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

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

On Mon, Sep 17, 2018 at 7:56 AM Nick Clifton <nickc@redhat.com> wrote:
>
> Hi Jordan,
>
> > 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.
>
> To be fair, this code was never planned out fully, rather it grew organically...
All good software is :)

>
> > * Weak symbols are not globalized. --help/man pages never mention
> > this. Is this intentional?
>
> Yes. There is no such thing as a weak local symbol.  (Well there shouldn't
> be anyway).  So when promoting from LOCAL to GLOBAL there is no need to also
> check for WEAK.
>
Technically a weak symbol isn't global either though -- i.e. the
binding type is an enum of STB_LOCAL/STB_GLOBAL/STB_WEAK (plus enum
masks).

Still, I'm not sure why anyone would want to take a weak symbol and
make it global. Maybe someone would want to take an object file with a
weak symbol and make sure it's a link error to also include some other
object file that has that symbol as global? Anyway, it's purely
hypothetical.

>
> > * 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.
>
> All true.
>
> >  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).
>
> Actually I think that it should be an error to mix the two options.
> However you code it, somebody is going to get confused about the behaviour
> when both options are specified.  Best to keep things simple and consider
> the two options to be mutually exclusive.
Agree, probably worth just making it an error (or at least a warning)
to use both flags.

>
> Cheers
>   Nick
>

Thanks!
-- Jordan

Attachment: smime.p7s
Description: S/MIME Cryptographic Signature