[LLD][COFF] Fix out-of-bounds write when filling gaps with INT3 in code sections by wbenny · Pull Request #180411 · llvm/llvm-project

wbenny · 2026-02-08T14:54:57Z

When merging .bss into a code section (e.g., /MERGE:.bss=.text), the INT3 gap-filling loop in writeSections() would write past the output buffer. This happens because .bss chunks have hasData=false, so they contribute to VirtualSize but not SizeOfRawData. The loop was using chunk RVAs without checking if they exceeded the raw data region.

This caused a crash on Windows with /FILEALIGN:1 (access violation 0xC0000005). The tight alignment leaves no slack in the mapped buffer, so the overflow immediately hits unmapped memory.

The fix bounds all memset operations to rawSize and exits early when encountering chunks beyond the raw data boundary.

Fixes #180406

github-actions · 2026-02-08T14:55:16Z

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llvmbot · 2026-02-08T14:55:45Z

@llvm/pr-subscribers-lld-coff

@llvm/pr-subscribers-lld

Author: Petr Beneš (wbenny)

Changes

When merging .bss into a code section (e.g., /MERGE:.bss=.text), the INT3 gap-filling loop in writeSections() would write past the output buffer. This happens because .bss chunks have hasData=false, so they contribute to VirtualSize but not SizeOfRawData. The loop was using chunk RVAs without checking if they exceeded the raw data region.

This caused a crash on Windows with /FILEALIGN:1 (access violation 0xC0000005). The tight alignment leaves no slack in the mapped buffer, so the overflow immediately hits unmapped memory.

The fix bounds all memset operations to rawSize and exits early when encountering chunks beyond the raw data boundary.

Fixes #180406

Full diff: https://github.com/llvm/llvm-project/pull/180411.diff

1 Files Affected:

(modified) lld/COFF/Writer.cpp (+7-2)

diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 559bd387fa9cb..a5e30e26b9e5b 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -2619,12 +2619,17 @@ void Writer::writeSections() {
     if ((sec->header.Characteristics & IMAGE_SCN_CNT_CODE) &&
         (ctx.config.machine == AMD64 || ctx.config.machine == I386)) {
       uint32_t prevEnd = 0;
+      uint32_t rawSize = sec->getRawSize();
       for (Chunk *c : sec->chunks) {
         uint32_t off = c->getRVA() - sec->getRVA();
+        // Chunks without data (e.g., .bss) have virtual addresses beyond
+        // rawSize; stop filling when we reach the end of raw data.
+        if (off >= rawSize)
+          break;
         memset(secBuf + prevEnd, 0xCC, off - prevEnd);
-        prevEnd = off + c->getSize();
+        prevEnd = std::min(off + static_cast<uint32_t>(c->getSize()), rawSize);
       }
-      memset(secBuf + prevEnd, 0xCC, sec->getRawSize() - prevEnd);
+      memset(secBuf + prevEnd, 0xCC, rawSize - prevEnd);
     }
 
     parallelForEach(sec->chunks, [&](Chunk *c) {

aganea · 2026-02-08T17:56:54Z

This fix looks good, thanks. Are you able to craft a testcase please?

wbenny · 2026-02-08T18:21:23Z

Amended the commit with a test case and verified it doesn't pass without the fix and passes with it.

If I understand correctly, the # REQUIRES: x86 applies to AMD64 too, and it should be there, because the CCs are guarded with if .. (ctx.config.machine == AMD64 || ctx.config.machine == I386)... right?

aganea

Looks good, just a small nit.

aganea · 2026-02-09T00:02:39Z

+# RUN: llvm-readobj --sections %t.exe | FileCheck %s
+
+# CHECK:      Name: .text
+# CHECK-NEXT: VirtualSize: 0x


nit: I guess we can write the virtual size that we're expecting here: 0x104

…de sections

aganea · 2026-02-10T02:27:57Z

        uint32_t off = c->getRVA() - sec->getRVA();
+        // Chunks without data (e.g., .bss) have virtual addresses beyond
+        // rawSize; stop filling when we reach the end of raw data.
+        if (off >= rawSize)


I am wondering if this check could prevent from filling the gap between the last .text chunk and the .bss, if the file alignement is larger than the last .text chunk?

