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Support --unresolved-symbols=@<file> option in LLD for ELF #142917

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Support --unresolved-symbols=@<file> option in LLD for ELF #142917

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e9f2733
Support --unresolved-symbols=@/path/to/file
Nechda Jun 5, 2025
2542254
Add item to release notes
Nechda Jun 5, 2025
2e52861
Fix style
Nechda Jun 5, 2025
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1 change: 1 addition & 0 deletions lld/ELF/Config.h
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Original file line number Diff line number Diff line change
Expand Up @@ -397,6 +397,7 @@ struct Config {
SortSectionPolicy sortSection;
StripPolicy strip;
UnresolvedPolicy unresolvedSymbols;
llvm::SmallVector<std::string> unresolvedSymbolsList;
UnresolvedPolicy unresolvedSymbolsInShlib;
Target2Policy target2;
GcsPolicy zGcs;
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8 changes: 8 additions & 0 deletions lld/ELF/Driver.cpp
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Expand Up @@ -714,6 +714,14 @@ static void setUnresolvedSymbolPolicy(Ctx &ctx, opt::InputArgList &args) {
} else if (s == "report-all") {
diagRegular = true;
diagShlib = true;
} else if (s.starts_with("@")) {
// Read file with set of unresolved symbols
StringRef filename(s.substr(1ULL));
std::optional<MemoryBufferRef> buffer = readFile(ctx, filename);
if (!buffer)
continue;
for (auto [_, line] : llvm::enumerate(args::getLines(*buffer)))
ctx.arg.unresolvedSymbolsList.emplace_back(line);
} else {
ErrAlways(ctx) << "unknown --unresolved-symbols value: " << s;
}
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7 changes: 7 additions & 0 deletions lld/ELF/Relocations.cpp
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Expand Up @@ -804,6 +804,13 @@ static bool maybeReportUndefined(Ctx &ctx, Undefined &sym,
if (ctx.arg.unresolvedSymbols == UnresolvedPolicy::Ignore && canBeExternal)
return false;

// Skip undefined symbols from list
for (const auto &ignoredUndef : ctx.arg.unresolvedSymbolsList) {
if (ignoredUndef == sym.getName()) {
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@MaskRay MaskRay Jun 5, 2025 •
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omit braces in this case.

ctx.arg.unresolvedSymbolsList can be a DenseSet to improve performance when the list is long.

This should only apply to canBeExternal symbols.
Peter has a comment that you might be able to use --defsym=bar=0 instead.

I'd file a binutils feature request before rushing to add an option

return false;
}
}

// clang (as of 2019-06-12) / gcc (as of 8.2.1) PPC64 may emit a .rela.toc
// which references a switch table in a discarded .rodata/.text section. The
// .toc and the .rela.toc are incorrectly not placed in the comdat. The ELF
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3 changes: 3 additions & 0 deletions lld/docs/ReleaseNotes.rst
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Expand Up @@ -58,6 +58,9 @@ ELF Improvements
on executable sections.
(`#128883 <https://github.com/llvm/llvm-project/pull/128883>`_)

* Added ``--unresolved-symbols=@<file>`` flag to specify path to a file with the
list of unresolved symbols that will not trigger an error during lininking.
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typo lininking -> linking

You may also want to find a more permanent place to document this. Right now it is not in the help or man-page. LLD is piggybacking on top of GNU ld's docs at https://sourceware.org/binutils/docs/ld/Options.html and that won't mention it.


Breaking changes
----------------
* Executable-only and readable-executable sections are now allowed to be placed
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1 change: 1 addition & 0 deletions lld/test/ELF/Inputs/unresolved-symbols.s
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@@ -1,3 +1,4 @@
.globl _shared
_shared:
callq undef@PLT
callq undef2@PLT
1 change: 1 addition & 0 deletions lld/test/ELF/Inputs/unresolved.ignore
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@@ -0,0 +1 @@
undef
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New lld tests tend to favour using split-file to put all the inputs for a test in the same file.

I'd also recommend having more test cases. For example an empty file, multiple symbols on separate lines.

6 changes: 6 additions & 0 deletions lld/test/ELF/unresolved-symbols.s
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Expand Up @@ -63,5 +63,11 @@
# RUN: FileCheck -check-prefix=ERRUND %s
# RUN: not ld.lld %t2.o -o /dev/null 2>&1 | FileCheck -check-prefix=ERRUND %s

## Ignoring undefines in objects should not produce error for symbol from object.
# RUN: not ld.lld %t2.o -o /dev/null --unresolved-symbols=@%p/Inputs/unresolved.ignore 2>&1 | \
# RUN: FileCheck %s --check-prefix=IGNLST

# IGNLST: error: undefined symbol: undef2

.globl _start
_start:
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