute the CPU flags.
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CSVs are used by the generated code and by the helper functions. This is also
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the reason why they cannot be promoted to local variables in the `root`
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function
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Note that since they are global variables, the generated code interacts with
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them using load and store operations, which might sound unusual for registers.
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Segment variables
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-----------------
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The translated program expects the memory layout to be exactly as the one in the
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original binary. This means that all the segments have to be loaded at the
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original addresses. In the generated module, they are encoded as global
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variables containing all the data of the segments. These variables have a name
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similar to ``.o_permissions_address`` (e.g., ``.o_rx_0x10000``), where
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*permissions* it's a string representing what type of accesses are allowed to
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that segment (read, execute, write), and *address* is the starting address.
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These variables are associated to special sections which will be assigned to the
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appropriate virtual address at link-time.
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In our example we have single segment, readable and writable:
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.. code-block:: llvm
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@.o_rx_0x400000 = constant [344 x i8] c"\7FELF\02\01\01\0...", section ".o_rx_0x400000", align 1
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As you can see it is initalized with a copy of the original segment and its
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assigned to the `.o_rx_0x400000` section.
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Other global variables
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----------------------
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Apart from CSVs and segment variables, the output module will contain a number
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of other global variables, mainly for loading purposes (see ``support.c``). In
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the following we report the most relevant ones.
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:.elfheaderhelper: a variable whose only purpose is to create the
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`.elfheaderhelper` section, which is employed to force an
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appropriate layout at link-time. It isn't of general
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interest.
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:e_phentsize: size of the ELF program header structure of the input binary.
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:e_phnum: number of ELF program headers in the input binary.
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:phdr_address: virtual address where the ELF program headers are loaded.
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For more information on the ELF program headers, see ``man elf``. In the
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example program we have three program headers of 56 bytes, loaded at
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`0x400040`:
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.. code-block:: llvm
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@.elfheaderhelper = constant i8 0, section ".elfheaderhelper", align 1
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@e_phentsize = constant i64 56
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@e_phnum = constant i64 3
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@phdr_address = constant i64 4194368
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Input architecture description
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==============================
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The generated module also contains a *named metadata node*:
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`revamb.input.architecture`. Currently it's composed by a metadata tuple with
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two values:
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:u32 DelaySlotSize: the size, in number of instructions of the delay slot of the
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input architecture.
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:string PCRegName: the name of the CSV representing the program counter.
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Here's how this information appears in our example:
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.. code-block:: llvm
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!revamb.input.architecture = !{!0}
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!0 = !{i32 0, !"pc"}
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There is no delay slot on x86-64 and the CSV representing the program counter is
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`@pc`.
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The `root` function
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===================
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This section describes how the function collecting all the translated code is
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organized. This fuction is knonw as the `root` function:
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.. code-block:: llvm
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define void @root(i64) {
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; ...
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}
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The `root` function takes a single argument, which is a pointer to the stack
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that the translated program has to use. This stack must have been properly set
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up by the caller, for more information see `FromIRToExecutable.rst`_.
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First of all, the `root` function must set up two key CSVs: the stack pointer
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and the program counter:
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.. code-block:: llvm
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define void @root(i64) {
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entrypoint:
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; ...
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store i64 4194542, i64* @pc
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store i64 %0, i64* @rsp
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; ...
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}
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The program counter is obtained from the entry point of the input program and
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it's therefore statically available, while the stack pointer (the `rsp` register
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in x86-64), is taken from the first argument of the `root` function.
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The dispatcher
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--------------
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The first set of basic blocks are related to the dispatcher. Every time we have
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an indirect branch that we cannot fully handle we jump to the *dispatcher*,
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which basically maps (with a huge ``switch`` statement) the starting address of
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each basic block A in the input program to the first basic block containing the
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code generated due to A.
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:``dispatcher.entry``: the body of the dispatcher. Containes the ``switch``
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statement. If the requested address has not been
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translated, execution is diverted to
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``dispatcher.default``.
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:``dispatcher.default``: calls the `unknownPC` function, whose definition is
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left to the user.
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:``anypc``: handles the situation in which we were not able to fully enumerate
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all the possible jump targets of an indirect jump. Typically will
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just jump to ``dispatcher.entry``.
