Date: Wed, 26 Feb 2020 20:53:03 +0100 From: Florian Weimer <fw@...eb.enyo.de> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: [PATCH] Add REL_COPY size change detection * 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. > 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. Even if we did that, the ABI incompatibility will still be there. There is also a similar truncation issue for TLS variables, I think.
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