[llvm-bugs] [Bug 51082] New: link error when overriding common symbols with weak symbols in library

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51082

            Bug ID: 51082
           Summary: link error when overriding common symbols with weak
                    symbols in library
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: yabinc at google.com
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com

If I define a common symbol in main object file, and define a weak symbol with
the same name in a library. lld may not be able to override the common symbol
correctly with the weak symbol, and report link error.

Below is an example:

main.c:

#include <stdio.h>

extern int A;
const char* B;

int main() {
  printf("A = %d, B = %s\n", A, B);
}

lib.c:

int A;
const char* B __attribute__((weak));

build.sh:

CLANG_DIR=/ssd/opensource/llvm/llvm-build/install/bin

$CLANG_DIR/clang -c -o main.o main.c -fPIE -fcommon
$CLANG_DIR/clang -c -o lib.o lib.c -fPIC
$CLANG_DIR/llvm-ar crsPD -format=gnu lib.a lib.o
$CLANG_DIR/clang main.o lib.a -pie -fuse-ld=lld -v

link error:

ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol B;
recompile with -fPIC
>>> defined in lib.a
>>> referenced by main.c
>>>               main.o:(main)


The cause of the error seems to be below:

1. lld parses main.o, and adds undefined symbol A and common symbol B.
2. lld parses lib.a, reads LazyArchive symbol A, and decides to read lib.o
   to fetch symbol A. By the way, it reads weak symbol B in lib.o, but
   decide not to override common symbol B.
3. lld parses lib.a, reads lazyArchive symbol B, and decides to override
   common symbol B. So it replace common symbol B with lazyArchive symbol B.
   But not further read weak symbol B, because lib.o has been read before.
4. As a result, we have a LazyArchive symbol B, which can't do relocation.

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