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Date: Fri, 7 Dec 2018 15:14:53 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: aio_cancel segmentation fault for in progress write
 requests

On Fri, Dec 07, 2018 at 09:06:18PM +0100, Florian Weimer wrote:
> * Rich Felker:
> 
> > I don't think so. I'm concerned that it's a stack overflow, and that
> > somehow the kernel folks have managed to break the MINSIGSTKSZ ABI.
> 
> Probably:
> 
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=20305>
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=22636>
> 
> It's a nasty CPU backwards compatibility problem.  Some of the
> suggestions I made to work around this are simply wrong; don't take them
> too seriously.
> 
> Nowadays, the kernel has a way to disable the %zmm registers, but it
> unfortunately does not reduce the save area size.

How large is the saved context with the %zmm junk? I measured just
~768 bytes on normal x86_64 without it, and since 2048 is rounded up
to a whole page (4096), overflow should not happen until the signal
context is something like 3.5k (allowing ~512 bytes for TCB (~128) and
2 simple call frames).

Rich

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