[llvm] b3336bf - [llvm-objcopy][ELF] --only-keep-debug: set offset/size of segments with no sections to zero

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2021-05-05T10:26:57-07:00
New Revision: b3336bfa2e6a38f16c4ecf4d77bd0f97ec5a46eb

URL: https://github.com/llvm/llvm-project/commit/b3336bfa2e6a38f16c4ecf4d77bd0f97ec5a46eb
DIFF: https://github.com/llvm/llvm-project/commit/b3336bfa2e6a38f16c4ecf4d77bd0f97ec5a46eb.diff

LOG: [llvm-objcopy][ELF] --only-keep-debug: set offset/size of segments with no sections to zero

PR50160: we currently ignore non-PT_PHDR segments with no sections, not
accounting for its p_offset and p_filesz: this can cause an out-of-bounds write
in `writeSegmentData` if the p_offset+p_filesz is larger than the total file
size.

This can be fixed by setting p_offset=p_filesz=0. The logic nicely unifies with
the logic added in D90897.

Reviewed By: jhenderson, rupprecht

Differential Revision: https://reviews.llvm.org/D101560

Added: 
    

Modified: 
    llvm/test/tools/llvm-objcopy/ELF/only-keep-debug.test
    llvm/tools/llvm-objcopy/ELF/Object.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/test/tools/llvm-objcopy/ELF/only-keep-debug.test b/llvm/test/tools/llvm-objcopy/ELF/only-keep-debug.test
index 64de90ce27e9..3c41afefb644 100644
--- a/llvm/test/tools/llvm-objcopy/ELF/only-keep-debug.test
+++ b/llvm/test/tools/llvm-objcopy/ELF/only-keep-debug.test
@@ -129,11 +129,11 @@ ProgramHeaders:
 # CHECK2-NEXT: [ 4] .strtab     STRTAB   0000000000000000 000221 000001 00      0   0  1
 # CHECK2-NEXT: [ 5] .shstrtab   STRTAB   0000000000000000 000222 00002b 00      0   0  1
 
-## Check that p_offset or p_filesz of empty segments or PT_PHDR are not modified.
+## Check that p_offset or p_filesz of PT_PHDR are not modified.
 # CHECK2:      Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
 # CHECK2-NEXT: PHDR 0x000040 0x0000000000000040 0x0000000000000040 0x0000a8 0x0000a8 R   0x8
 # CHECK2-NEXT: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000202 0x000202 R E 0x1000
-# CHECK2-NEXT: LOAD 0x000202 0x0000000000000202 0x0000000000000202 0x00000e 0x00000e RW  0x1
+# CHECK2-NEXT: LOAD 0x000000 0x0000000000000202 0x0000000000000202 0x000000 0x00000e RW  0x1
 
 --- !ELF
 FileHeader:
@@ -280,3 +280,41 @@ ProgramHeaders:
     VAddr:    0x1240
     FirstSec: .tdata
     LastSec:  .tdata
+
+## The offset and size fields of segments which contain no section and have no
+## parent segment are set to zeros, so that we can decrease the file size. Such
+## segments are not useful for debugging.
+# RUN: yaml2obj --docnum=5 %s -o %t5
+# RUN: llvm-objcopy --only-keep-debug %t5 %t5.dbg
+# RUN: llvm-readelf -S -l %t5.dbg | FileCheck --check-prefix=CHECK5 %s
+# RUN: llvm-objcopy --strip-sections %t5 %t5s
+# RUN: llvm-objcopy --only-keep-debug %t5s %t5s.dbg
+# RUN: llvm-readelf -S -l %t5s.dbg | FileCheck --check-prefix=CHECK5S %s
+
+# CHECK5:      [Nr] Name        Type     Address          Off    Size   ES Flg Lk Inf Al
+# CHECK5:      [ 1] .foo        NOBITS   0000000000000000 000078 001000 00   A  0   0  0
+# CHECK5-NEXT: [ 2] .strtab     STRTAB   0000000000000000 000078 000001 00      0   0  1
+# CHECK5-NEXT: [ 3] .shstrtab   STRTAB   0000000000000000 000079 000018 00      0   0  1
+
+# CHECK5:      Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# CHECK5-NEXT: NULL 0x000000 0x0000000000000000 0x0000000000000000 0x000078 0x001000     0x1
+# CHECK5-EMPTY:
+
+# CHECK5S:      Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# CHECK5S-NEXT: NULL 0x000000 0x0000000000000000 0x0000000000000000 0x000078 0x001000     0x1
+# CHECK5S-EMPTY:
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+Sections:
+  - Name:    .foo
+    Type:    SHT_PROGBITS
+    Flags:   [ SHF_ALLOC ]
+    Size:    0x01000
+ProgramHeaders:
+  - Type:     PT_NULL
+    Flags:    []
+    FileSize: 0x01000
+    MemSize:  0x01000

diff  --git a/llvm/tools/llvm-objcopy/ELF/Object.cpp b/llvm/tools/llvm-objcopy/ELF/Object.cpp
index a8bd135c3d46..0ac8b3a0c212 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/Object.cpp
@@ -2315,18 +2315,19 @@ static uint64_t layoutSegmentsForOnlyKeepDebug(std::vector<Segment *> &Segments,
                                                uint64_t HdrEnd) {
   uint64_t MaxOffset = 0;
   for (Segment *Seg : Segments) {
-    // An empty segment contains no section (see sectionWithinSegment). If it
-    // has a parent segment, copy the parent segment's offset field. This works
-    // for empty PT_TLS. We don't handle empty segments without a parent for
-    // now.
-    if (Seg->ParentSegment != nullptr && Seg->MemSize == 0)
-      Seg->Offset = Seg->ParentSegment->Offset;
-
-    const SectionBase *FirstSec = Seg->firstSection();
-    if (Seg->Type == PT_PHDR || !FirstSec)
+    if (Seg->Type == PT_PHDR)
       continue;
 
-    uint64_t Offset = FirstSec->Offset;
+    // The segment offset is generally the offset of the first section.
+    //
+    // For a segment containing no section (see sectionWithinSegment), if it has
+    // a parent segment, copy the parent segment's offset field. This works for
+    // empty PT_TLS. If no parent segment, use 0: the segment is not useful for
+    // debugging anyway.
+    const SectionBase *FirstSec = Seg->firstSection();
+    uint64_t Offset =
+        FirstSec ? FirstSec->Offset
+                 : (Seg->ParentSegment ? Seg->ParentSegment->Offset : 0);
     uint64_t FileSize = 0;
     for (const SectionBase *Sec : Seg->Sections) {
       uint64_t Size = Sec->Type == SHT_NOBITS ? 0 : Sec->Size;