[lld][elf] Skip BP ordering input sections with null data #149265

Colibrow · 2025-07-17T06:57:14Z

LLD crashes when BP (Balanced Partitioning) ordering is applied to the .bss section, since .bss has a non-zero size but its data pointer is null.

This patch adds a null check for section data in ELF's BPSectionOrderer to avoid crash.

The fix is consistent with the Mach-O implementation in #137906.

llvmbot · 2025-07-17T06:57:46Z

@llvm/pr-subscribers-lld-elf

@llvm/pr-subscribers-lld

Author: Pengying Xu (Colibrow)

Changes

Same as #137906.

Full diff: https://github.com/llvm/llvm-project/pull/149265.diff

2 Files Affected:

(modified) lld/ELF/BPSectionOrderer.cpp (+2-2)
(modified) lld/test/ELF/bp-section-orderer.s (+15-6)

diff --git a/lld/ELF/BPSectionOrderer.cpp b/lld/ELF/BPSectionOrderer.cpp
index f464b1d4518a4..c471ab163708f 100644
--- a/lld/ELF/BPSectionOrderer.cpp
+++ b/lld/ELF/BPSectionOrderer.cpp
@@ -76,10 +76,10 @@ DenseMap<const InputSectionBase *, int> elf::runBalancedPartitioning(
     if (!d)
       return;
     auto *sec = dyn_cast_or_null<InputSection>(d->section);
-    // Skip empty, discarded, ICF folded sections. Skipping ICF folded sections
+    // Skip empty, discarded, ICF folded sections, .bss. Skipping ICF folded sections
     // reduces duplicate detection work in BPSectionOrderer.
     if (!sec || sec->size == 0 || !sec->isLive() || sec->repl != sec ||
-        !orderer.secToSym.try_emplace(sec, d).second)
+        !orderer.secToSym.try_emplace(sec, d).second || !sec->content().data())
       return;
     rootSymbolToSectionIdxs[CachedHashStringRef(
                                 lld::utils::getRootSymbol(sym.getName()))]
diff --git a/lld/test/ELF/bp-section-orderer.s b/lld/test/ELF/bp-section-orderer.s
index 4df2e8d43022e..438d7c2da0f76 100644
--- a/lld/test/ELF/bp-section-orderer.s
+++ b/lld/test/ELF/bp-section-orderer.s
@@ -26,28 +26,28 @@
 
 # RUN: ld.lld -o out.s a.o --irpgo-profile=a.profdata --bp-startup-sort=function
 # RUN: llvm-nm -jn out.s | tr '\n' , | FileCheck %s --check-prefix=STARTUP
-# STARTUP: s5,s4,s3,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d3,d2,d1,{{$}}
+# STARTUP: s5,s4,s3,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d3,d2,d1,g1,{{$}}
 
 # RUN: ld.lld -o out.os a.o --irpgo-profile=a.profdata --bp-startup-sort=function --symbol-ordering-file a.txt
 # RUN: llvm-nm -jn out.os | tr '\n' , | FileCheck %s --check-prefix=ORDER-STARTUP
-# ORDER-STARTUP: s2,s1,s5,s4,s3,A,F,E,D,B,C,merged1,merged2,_start,d3,d2,d4,d1,{{$}}
+# ORDER-STARTUP: s2,s1,s5,s4,s3,A,F,E,D,B,C,merged1,merged2,_start,d3,d2,d4,d1,g1,{{$}}
 
 # RUN: ld.lld -o out.cf a.o --verbose-bp-section-orderer --bp-compression-sort=function 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-FUNC
 # RUN: ld.lld -o out.cf.icf a.o --verbose-bp-section-orderer --bp-compression-sort=function --icf=all --gc-sections 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-ICF-FUNC
 # RUN: llvm-nm -jn out.cf | tr '\n' , | FileCheck %s --check-prefix=CFUNC
-# CFUNC: s5,s4,s3,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d3,d2,d1,{{$}}
+# CFUNC: s5,s4,s3,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d3,d2,d1,g1,{{$}}
 
 # RUN: ld.lld -o out.cd a.o --verbose-bp-section-orderer --bp-compression-sort=data 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-DATA
 # RUN: llvm-nm -jn out.cd | tr '\n' , | FileCheck %s --check-prefix=CDATA
-# CDATA: s5,s3,s4,s2,s1,F,C,E,D,B,A,merged1,merged2,_start,d4,d1,d3,d2,{{$}}
+# CDATA: s5,s3,s4,s2,s1,F,C,E,D,B,A,merged1,merged2,_start,d4,d1,d3,d2,g1,{{$}}
 
 # RUN: ld.lld -o out.cb a.o --verbose-bp-section-orderer --bp-compression-sort=both 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-BOTH
 # RUN: llvm-nm -jn out.cb | tr '\n' , | FileCheck %s --check-prefix=CBOTH
-# CBOTH: s5,s3,s4,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d1,d3,d2,{{$}}
+# CBOTH: s5,s3,s4,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d1,d3,d2,g1,{{$}}
 
