[Bug ld/27871] New: ld: Add -Bsymbolic-global-functions variant which only applies to STB_GLOBAL STT_FUNC

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27871

            Bug ID: 27871
           Summary: ld: Add -Bsymbolic-global-functions variant which only
                    applies to STB_GLOBAL STT_FUNC
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

As a poor man's direct binding feature, -Bsymbolic-functions is incompatible
with two things:

(1) canonical PLT entries with -fno-pic code. This should be fixed on GCC's
side https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100593
(2) some vague linkage function definitions. See below


cat > ./a.cc <<eof
inline void f() {}
void *g() { return (void *)&f; }
eof
cat > ./a.sh <<eof
gcc -fpic -fuse-ld=bfd -shared -Wl,-Bsymbolic-functions a.cc -o a.so
gcc -fpie -fuse-ld=bfd b.cc ./a.so
./a.out
eof
cat > ./b.cc <<eof
#include <stdio.h>

inline void f() {}
void *g();
int main() {
  printf("exe: %p\n", (void *)&f);
  printf("DSO: %p\n", g());
}
eof

On Mach-O, such symbols are placed into __LINKEDIT,__weak_binding so that dyld
can coalesce the definitions across dylibs.

For ELF, we can introduce -Bsymbolic-global-functions to exclude STB_WEAK
function definitions, avoiding the pointer equality issue.

For more context, see
https://maskray.me/blog/2021-05-16-elf-interposition-and-bsymbolic#the-last-alliance-of-elf-and-men

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