No, the memset after the loop handles that.

this exposed a new bug I guess:

clang_release_ble/DM40Native.exe //20.1.8 FileSize=55550 EP=0xd8b5 SizeOfImage=0xd8fe SizeOfHeaders=0x198 [.data] VA=0x198 VSize=0xd766 RawOff=0x198 RawSz=0xd766 Chars=0xe0000020 VSize-RawSz=0 clang_release_ble/DM40Native_broken.exe //trunk FileSize=55710 EP=0xd949 SizeOfImage=0xd9a4 SizeOfHeaders=0x198 [.data] VA=0x198 VSize=0xd80c RawOff=0x198 RawSz=0xd806 Chars=0xe0000020 VSize-RawSz=6 clang_release_bt/DM40Native.exe //20.1.8 FileSize=52542 EP=0xccc2 SizeOfImage=0xcd3e SizeOfHeaders=0x198 [.data] VA=0x198 VSize=0xcba6 RawOff=0x198 RawSz=0xcba6 Chars=0xe0000020 VSize-RawSz=0 clang_release_bt/DM40Native_broken.exe //trunk FileSize=52718 EP=0xcd6a SizeOfImage=0xcdf4 SizeOfHeaders=0x198 [.data] VA=0x198 VSize=0xcc5c RawOff=0x198 RawSz=0xcc56 Chars=0xe0000020 VSize-RawSz=6

PE loader rejects the exe

seems to be due to:

if (c->hasData) rawSize = alignTo(virtualSize, config->fileAlign);

since bss is not handled in this block it gets excluded by this, locally for me this fixed it but this probably isn't the correct solution since filealign 2 and 4 would still probably be outside the rawSize:

if (config->fileAlign == 1 && rawSize < virtualSize) rawSize = virtualSize;

so maybe something like this would be better:

if (rawSize < alignTo(virtualSize, config->fileAlign)) rawSize = alignTo(virtualSize, config->fileAlign);

Are you sure that this issue that you're observing is related to this commit? As far as I can see, this change shouldn't affect the final layout of the PE image (which can cause the image to be rejected), only what is filled in it.

Can you file a separate issue about the problem you're seeing, so we can try to track it down? Ideally, can you provider a reproducer? If you add -reproduce:repro.tar to the link command, it'll produce a file repro.tar which contains everything required for rerunning the link command. That would allow others to pinpoint exactly which LLD change is causing the PE loader to reject it. (If linking through the compiler driver in mingw mode, pass -Wl,--reproduce=repro.tar instead.)

github-actions · 2026-02-10T13:26:26Z

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Andarwinux · 2026-05-16T21:48:34Z

#198126

Would be nice yeah, the VS bundled clang/lld barely gets updated I'd rather not wait a year for this...

Can we backport this to release/22?

mstorsjo · 2026-05-17T19:53:46Z

/cherry-pick 6558595

mstorsjo · 2026-05-17T19:54:31Z

Would be nice yeah, the VS bundled clang/lld barely gets updated I'd rather not wait a year for this...

Can we backport this to release/22?

We can try; we're already kinda far in the 22 release series, so the release managers are getting more selective in what fixes they approve for backports, but we can see what they say about this.

llvmbot · 2026-05-17T20:01:20Z

/pull-request #198212

llvmbot added lld lld:COFF platform:windows labels Feb 8, 2026

aganea approved these changes Feb 9, 2026

View reviewed changes

[LLD][COFF] Fix out-of-bounds write when filling gaps with INT3 in co…

4f7d17e

…de sections

wbenny requested a review from aganea February 9, 2026 15:59

aganea reviewed Feb 10, 2026

View reviewed changes

aganea enabled auto-merge (squash) February 10, 2026 13:17

aganea merged commit 6558595 into llvm:main Feb 10, 2026
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mstorsjo added this to the LLVM 22.x Release milestone May 17, 2026

github-project-automation Bot added this to LLVM Release Status May 17, 2026

github-project-automation Bot moved this to Needs Triage in LLVM Release Status May 17, 2026

github-project-automation Bot moved this from Needs Triage to Done in LLVM Release Status May 17, 2026

Andarwinux mentioned this pull request May 26, 2026

release/22.x: [LLD][COFF] Fix out-of-bounds write when filling gaps with INT3 in code sections (#180411) #198212

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