From 63e333184093d8a85bf3b147ebd14dfe6306e826 Mon Sep 17 00:00:00 2001
From: Austin Clements <austin@google.com>
Date: Tue, 1 Dec 2015 22:55:28 -0500
Subject: [PATCH] dwbug/elf: support old-style compressed DWARF

GCC and LLVM support zlib-compressing DWARF debug sections (and
there's some evidence that this may be happening by default in some
circumstances now).

Add support for reading compressed DWARF sections. Since ELF
relocations apply to the decompressed data, decompression is done
before applying relocations. Since relcations are applied by
debug/elf, decompression must also be handled there.

Note that this is different from compressed ELF sections, which is a
more general mechanism used by very recent versions of GCC.

Updates #11773.

Change-Id: I3f4bf1b04d0802cc1e8fcb7c2a5fcf6c467c5089
Reviewed-on: https://go-review.googlesource.com/17340
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
---
 elf/file.go                                |  29 ++++++++++++++++++---
 elf/file_test.go                           |  28 ++++++++++++++++++++
 elf/testdata/zdebug-test-gcc484-x86-64.obj | Bin 0 -> 3216 bytes
 3 files changed, 54 insertions(+), 3 deletions(-)
 create mode 100644 elf/testdata/zdebug-test-gcc484-x86-64.obj

diff --git a/elf/file.go b/elf/file.go
index 3e766af..a42bde9 100644
--- a/elf/file.go
+++ b/elf/file.go
@@ -7,6 +7,7 @@ package elf
 
 import (
 	"bytes"
+	"compress/zlib"
 	"debug/dwarf"
 	"encoding/binary"
 	"errors"
@@ -863,6 +864,22 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 			return nil, err
 		}
 
+		if len(b) >= 12 && string(b[:4]) == "ZLIB" {
+			dlen := binary.BigEndian.Uint64(b[4:12])
+			dbuf := make([]byte, dlen)
+			r, err := zlib.NewReader(bytes.NewBuffer(b[12:]))
+			if err != nil {
+				return nil, err
+			}
+			if _, err := io.ReadFull(r, dbuf); err != nil {
+				return nil, err
+			}
+			if err := r.Close(); err != nil {
+				return nil, err
+			}
+			b = dbuf
+		}
+
 		for _, r := range f.Sections {
 			if r.Type != SHT_RELA && r.Type != SHT_REL {
 				continue
@@ -887,17 +904,23 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 	// Don't bother loading others.
 	var dat = map[string][]byte{"abbrev": nil, "info": nil, "str": nil, "line": nil}
 	for i, s := range f.Sections {
-		if !strings.HasPrefix(s.Name, ".debug_") {
+		suffix := ""
+		switch {
+		case strings.HasPrefix(s.Name, ".debug_"):
+			suffix = s.Name[7:]
+		case strings.HasPrefix(s.Name, ".zdebug_"):
+			suffix = s.Name[8:]
+		default:
 			continue
 		}
-		if _, ok := dat[s.Name[7:]]; !ok {
+		if _, ok := dat[suffix]; !ok {
 			continue
 		}
 		b, err := sectionData(i, s)
 		if err != nil {
 			return nil, err
 		}
-		dat[s.Name[7:]] = b
+		dat[suffix] = b
 	}
 
 	d, err := dwarf.New(dat["abbrev"], nil, nil, dat["info"], dat["line"], nil, nil, dat["str"])
diff --git a/elf/file_test.go b/elf/file_test.go
index cd1a457..6864b6d 100644
--- a/elf/file_test.go
+++ b/elf/file_test.go
@@ -514,6 +514,34 @@ func TestDWARFRelocations(t *testing.T) {
 	}
 }
 
+func TestCompressedDWARF(t *testing.T) {
+	// Test file built with GCC 4.8.4 and as 2.24 using:
+	// gcc -Wa,--compress-debug-sections -g -c -o zdebug-test-gcc484-x86-64.obj hello.c
+	f, err := Open("testdata/zdebug-test-gcc484-x86-64.obj")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dwarf, err := f.DWARF()
+	if err != nil {
+		t.Fatal(err)
+	}
+	reader := dwarf.Reader()
+	n := 0
+	for {
+		entry, err := reader.Next()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if entry == nil {
+			break
+		}
+		n++
+	}
+	if n != 18 {
+		t.Fatalf("want %d DWARF entries, got %d", 18, n)
+	}
+}
+
 func TestNoSectionOverlaps(t *testing.T) {
 	// Ensure 6l outputs sections without overlaps.
 	if runtime.GOOS != "linux" && runtime.GOOS != "freebsd" {
diff --git a/elf/testdata/zdebug-test-gcc484-x86-64.obj b/elf/testdata/zdebug-test-gcc484-x86-64.obj
new file mode 100644
index 0000000000000000000000000000000000000000..a595a01df45c31a5f266f71103eca7a835242996
GIT binary patch
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literal 0
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