Re: objcopy behavior for --globalize-symbol and --keep-global-symbol (-G)

["Jordan Rupprecht via binutils" <binutils at sourceware dot org>]

As part of work on LLVM binutils, I've been spending time looking at
GNU objcopy. One thing I'm confused by is how --globalize-symbol and
--keep-global-symbol are supposed to work, especially when comparing
their documented effects vs reading the code.

My understanding from documentation is that --globalize-symbol is
supposed to promote a symbol to global visibility, and
--keep-global-symbol is supposed to localize all symbols except those
retained by --keep-global-symbol. However, the way it's written
(simplified for purposes of this thread) looks like this:

for (symbol *sym : symbols) {
  flagword flags = sym->flags;
  char* name = sym->name;

  // If flag is global/weak, and --keep-global-symbol includes it,
  // make it local
  if ((flags & (GLOBAL | WEAK)) &&
      (keep_globals.size() != 0 && !list_contains(keep_globals, name))) {
    sym->flags &= ~(GLOBAL | WEAK);
    sym->flags |= LOCAL;
  }

  // If flag is local and --globalize-symbol is set, make it global
  // Note: checks flags, not sym->flags
  if ((flags & LOCAL) && list_contains(globalize, name)) {
    sym->flags &= ~LOCAL;
    sym->flags |= GLOBAL;
  }
}

The actual snippet I'm talking about is here:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=binutils/objcopy.c;h=6bd933993b39e9f4d3038d3045a1d4096705be3e;hb=HEAD#l1627

The two things I'd like to ask about this:

* Weak symbols are not globalized. --help/man pages never mention
this. Is this intentional?

* Supposing we had an object file with two globals, SomeGlobal and
SomeOtherGlobal, if one were to do "--globalize-symbol SomeGlobal
--keep-global-symbol SomeOtherGlobal", you might expect that both
SomeGlobal and SomeOtherGlobal are global in the output file... but it
isn't. Because --keep-global-symbol is set and doesn't include
SomeGlobal, SomeGlobal will be demoted to a local symbol. And because
the check to see if we should apply the --globalize-symbol flag checks
"flags" (the original flag set), and not "sym->flags", it decides not
to do anything, so SomeGlobal remains a local symbol. Although this is
a weird edge case, should this be changed so that --keep-global-symbol
implicitly keeps anything also specified via --globalize-symbol? (The
code seems technically correct with respect to the documentation, but
IMO the behavior is counter-intuitive).

Note that I haven't (yet) seen anyone actually try to use objcopy in
either of these ways, this is just something I've noticed from reading
documentation and code. I'm wondering if this behavior is intentional,
and if anyone is known to be relying on objcopy working this way.

Thanks -- Jordan

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