Summary: | Segfault in AVX-2 strncat implementation (gnulib) | ||
---|---|---|---|
Product: | glibc | Reporter: | Simon Chopin <simon.chopin> |
Component: | string | Assignee: | Not yet assigned to anyone <unassigned> |
Status: | RESOLVED FIXED | ||
Severity: | normal | CC: | carlos, goldstein.w.n, hjl.tools |
Priority: | P2 | ||
Version: | 2.37 | ||
Target Milestone: | 2.37 | ||
Host: | Target: | ||
Build: | Last reconfirmed: |
Description
Simon Chopin
2023-01-31 15:55:16 UTC
(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 https://sourceware.org/glibc/wiki/Testing/Gnulib Those directions seem a bit outdated, any chance you can share your build steps? > > > 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 > #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. So far unable to reproduce, haven't been able to build gnulib against installed GLIBC, but pulled out the u8/u32 strncat and tested them both. As well have done exhaustive strncat/wcsncpy at the end of page (exhaustive for len {0..128} with all alignments {4096-128...4095} for s1/s2. So far unable to reproduce. Maybe the issue is uninitialized register. (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. (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. (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* The second argument of strncat is not required to be a string, and the function shall not access more bytes than indicated by the third argument. (In reply to Andreas Schwab from comment #5) > The second argument of strncat is not required to be a string, and the > function shall not access more bytes than indicated by the third argument. You're right, re-read man page. Have fix coming up shortly + test. Bugfix patch posted. Fix pushed: https://sourceware.org/git/?p=glibc.git;a=commit;h=b2c474f8de4c92bfe7435853a96805ec32d68dfa Marking this as fixed for 2.37. |