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:``unexpectedpc``: handles the situation in which we though we were able to
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enumerate all the possible jump targets, but an unexpected
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program counter was requested. This indicates the presence of
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a bug. It can either try to proceed with execution going to
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``dispatcher.entry`` or simply abort.
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The very first basic block is `entrypoint`. Its main purpose is to create all
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the required local variables (``alloca`` instructions) and ensure that all the
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basic blocks are reachable. In fact, it is terminated by a ``switch``
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instruction which make all the previously mentioned basic blocks reachable. This
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ensures that we can compute a proper dominator tree and no basic blocks are
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collected as dead code.
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Here's how it looks like in our example:
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.. code-block:: llvm
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define void @root(i64) !dbg !4 {
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entrypoint:
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%1 = alloca i64
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%2 = bitcast i64* %1 to i8*
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store i64 4194542, i64* @pc
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store i64 %0, i64* @rsp
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switch i8 0, label %dispatcher.entry [
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i8 1, label %anypc
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i8 2, label %unexpectedpc
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]
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dispatcher.entry: ; preds = %unexpectedpc, %anypc, %bb.myfunction, %bb._start.0x11, %entrypoint
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%3 = load i64, i64* @pc
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switch i64 %3, label %dispatcher.default [
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i64 4194536, label %bb.myfunction
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i64 4194542, label %bb._start
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i64 4194559, label %bb._start.0x11
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], !revamb.block.type !1
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dispatcher.default: ; preds = %dispatcher.entry
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call void @unknownPC()
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unreachable
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anypc: ; preds = %entrypoint
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br label %dispatcher.entry, !revamb.block.type !2
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unexpectedpc: ; preds = %entrypoint
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br label %dispatcher.entry, !revamb.block.type !3
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; ...
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}
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As you can see, we have three jump targets: `myfunction`, `_start` and
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`_start+0x11` (the return address after the function call. In this specific
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example we decide to divert execution to the dispatcher both in `anypc` and
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`unexpectedpc`.
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The translated basic blocks
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---------------------------
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The rest of the function is composed by basic blocks containing the translated
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code. If symbols are available in the input binary, each basic block has name in
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the form ``bb.closest_symbol.distance`` (e.g., ``bb.main.0x4`` means 4 bytes
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after the symbol `main`). Otherwise the name is simply in the form
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``bb.absolute_address`` (e.g., ``bb.0x400000``).
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In our example we have three basic blocks:
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.. code-block:: llvm
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define void @root(i64) {
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; ...
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bb._start: ; preds = %dispatcher.entry, %entrypoint
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; ...
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bb._start.0x11: ; preds = %dispatcher.entry
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; ...
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bb.myfunction: ; preds = %dispatcher.entry, %bb._start
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; ...
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}
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Debug metadata
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--------------
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Each instruction we generate is associated with three types of metadata:
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:dbg: LLVM debug metadata, used to be able to step through the generated LLVM IR
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(or input assembly or tiny code).
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:oi: *original instruction* metadata, contains a string-integer pair. The string
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represents the disassembled input instruction that generated the current
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instruction. The integer is the program counter associated to that
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instruction.
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:pi: *portable tiny code instruction* metadata, contains a string representing
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the textual representation of the TCG instruction that generated the
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current instruction.
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Note: some optimizations passes might remove the metadata.
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For debugging purposes, the generated LLVM IR contains comments with information
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derived from these metadata.
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As an example, let's see the first instruction of `myfunction`, ``mov
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eax,0x2a``:
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.. code-block:: llvm
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define void @root(i64) {
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; ...
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bb.myfunction: ; preds = %dispatcher.entry, %bb._start
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; 0x00000000004000e8: mov eax,0x2a
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; movi_i64 tmp0,$0x2a
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; ext32u_i64 rax,tmp0
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store i64 42, i64* @rax, !dbg !135, !oi !133, !pi !136
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; ...
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}
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; ...
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!4 = distinct !DISubprogram(name: "root", ...)
