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[ELF] resolveUndefined: ignore undefined symbols in SharedFile for Undefined and SharedSymbol 

If %t1.o has a weak reference on foo, and %t2.so has a non-weak
reference on foo: `ld.lld %t1.o %t2.so -o %t`

We incorrectly set the binding of the undefined foo to STB_GLOBAL.
Fix this by ignoring undefined symbols in a SharedFile for Undefined and
SharedSymbol.

This fixes the binding of pthread_once when the program links against
both librt.so and libpthread.so

```
a.o: STB_WEAK reference to pthread_once
librt.so: STB_GLOBAL reference to pthread_once    # should be ignored
libstdc++.so: STB_WEAK reference to pthread_once  # should be ignored
libgcc_s.so.1: STB_WEAK reference to pthread_once # should be ignored
```

The STB_GLOBAL pthread_once issue (not fixed by D63974) can cause a link error when the result
DSO is used to link another DSO with -z defs if -lpthread is not specified. (libstdc++.so.6 not having a dependency on libpthread.so is a really nasty hack...)

We happened to create a weak undef before D63974 because libgcc_s.so.1
was linked the last and it changed the binding again to weak.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D64136

llvm-svn: 365129
This commit is contained in:
Fangrui Song 2019-07-04 10:38:04 +00:00
parent 555d743fcf
commit f347541fbc
3 changed files with 32 additions and 13 deletions
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31
lld/ELF/Symbols.cpp
View File

@ -410,18 +410,7 @@ void Symbol::resolveUndefined(const Undefined &Other) {
if (Traced)
printTraceSymbol(&Other);
if (isUndefined()) {
// The binding may "upgrade" from weak to non-weak.
if (Other.Binding != STB_WEAK)
Binding = Other.Binding;
} else if (auto *S = dyn_cast<SharedSymbol>(this)) {
// The binding of a SharedSymbol will be weak if there is at least one
// reference and all are weak. The binding has one opportunity to change to
// weak: if the first reference is weak.
if (Other.Binding != STB_WEAK || !S->Referenced)
Binding = Other.Binding;
S->Referenced = true;
} else if (isLazy()) {
if (isLazy()) {
// An undefined weak will not fetch archive members. See comment on Lazy in
// Symbols.h for the details.
if (Other.Binding == STB_WEAK) {
@ -489,6 +478,24 @@ void Symbol::resolveUndefined(const Undefined &Other) {
if (Backref && !isWeak())
warn("backward reference detected: " + Other.getName() + " in " +
toString(Other.File) + " refers to " + toString(File));
return;
}
// Undefined symbols in a SharedFile do not change the binding.
if (dyn_cast_or_null<SharedFile>(Other.File))
return;
if (isUndefined()) {
// The binding may "upgrade" from weak to non-weak.
if (Other.Binding != STB_WEAK)
Binding = Other.Binding;
} else if (auto *S = dyn_cast<SharedSymbol>(this)) {
// The binding of a SharedSymbol will be weak if there is at least one
// reference and all are weak. The binding has one opportunity to change to
// weak: if the first reference is weak.
if (Other.Binding != STB_WEAK || !S->Referenced)
Binding = Other.Binding;
S->Referenced = true;
}
}

9
lld/test/ELF/archive-fetch.s
View File

@ -9,7 +9,14 @@
# RUN: llvm-ar rcs %t.a %tfoo.o %tbar.o
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.a %t.o -o /dev/null
# RUN: ld.lld %t.a %t.o -o %t
# RUN: llvm-nm %t | FileCheck %s
# RUN: ld.lld -shared %t.o -o %t.so
# RUN: ld.lld %t.a %t.so -o %t
# RUN: llvm-nm %t | FileCheck %s
# CHECK: T foo
_start:
callq foo

5
lld/test/ELF/weak-undef-shared.s
View File

@ -24,6 +24,11 @@
# RUN: ld.lld %t2.o %t.so %t1.o -o %t
# RUN: llvm-readelf --dyn-syms %t | FileCheck --check-prefix=GLOBAL %s
## Check the binding (weak) is not affected by the STB_GLOBAL undefined
## reference in %t2.so
# RUN: ld.lld %t1.o %t2.so -o %t
# RUN: llvm-readelf --dyn-syms %t | FileCheck --check-prefix=WEAK %s
# WEAK: NOTYPE WEAK DEFAULT UND foo
# GLOBAL: NOTYPE GLOBAL DEFAULT UND foo