[Bug string/30065] Segfault in AVX-2 strncat implementation (gnulib)

[goldstein.w.n at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30065

--- Comment #4 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
(In reply to Noah Goldstein from comment #3)
> (In reply to Noah Goldstein from comment #2)
> > (In reply to Simon Chopin from comment #0)
> > > I did some tests on master (2f39e44a84) for the upcoming 2.37 release, and I
> > > found a regression in libunistring test suite on amd64 with AVX-2
> > > instructions, more specifically on gnulib's test-strncat. It can be
> > > reproduced using these instructions:
> > > 
> > > https://sourceware.org/glibc/wiki/Testing/Gnulib
> > > 
> > > 
> > > I bisected the issue to
> > > commit 642933158e7cf072d873231b1a9bb03291f2b989
> > > Author: Noah Goldstein <goldstein.w.n@gmail.com>
> > > Date:   Tue Nov 8 17:38:39 2022 -0800
> > > 
> > >     x86: Optimize and shrink st{r|p}{n}{cat|cpy}-avx2 functions
> > >     
> > >     Optimizations are:
> > >         1. Use more overlapping stores to avoid branches.
> > >         2. Reduce how unrolled the aligning copies are (this is more of a
> > >            code-size save, its a negative for some sizes in terms of
> > >            perf).
> > >         3. For st{r|p}n{cat|cpy} re-order the branches to minimize the
> > >            number that are taken.
> > > 
> > > I get the following backtrace:
> > > 
> > > #0  __strncat_avx2 () at ../sysdeps/x86_64/multiarch/strncat-avx2.S:76
> > 
> > Do you know what instruction its segfaulting at?
> > > #1  0x00005555555555d7 in strncat (__len=0, __src=0x7ffff7de4000 "",
> > > __dest=0x55555555c2a1 "") at
> > > /tmp/glibc-dev/include/bits/string_fortified.h:138
> > > #2  check_single (input=input@entry=0x7ffff7de4000 "", n=n@entry=0,
> > > length=90) at unistr/test-strncat.h:41
> > > #3  0x0000555555555352 in check (input=0x555555559100 <input> "Grüß Gott.
> > > Здравствуйте! x=(-b±sqrt(b²-4ac))/(2a)  日本語,中文,한글", input_length=91)
> > >     at unistr/test-strncat.h:86
> > > #4  main () at test-strncat.c:58
> > > 
> > > I'm out of my depth in the assembler code.
> 
> Reproduced. Changed the `malloc` to mmap and put it at the end of the code.
> 
> Issue is:
> ```
> 	test	%rdx, %rdx
> 	jl	L(zero_len)
> ```
> 
> Needs to be `jle` (was `decq` at some point, when changed didn't update
> flag).
> I think the test is actually UB b.c `dst` is not a valid null-terminated
> string (even though zero length), but will fix.
`src` is not a valid null-terminated string*

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