 # RUN: ld.lld -o out.cbs a.o --verbose-bp-section-orderer --bp-compression-sort=both --irpgo-profile=a.profdata --bp-startup-sort=function 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-BOTH
 # RUN: llvm-nm -jn out.cbs | tr '\n' , | FileCheck %s --check-prefix=CBOTH-STARTUP
-# CBOTH-STARTUP: s5,s3,s4,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d1,d3,d2,{{$}}
+# CBOTH-STARTUP: s5,s3,s4,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d1,d3,d2,g1,{{$}}
 
 # BP-COMPRESSION-FUNC: Ordered 9 sections ([[#]] bytes) using balanced partitioning
 # BP-COMPRESSION-ICF-FUNC: Ordered 8 sections ([[#]] bytes) using balanced partitioning
@@ -108,6 +108,7 @@ d3
 d2
 
 #--- a.c
+int g1;
 const char s5[] = "engineering";
 const char s4[] = "computer program";
 const char s3[] = "hardware engineer";
@@ -377,6 +378,14 @@ d1:
 	.word	6                               // 0x6
 	.size	d1, 16
 
+	.type	g1,@object                      // @g1
+	.section	.bss.g1,"aw",@nobits
+	.globl	g1
+	.p2align	2, 0x0
+g1:
+	.word	0                               // 0x0
+	.size	g1, 4
+
 	.section	".note.GNU-stack","",@progbits
 	.addrsig
 	.addrsig_sym F

github-actions · 2025-07-17T07:00:37Z

✅ With the latest revision this PR passed the C/C++ code formatter.

Colibrow · 2025-07-17T07:25:47Z

@ellishg @MaskRay Can you take a look at this PR?

ellishg · 2025-07-17T15:41:03Z

lld/ELF/BPSectionOrderer.cpp

    if (!sec || sec->size == 0 || !sec->isLive() || sec->repl != sec ||
-        !orderer.secToSym.try_emplace(sec, d).second)
+        !orderer.secToSym.try_emplace(sec, d).second || !sec->content().data())


Why not move the check to before we add the section to secToSym?

Yeah. Fixed it.

MaskRay · 2025-07-17T16:08:09Z

Same as #137906.

The description is too brief. Does a .bss lead to a crash?

Colibrow · 2025-07-17T16:23:18Z

@MaskRay Yes. I encountered a crash after updating lld to the latest version and enabling BP (Balanced Partitioning) ordering for the .data section in the ELF output.

The ld.lld command returned non-zero exit code: 139.
output:
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: ld.lld --export ...
1.	0x00000001025f7432 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (ld.lld+0x101934324)
2.	0x00000001025f4998 llvm::sys::RunSignalHandlers() (ld.lld+0x101932768)
3.	0x00000001025f4998 SignalHandler(int) (ld.lld+0x101932768)
4.	0x0000000193ecfe04 (/usr/lib/system/libsystem_platform.dylib+0x10842e04)
5.	0x0000000101ac1f1c llvm::SmallVectorBase::unsigned int, 12u>::base::grow_pod(void*, unsigned long, unsigned long) (ld.lld+0x100b6271c)
6.	0x0000000101e56f14 lld::elf::runBalancedPartitioning(lld::elf::PartitioningParams const&) (ld.lld+0x100eccc14)
7.	0x0000000101e58204 lld::elf::Ctx::Ctx() (ld.lld+0x100ecde14)
8.	0x0000000101e5d268 lld::elf::LinkerDriver::LinkerDriver() (ld.lld+0x100ed2c78)
9.	0x0000000101e5eb74 void lld::elf::writeResult<llvm::object::ELFType<llvm::endianness::little, true>>(lld::elf::Ctx&) (ld.lld+0x100ed4644)
10.	0x0000000101e5ed3c lld::elf::LinkerDriver::link(llvm::ArrayRef<char const*>) (ld.lld+0x100ed47f4)
11.	0x0000000101e5ef54 lld::elf::Link(llvm::ArrayRef<char const*>, bool, llvm::raw_ostream&, bool, bool) (ld.lld+0x100ed4a0c)
12.	0x0000000100003798 lld_main(int, char**, llvm::ToolContext const&) (ld.lld+0x100003798)
13.	0x0000000100003c98 main (ld.lld+0x100003c98)
14.	0x000000019fb0274b (/usr/lib/dyld+0x10004274b)

After investigation, I found that the crash occurs in BPSectionOrderer.cpp, where data.data() is nullptr, leading to a segmentation fault.

Colibrow · 2025-07-18T00:13:33Z

@MaskRay I don't have commit access — could you please merge this PR on my behalf? Thank you!

ellishg · 2025-07-18T00:51:51Z

I can land it tomorrow morning if it hasn't already (I like to wait until mornings to land changes to give myself time to revert or fix if needed).

llvmbot added lld lld:ELF labels Jul 17, 2025

[lld][elf] Skip BP ordering input sections with null data

4fba85b

Colibrow force-pushed the colibrow/fix-elf-bss-bp branch from e7afef3 to 4fba85b Compare July 17, 2025 07:01

ellishg reviewed Jul 17, 2025

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Address comments.

f0944cc

MaskRay approved these changes Jul 17, 2025

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ellishg approved these changes Jul 17, 2025

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ellishg merged commit 6c705d1 into llvm:main Jul 18, 2025
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[lld][elf] Skip BP ordering input sections with null data #149265

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[lld][elf] Skip BP ordering input sections with null data #149265

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