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[SanitizerCoverage] Make __start_/__stop_ symbols extern_weak 

On ELF, we place the metadata sections (`__sancov_guards`, `__sancov_cntrs`,
`__sancov_bools`, `__sancov_pcs` in section groups (either `comdat any` or
`comdat noduplicates`).

With `--gc-sections`, LLD since D96753 and GNU ld `-z start-stop-gc` may garbage
collect such sections. If all `__sancov_bools` are discarded, LLD will error
`error: undefined hidden symbol: __start___sancov_cntrs` (other sections are similar).

```
% cat a.c
void discarded() {}
% clang -fsanitize-coverage=func,trace-pc-guard -fpic -fvisibility=hidden a.c -shared -fuse-ld=lld -Wl,--gc-sections
...
ld.lld: error: undefined hidden symbol: __start___sancov_guards
>>> referenced by a.c
>>>               /tmp/a-456662.o:(sancov.module_ctor_trace_pc_guard)
```

Use the `extern_weak` linkage (lowered to undefined weak symbols) to avoid the
undefined error.

Differential Revision: https://reviews.llvm.org/D98903
This commit is contained in:
Fangrui Song 2021-03-18 16:46:04 -07:00
parent c9861f722e
commit 9558456b53
4 changed files with 14 additions and 5 deletions
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10
llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
View File

@ -328,13 +328,15 @@ PreservedAnalyses ModuleSanitizerCoveragePass::run(Module &M,
std::pair<Value *, Value *>
ModuleSanitizerCoverage::CreateSecStartEnd(Module &M, const char *Section,
Type *Ty) {
// Use ExternalWeak so that if all sections are discarded due to section
// garbage collection, the linker will not report undefined symbol errors.
GlobalVariable *SecStart = new GlobalVariable(
M, Ty->getPointerElementType(), false, GlobalVariable::ExternalLinkage,
nullptr, getSectionStart(Section));
M, Ty->getPointerElementType(), false,
GlobalVariable::ExternalWeakLinkage, nullptr, getSectionStart(Section));
SecStart->setVisibility(GlobalValue::HiddenVisibility);
GlobalVariable *SecEnd = new GlobalVariable(
M, Ty->getPointerElementType(), false, GlobalVariable::ExternalLinkage,
nullptr, getSectionEnd(Section));
M, Ty->getPointerElementType(), false,
GlobalVariable::ExternalWeakLinkage, nullptr, getSectionEnd(Section));
SecEnd->setVisibility(GlobalValue::HiddenVisibility);
IRBuilder<> IRB(M.getContext());
if (!TargetTriple.isOSBinFormatCOFF())

5
llvm/test/Instrumentation/SanitizerCoverage/inline-8bit-counters.ll
View File

@ -2,11 +2,14 @@
; RUN: opt < %s -sancov -sanitizer-coverage-level=1 -sanitizer-coverage-inline-8bit-counters=1 -S -enable-new-pm=0 | FileCheck %s
; RUN: opt < %s -passes='module(sancov-module)' -sanitizer-coverage-level=1 -sanitizer-coverage-inline-8bit-counters=1 -S | FileCheck %s
; CHECK: @__sancov_gen_ = private global [1 x i8] zeroinitializer, section "__sancov_cntrs", comdat($foo), align 1
; CHECK: @__start___sancov_cntrs = extern_weak hidden global i8
; CHECK-NEXT: @__stop___sancov_cntrs = extern_weak hidden global i8
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"
define void @foo() {
entry:
; CHECK: section "__sancov_cntrs", comdat($foo), align 1
; CHECK: %0 = load i8, i8* getelementptr inbounds ([1 x i8], [1 x i8]* @__sancov_gen_, i64 0, i64 0), align 1, !nosanitize
; CHECK: %1 = add i8 %0, 1
; CHECK: store i8 %1, i8* getelementptr inbounds ([1 x i8], [1 x i8]* @__sancov_gen_, i64 0, i64 0), align 1, !nosanitize

2
llvm/test/Instrumentation/SanitizerCoverage/inline-bool-flag.ll
View File

@ -4,6 +4,8 @@
; CHECK: $foo = comdat noduplicates
; CHECK: @__sancov_gen_ = private global [1 x i1] zeroinitializer, section "__sancov_bools", comdat($foo), align 1{{$}}
; CHECK: @__start___sancov_bools = extern_weak hidden global i1
; CHECK-NEXT: @__stop___sancov_bools = extern_weak hidden global i1
; CHECK-NOT: @llvm.used =
; CHECK: @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast ([1 x i1]* @__sancov_gen_ to i8*)], section "llvm.metadata"

2
llvm/test/Instrumentation/SanitizerCoverage/pc-table.ll
View File

@ -22,6 +22,8 @@ entry:
}
; CHECK: private constant [6 x i64*] [{{.*}}@foo{{.*}}blockaddress{{.*}}blockaddress{{.*}}], section "__sancov_pcs", comdat($foo), align 8
; CHECK: @__start___sancov_pcs = extern_weak hidden global i64
; CHECK-NEXT: @__stop___sancov_pcs = extern_weak hidden global i64
; CHECK: define internal void @sancov.module_ctor
; CHECK: call void @__sanitizer_cov
; CHECK: call void @__sanitizer_cov_pcs_init