[Bug binutils/31000] New: objcopy: add support for changing ELF symbol visibility

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=31000

            Bug ID: 31000
           Summary: objcopy: add support for changing ELF symbol
                    visibility
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

Recently I have been dealing with a benign multiple definition related to
compiler-rt/lib/builtins and rust-lang/compiler-builtins.
I need a workaround to convert certain symbols with hidden visibility to
default visibility.
(If you are curious, see
https://discourse.llvm.org/t/relocatable-file-definitions-shared-with-dso/74391
related to an improved linker error --no-allow-shlib-undefined)

It appears that objcopy does not provide a direct way to set symbol visibility,
but I have found a workaround:

    objcopy --strip-symbol __udivmodti4 --add-symbol
__udivmodti4=.text.__udivmodti4:0,weak udivmodti4.c.o

Running this command on an archive will add __udivmodti4 to every archive
member, which is not desired. So, I needed to determine which archive member
defines __udivmodti4. I ended up implementing something like this in Bazel:

    """$(AR) x --output=$(@D)/lib $(OUTS)
    for o in $(@D)/lib/*.o; do
      $(NM) -gU $$o | grep -qw __udivmodti4 && $(OBJCOPY) --strip-symbol
__udivmodti4 --add-symbol __udivmodti4=.text.__udivmodti4:0,weak $$o
    done
    $(AR) r $(OUTS) $(@D)/lib/*.o
    rm -rf $(@D)/lib"""

This is cumbersome.

Suppose you want to convert a symbol from default visibility to hidden
visibility. Unfortunately, I couldn't find a straightforward way to do it.
However, llvm-objcopy provides two extension:

    # objcopy --strip-symbol __udivmodti4 --add-symbol
__udivmodti4=.text.__udivmodti4:0,weak,hidden udivmodti4.c.o # unrecognized
symbol flag `hidden'
    llvm-objcopy --strip-symbol __udivmodti4 --add-symbol
__udivmodti4=.text.__udivmodti4:0,weak,hidden udivmodti4.c.o
    llvm-objcopy --strip-symbol __udivmodti4 --add-symbol
__udivmodti4=.text.__udivmodti4:0,weak --new-symbol-visibility=hidden
udivmodti4.c.o

https://llvm.org/docs/CommandGuide/llvm-objcopy.html#cmdoption-llvm-objcopy-new-symbol-visibility

--new-symbol-visibility also affects `_binary_*_{start,end,size}` symbols
created by
llvm-objcopy --new-symbol-visibility hidden -I binary -B i386:x86-64 a.txt a.o

This feature request tracks these possible extensions:

* 'hidden' in --add-symbol
* --new-symbol-visibility=hidden
* An option like --set-symbol-flags that can operate on an existing symbol

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