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!133 = distinct !{!"0x00000000004000e8: mov eax,0x2a\0A", i64 4194536}
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!134 = distinct !{!"movi_i64 tmp0,$0x2a\0A"}
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!135 = !DILocation(line: 244, scope: !4)
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!136 = distinct !{!"ext32u_i64 rax,tmp0,\0A"}
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The `!dbg` metadata points to a `DILocation` object, which tells us that we're
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at line 244 within the `root` function. This information will allow the debugger
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(e.g., `gdb`) to perform step-by-step debugging. `!oi` points to a metadata node
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containing the diassembled instruction that lead to generate this instruction
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and its address (`4194536`). Finally, `!pi` points to the TCG instruction
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leading to the creation of this instruction.
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Above the instruction, we also have, for easier reading, the corresponding
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original and TCG instructions.
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Delimiting generated code
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-------------------------
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The code generated due to a certain input instruction is delimited by calls to a
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marker function `newpc`. This function takes three arguments plus a set of
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variadic arguments:
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:u64 Address: the address of the instruction leading to the generation of the
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code coming after the call of `newpc`.
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:u64 InstructionSize: the size of the instruction at `Address`.
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:u1 isJT: a boolean flag indicating whether the instruction at `Address` is a
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jump target or not.
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:u8 \*LocalVariables: a series of pointer to all the local variables used by
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this instruction.
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The call to `newpc` prevents the optimizer to reorder instructions across its
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boundaries and perform other optimizations. This is useful during analysis and
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for debugging purposes, but to achieve optimal performances all these function
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calls should be removed.
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Let's see how this works for the `bb.myfunction` basic block:
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.. code-block:: llvm
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bb.myfunction: ; preds = %dispatcher.entry, %bb._start
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; 0x00000000004000e8: mov eax,0x2a
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call void (i64, i64, i32, i8*, ...) @newpc(i64 4194536, i64 5, i32 1, i8* null), !oi !55, !pi !56
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; ...
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; 0x00000000004000ed: ret
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call void (i64, i64, i32, i8*, ...) @newpc(i64 4194541, i64 1, i32 0, i8* null), !oi !58, !pi !59
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; ...
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As you cane see there are two calls to `newpc`, the first for the ``mov``
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instruction at ``0x4000e8`` (5 bytes long) and the second one for the `ret`
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instruction at ``0x4000ed`` (1 byte long). Note that the first instruction is a
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jump target, in fact `newpc`'s third parameter is set to ``1``, unlike the
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second call.
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Function calls
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--------------
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revamb can detect function calls. The terminator of a basic block can be
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considered a function call if it's preceeded by a call to a function called
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`function_call`. This function take three parameters:
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:BlockAddress Callee: reference to the callee basic block. The target of the
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function call, most likely a function.
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:BlockAddress Return: reference to the return basic block. It's the basic block
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associated to the return address.
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:u64 ReturnPC: the return address.
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In our example we had a function call in the `_start` basic block:
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.. code-block:: llvm
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bb._start: ; preds = %dispatcher.entry, %entrypoint
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; ...
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; 0x00000000004000fa: call 0x4000e8
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; ...
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store i64 4194536, i64* @pc, !dbg !58, !oi !46, !pi !59
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call void @function_call(i8* blockaddress(@root, %bb.myfunction), i8* blockaddress(@root, %bb._start.0x11), i32 4194559), !dbg !60
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br label %bb.myfunction, !dbg !61, !func.entry !62, !func.member.of !63
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As expected, before the branch instruction representing the function call, we
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have a call to `@function_call`. The first argument is the callee basic block
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(`bb.myfunction`), the second argument is the return basic block (`_start+0x11`)
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and the third one is the return address (``0x4000ff``).
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Function boundaries
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-------------------
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revamb can identify function boundaries. This information is also encoded in the
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generated module by associating two types of metadata (`func_entry` and `func`)
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to the terminator instruction of each basic block.
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:func.entry: denotes that the current basic block is the entry block of a
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certain function. The associated metadata tuple contains a single
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string node representing the name assigned to the function.
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:func.member.of: denotes that the current basic block is part of a set of
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functions. The associated metadata tuple contains a set of
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string nodes representing the name assigned to the
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corresponding functions.
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In our example we had three basic blocks: `_start`, `_start+0x11` and
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`myfunction`. Let's see to what function they belong:
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.. code-block:: llvm
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define void @root(i64) !dbg !4 {
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; ...
