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Fangrui Song 1c70d136fb [ELF] Only allow the binding of SharedSymbol to change for the first undef ref 
Fixes PR42442

t.o has a STB_GLOBAL undef ref to f
t2.so has a STB_WEAK undef ref to f
t1.so defines f

ld.lld t.o t1.so t2.so currently sets the binding of `f` to STB_WEAK.
This is not correct because there exists a STB_GLOBAL undef ref from a
regular object. The problem is that resolveUndefined() doesn't check
if the undef ref is seen for the first time:

    if (isShared() || isLazy() || (isUndefined() && Other.Binding != STB_WEAK))
      Binding = Other.Binding;

The isShared() condition should be `isShared() && !Referenced`
where Referenced is set to true after an undef ref is seen.

In practice, when linking a pthread program with glibc:

    // a.o
    #include <pthread.h>
    pthread_mutex_t mu = PTHREAD_MUTEX_INITIALIZER;
    int main() { pthread_mutex_unlock(&mu); }

{clang,gcc} -fuse-ld=lld a.o -lpthread # libpthread.so is linked before libgcc_s.so.1

The weak undef pthread_mutex_unlock in libgcc_s.so.1 makes the result
weak, which diverges from GNU linkers where STB_DEFAULT is used:

    23: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND pthread_mutex_lock

(Note, if -pthread is used instead, libpthread.so will be linked **after**
libgcc_s.so.1 . lld sets the binding to the expected STB_GLOBAL)

Similar linking sequences (ld.lld t.o t1.so t2.so) appear to be used by
Go, which cause a build error https://github.com/golang/go/issues/31912.

Reviewed By: grimar, ruiu

Differential Revision: https://reviews.llvm.org/D63974

llvm-svn: 364913
 		2019-07-02 11:37:21 +00:00
..
COFF [COFF] Fix .rsrc sections with differing permissions 2019-06-28 17:13:52 +00:00
Common [lld] Allow args::getInterger to parse args larger than 2^31-1 2019-06-07 06:05:26 +00:00
ELF [ELF] Only allow the binding of SharedSymbol to change for the first undef ref 2019-07-02 11:37:21 +00:00
MinGW [MinGW] Support the --subsystem=val option in joined form 2019-06-14 17:50:39 +00:00
cmake/modules [CMake] Properly conditionalize installation of lld libraries 2018-03-09 13:09:36 +00:00
docs Add --undefined-glob which is an --undefined with wildcard pattern match 2019-06-14 14:00:59 +00:00
include/lld [lld] Allow args::getInterger to parse args larger than 2^31-1 2019-06-07 06:05:26 +00:00
lib Use llvm::stable_sort 2019-04-23 02:42:06 +00:00
test [ELF] Only allow the binding of SharedSymbol to change for the first undef ref 2019-07-02 11:37:21 +00:00
tools/lld Fix -DBUILD_SHARED_LIBS=on build after r362977 2019-06-11 01:50:58 +00:00
unittests Update the file headers across all of the LLVM projects in the monorepo 2019-01-19 08:50:56 +00:00
utils Python 2/3 compatibility 2019-03-20 07:42:13 +00:00
wasm [wasm-ld] Add __global_base symbol to mark the value of --global-base 2019-06-26 20:12:33 +00:00
.arcconfig [lld] Set up .arcconfig to point to new Diffusion LLD repository 2017-12-06 20:56:23 +00:00
.clang-format
…
.gitignore
…
CMakeLists.txt
…
CODE_OWNERS.TXT
…
LICENSE.TXT Fix typos throughout the license files that somehow I and my reviewers 2019-01-21 09:52:34 +00:00
README.md Note that the test .tar.xz has been updated. 2017-12-22 19:37:32 +00:00

README.md

LLVM Linker (lld)

This directory and its subdirectories contain source code for the LLVM Linker, a modular cross platform linker which is built as part of the LLVM compiler infrastructure project.

lld is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.

Benchmarking

In order to make sure various developers can evaluate patches over the same tests, we create a collection of self contained programs.

It is hosted at https://s3-us-west-2.amazonaws.com/linker-tests/lld-speed-test.tar.xz

The current sha256 is 10eec685463d5a8bbf08d77f4ca96282161d396c65bd97dc99dbde644a31610f.