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[ELF] Update st_size when merging a common symbol with a shared symbol

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When a common symbol is merged with a shared symbol, increase st_size if
the shared symbol has a larger st_size. At runtime, the executable's
symbol overrides the shared symbol.  The shared symbol may be created
from common symbols in a previous link.  This rule makes sure we pick
the largest size among all common symbols.

This behavior matches GNU ld. See
https://sourceware.org/bugzilla/show_bug.cgi?id=25236 for discussions.

A shared symbol does not hold alignment constraints. Ignore the
alignment update.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D71161
This commit is contained in:
Fangrui Song
2019-12-06 21:18:31 -08:00
parent ed4618edb3
commit 69d10d282e
2 changed files with 34 additions and 1 deletions
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19
lld/ELF/Symbols.cpp
View File

@@ -620,7 +620,18 @@ void Symbol::resolveCommon(const CommonSymbol &other) {
return;
if (cmp > 0) {
replace(other);
if (auto *s = dyn_cast<SharedSymbol>(this)) {
// Increase st_size if the shared symbol has a larger st_size. The shared
// symbol may be created from common symbols. The fact that some object
// files were linked into a shared object first should not change the
// regular rule that picks the largest st_size.
uint64_t size = s->size;
replace(other);
if (size > cast<CommonSymbol>(this)->size)
cast<CommonSymbol>(this)->size = size;
} else {
replace(other);
}
return;
}
@@ -661,6 +672,12 @@ template <class LazyT> void Symbol::resolveLazy(const LazyT &other) {
}
void Symbol::resolveShared(const SharedSymbol &other) {
if (isCommon()) {
// See the comment in resolveCommon() above.
if (other.size > cast<CommonSymbol>(this)->size)
cast<CommonSymbol>(this)->size = other.size;
return;
}
if (visibility == STV_DEFAULT && (isUndefined() || isLazy())) {
// An undefined symbol with non default visibility must be satisfied
// in the same DSO.

16
lld/test/ELF/common-shared.s Normal file
View File

@@ -0,0 +1,16 @@
# REQUIRES: x86
## When a common symbol is merged with a shared symbol, pick the larger st_size.
# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
# RUN: echo '.globl com; .comm com, 16' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
# RUN: ld.lld -shared %t1.o -o %t1.so
# RUN: ld.lld %t.o %t1.so -o %t
# RUN: llvm-readelf -s %t | FileCheck %s
# RUN: ld.lld %t1.so %t.o -o %t
# RUN: llvm-readelf -s %t | FileCheck %s
# CHECK: 16 OBJECT GLOBAL DEFAULT 7 com
.globl com
.comm com,1