[Bug ld/27973] New: ld x86: Allow direct access to protected function symbols

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=27973

            Bug ID: 27973
           Summary: ld x86: Allow direct access to protected function
                    symbols
           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: ---

// a.c
__attribute__((visibility("protected"))) void *foo () {
  return (void *)foo;
}

// This roughly maps to:
        .globl  foo
        .protected      foo
        .type   foo, @function
foo:
        leaq    foo(%rip), %rax

% gcc -fpic -shared -fuse-ld=bfd a.s
/usr/bin/ld.bfd: /tmp/ccWPJCLw.o: relocation R_X86_64_PC32 against protected
symbol `foo' can not be used when making a shared object
/usr/bin/ld.bfd: final link failed: bad value
collect2: error: ld returned 1 exit status

The diagnostic was probably added circa 2016/2017 to make copy relocations on
protected data symbols work with glibc.
It should not apply to function symbols.


The "copy relocations on protected data symbols" scheme has some fragile
support with GNU ld and glibc on x86 (acked by multiple glibc maintainers).
It definitely did not work before circa 2016, and should not work in the
future.
This scheme does not work with gold or LLD.
GNU ld for non-x86 architectures don't work.
I think no software will break if we drop the fragile glibc support.

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