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Date: Wed, 26 Feb 2020 17:48:20 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Add REL_COPY size change detection
On Wed, Feb 26, 2020 at 08:53:03PM +0100, Florian Weimer wrote:
> * Rich Felker:
>
> > 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?
>
> There's an NSS bug report:
>
> <https://bugzilla.mozilla.org/show_bug.cgi?id=1201900>
>
> It seems that the NSS situation is better than what I remembered.
Good to know.
> > 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.
>
> If I recall correctly, the optimization was a factor when rolling out
> PIE-by-default in Fedora. I do not know if we can revert it without
> switching back to fixed-address builds.
I think this is almost surely premature optimization. In almost all
cases, if there's software where the performance impact makes a
difference it can be avoided by giving the affected global data
objects visibility of hidden (if it's not used outside the main
program anyway) or protected (if it needs to be externally visible).
But on x86_64 and aarch64, and to some extent on 32-bit arm as well,
the performance difference of accessing globals via the got vs
pc-relative is negligible.
> Even if we did that, the ABI incompatibility will still be there.
Yes. But fixing it would avoid any bugs from fallout of the full
object not being copyable at runtime in any newly build programs.
> There is also a similar truncation issue for TLS variables, I think.
TLS variables never use copy relocations, except for a short period of
time on riscv64, where thankfully it was realized to be a mistake and
reverted. So I don't think this issue applies to TLS.
Rich
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