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Date: Sun, 30 Aug 2020 21:41:45 -0400
From: Rich Felker <>
Cc: Theodore Dubois <>
Subject: Re: i386 __set_thread_area will crash if the syscall fails

On Sun, Aug 30, 2020 at 09:07:10PM -0400, Rich Felker wrote:
> On Sun, Aug 30, 2020 at 05:34:09PM -0700, Theodore Dubois wrote:
> > Found a (small) bug in this file:
> >
> > 
> > If the syscall fails, the branch on line 20 is taken and %eax will
> > be a small negative number. Then "mov $123,%al" will make syscall
> > 0xffffff7b instead of 0x7b, since overwriting %al only overwrites
> > the low byte of %eax. So the modify_ldt fallback has apparently
> > never worked.
> Thanks! Indeed, systems where the first syscall fails are outside the
> actually-supported version range (before 2.6) so it's likely that this
> was never actually tested. Nowadays SECCOMP_FILTER makes it easy to
> test, so we should actually try to test some of these things. Have you
> checked if it works adding xor %eax,%eax before the byte mov or
> changing it to a 32-bit mov?

OK, I just confirmed it works after this change. I'll push a fix soon
along with a bunch of other pending commits. Thanks again for the


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