Re: [musl] [PATCH] Add REL_COPY size change detection

[Rich Felker <dalias@libc.org>]

On Wed, Feb 26, 2020 at 07:38:31PM +0100, Florian Weimer wrote:
> * Rich Felker:
> 
> > At the very least I think we ought to catch and error on the case
> > where def.sym->st_size>sym->st_size, since we can't honor it and
> > failure to honor it can produce silent memory corruption. I'm less
> > sure about what to do if def.sym->st_size<sym->st-size; this case
> > seems safe and might be desirable not to break (I vaguely recall an
> > intent that it be ok), but if you think there are reasons it's
> > dangerous I'm ok with disallowing it too. I'm having a hard time now
> > thinking of a reason it would really help to support that, anyway.
> 
> Unfortunately the Mozilla NSS people disagree that size mismatches for
> global symbols are an ABI break.  I don't know if this is relevant in
> the musl context, but it means that for glibc, we probably can't make
> it a hard error.
> 
> I want to have better diagnostics for this in glibc, but the current
> warning (which is poorly worded at that) is in the
> architecture-specific code, and I got side-tracked when I tried to
> clean this up the last time.

Thanks for the feedback. Do you have a source where we could read more
about this? What non-broken behavior do they expect to get when sizes
don't match?

As an aside, I think we should be encouraging distros that are using
PIE to get rid of copy relocations by passing whatever options are
needed (or building gcc with whatever options are needed) to avoid
emitting them in PIE. IIRC I looked this up once but I can't remember
what I found.

Rich