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bb._start: ; preds = %dispatcher.entry, %entrypoint
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; ...
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br label %bb.myfunction, !func.entry !62, !func.member.of !63
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bb._start.0x11: ; preds = %dispatcher.entry
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; ...
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br label %dispatcher.entry, !func.member.of !63
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bb.myfunction: ; preds = %dispatcher.entry, %bb._start
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; ...
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br label %dispatcher.entry, !func.entry !151, !func.member.of !152
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; ...
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}
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; ...
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!62 = !{!"bb._start"}
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!63 = !{!62}
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; ...
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!151 = !{!"bb.myfunction"}
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!152 = !{!151}
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As it can be seen, `bb._start` and `bb._start.0x11` belong to a single function,
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|
identified by `bb._start`. `bb._start` is also marked as the entry point of the
|
|
function. On the other hand, `bb.myfunction` also belongs to (and it's the entry
|
|
point of) a single function, with the same name.
|
|
|
|
Helper functions
|
|
================
|
|
|
|
Certain features of the input CPU would be to big to be expanded in TCG
|
|
instructions by QEMU (and therefore translate them in LLVM IR). For this reason,
|
|
call to *helper functions* are emitted. An example of an helper function is the
|
|
function handling a syscall or a floating point division. These functions can
|
|
take arguments and can read and modify freely all the CSV.
|
|
|
|
Helper functions are obtained from QEMU in the form of LLVM IR (e.g.,
|
|
``libtinycode-helpers-mips.ll``) and are statically linked by revamb before
|
|
emitting the module.
|
|
|
|
The presence of helper functions also import a quite large number of data
|
|
structures, which are not directly related to revamb's output.
|
|
|
|
Note that an helper function might be present multiple times with different
|
|
suffixes. This happens every time an helper function takes as an argument a
|
|
pointer to a CSV: for each different invocation we specialize that callee
|
|
function by fixing that argument. In this way, we can deterministically know
|
|
which parts of the CPU state is touched by an helper.
|
|
|
|
Currently, there is no complete documentation of all the helper functions. The
|
|
best way to understand which helper function does what, is to create a simple
|
|
assembly snippet using a specific feature (e.g., a performing a syscall) and
|
|
translate it using revamb.
|
|
|
|
Function isolation pass output reference
|
|
========================================
|
|
|
|
This section of the document aims to describe how to apply the function
|
|
isolation pass of revamb-dump to a simple example, to describe what to expect
|
|
as output of this pass and the assumptions made in the isolation pass.
|
|
|
|
All the following examples originate from the translation of the simple program
|
|
already shown in the beginning of this document.
|
|
|
|
Once we have applied the translation to the original binary we can apply the
|
|
function isolation pass using the `revamb-dump` utility like this:
|
|
|
|
.. code-block:: sh
|
|
|
|
revamb-dump --functions-isolation=example.isolated-functions.ll example.ll
|
|
|
|
As you can see by comparing the original IR and the one to which the function
|
|
isolation pass has been applied the main difference is that, on the basis of the
|
|
information recovered by the function boundaries analysis applied by revamb, now
|
|
the code is organized in different LLVM functions.
|
|
|
|
As a reference we can see that the basic block `bb.myfunction` that belonged to
|
|
the `root` function after the isolation is in the LLVM function
|
|
`bb.myfunction`.
|
|
|
|
.. code-block:: llvm
|
|
|
|
define void @bb.myfunction() {
|
|
bb.myfunction:
|
|
call void (i64, i64, i32, i8*, ...) @newpc(i64 4194536, i64 5, i32 1, i8* null), !dbg !96, !oi !97, !pi !98
|
|
; ...
|
|
ret void
|
|
}
|
|
|
|
Moreover, with this structure, instead of tagging the actual function calls with
|
|
a call to ``function_call`` we can place a real LLVM function call to the target
|
|
function.
|
|
Just after the function call we also add a branch to the identified return
|
|
address.
|
|
|
|
As a reference take the call to ``my_function``. In the original IR it appeared in
|
|
this form:
|
|
|
|
.. code-block:: llvm
|
|
|
|
call void @function_call(i8* blockaddress(@root, %bb.myfunction), i8* blockaddress(@root, %bb._start.0x11), i32 4194559), !dbg !60
|
|
br label %bb.myfunction, !dbg !61, !func.entry !62, !func.member.of !63
|
|
|
|
Now with the actual call appears like this:
|
|
|
|
.. code-block:: llvm
|
|
|
|
call void @bb.myfunction()
|
|
br label %bb._start.0x11
|
|
|
|
Always on the basis of the information recovered by the analysis performed by
|
|
revamb we are able to emit `ret` instructions where needed.
|
|
|
|
As a reference at the end of the basic block ``bb.myfunction`` the branch to the
|
|
dispatcher:
|
|
|
|
.. code-block:: llvm
|
|
|
|
br label %dispatcher.entry, !func.entry !151, !func.member.of !152, !func.return !151
|
|
|
|
has been substituted by the `ret` instruction:
|
|
|
|
.. code-block:: llvm
|
|
|
|
ret void
|
|
|
|
The fact that we are now not always operating inside the ```root`` function
|
|
means that we can't simply branch to the dispatcher when we need it.
|
|
For this purpose we have introduced a custom exception handling mechanism to be
|
|
able to restore the execution from the dispatcher when things do not go as
|
|
expected.
|
|
|
|
The main idea is to have a sort of separation between the world of the isolated
|
|
functions and the `root` function. In this way, as soon as possible after the
|
|
start of the execution of the program, we try to jump in the *isolated* world
|
|
and continue the execution from there. When we are not anymore able to continue
|
|
the execution in the *isolated* world we generate an exception that restores the
|
|
execution in the other world.
|
|
|
|
To do this we need to use the exception handling mechanism provided by the LLVM
|
|
framework, modifying it a little bit to suit our needs.
|
|
|
|
The first thing that we do is substitute the code of each `func.entry` block in
|
|
the `root` function with an `invoke` instruction that calls the isolated
|
|
function.
|
|
In our example, examining the ``bb._start`` function, we substitute the code of
|
|
the entry block with this:
|
|
|
|
.. code-block:: llvm
|
|
|
|
bb._start: ; preds = %dispatcher.entry
|
|
invoke void @bb._start()
|
|
to label %invoke_return unwind label %catchblock
|
|
|
|
In this way when we reach a point, inside the body of a function, where we need
|
|
the dispatcher we can use the ``_Unwind_RaiseException`` function provided by
|
|
``libunwind`` to restore the execution in the ``root`` function, where we take
|
|
care of doing the right action to correctly continue the execution(i.e. invoke)
|
|
the dispatcher.
|
|
|
|
Due to implementation details, we do not rely on the standard mechanism used by
|
|
the C++ excpetion handling mechanism. For this reason the ``catchblock`` is not
|
|
used, but we always transfer the execution to the ``invoke_return`` block, and
|
|
we then check for the value of ``ExceptionFlag`` for deciding where to transfer
|
|
the execution.
|
|
After this we transfer the control flow to the ``dispatcher.entry`` block for
|
|
resuming the execution in the correct manner.
|
|
|
|
We then need a ``function_dispatcher`` that acts as a normal dispatcher but is
|
|
used in presence of an indirect function call and assumes the form of a LLVM
|
|
function. Obviously the possible targets are only the function entry blocks,
|
|
since it is not possible that a function call requires to jump in the middle of
|
|
the code of a function.
|
|
|
|
We also add an extra check after each call to the ``function_dispatcher`` to
|
|
ensure that the program counter value is the one that we expect to have after
|
|
the call. This mechanism is usefull to avoid errors due to a bad identification
|
|
of ``ret`` instructions by the function boundaries analysis.
|
|
|
|
During the execution of the translated program, when an exception is raised, the
|
|
``exception_warning`` helper function is called, and it will print on ``stdout``
|
|
useful informations about the conditions that caused the exception (e.g. the
|
|
current program counter at the moment of the exception, the next program
|
|
counter, etc.).
|
|
|
|
.. _LLVM Language Reference Manual: http://llvm.org/docs/LangRef.html
|
|
.. _`FromIRToExecutable.rst`: FromIRToExecutable.rst
|