Hello Yury,

Yury Khrustalev via linaro-toolchain linaro-toolchain@lists.linaro.org writes:

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From: Christophe Lyon christophe.lyon@linaro.org Sent: 20 October 2025 12:21 PM To: Yury Khrustalev Cc: linaro-toolchain@lists.linaro.org Subject: Re: [Linaro-TCWG-CI] glibc-2.42.9000-284-g27effb3d50: 9 regressions on aarch64 ...

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Can I ask, which version of qemu was used for this build? does it support SME?

This build didn't use qemu. it was executed on a graviton 3 instance.

There are QEMU variables in the make check command... In any case, I cannot reproduce these failures, all relevant tests work correctly on both FVP and hardware aarch64 system.

Sorry for the delay. It took me a bit to reproduce it outside of our CI environment because I'm not used to glibc development, but I finally managed it on a QEMU VM with -cpu max running Ubuntu 24.04:

1. Build glibc and install according to https://sourceware.org/glibc/wiki/Testing/Builds#Building_glibc_with_intent_...

2. Change GDB testcase to wait for GDB to be attached:

--- a/gdb/testsuite/gdb.threads/foll-fork-other-thread.c +++ b/gdb/testsuite/gdb.threads/foll-fork-other-thread.c @@ -22,6 +22,7 @@ #include <errno.h> #include <assert.h> #include <limits.h> +#include <stdio.h>

/* Set by GDB. */ volatile int stop_looping = 0; @@ -66,6 +67,13 @@ main (void) int i; int ret; pthread_t thread; + volatile int gdb_attached = 0; + + + printf ("PID = %d\n", getpid ()); + + while (gdb_attached == 0) + sleep_a_bit ();

alarm (60);

I'm attaching the full .c file, to simplify things.

3. Build testcase according to https://sourceware.org/glibc/wiki/Testing/Builds#Compile_against_glibc_in_an...:

$ SYSROOT=/path/to/glibc-install $ gcc -L${SYSROOT}/usr/lib64 \ -I${SYSROOT}/include \ --sysroot=${SYSROOT} \ -Wl,-rpath=${SYSROOT}/lib64 \ -Wl,--dynamic-linker=${SYSROOT}/lib/ld-linux-aarch64.so.1 \ -g \ -pthread \ -o foll-fork-other-thread \ -DFORK_FUNC=fork \ foll-fork-other-thread.c

4. Create the following GDB commands file:

$ cat gdb-commands.txt set libthread-db-search-path /path/to/glibc-install/lib64 frame function main set gdb_attached = 1 set displaced-stepping auto catch fork continue thread 1 set scheduler-locking on break foll-fork-other-thread.c:85 continue set scheduler-locking off delete breakpoints set follow-fork child next bt

5. Run the testcase and attach GDB to it (you can use the distro's GDB):

$ ./foll-fork-other-thread & [1] 32660 PID = 32660 $ gdb -p 32660 -x gdb-commands.txt Attaching to process 32660 Reading symbols from /home/bauermann/scratchpad/GNU-1706/foll-fork-other-thread... Reading symbols from /home/thiago.bauermann/tmp/glibc-install/lib64/libc.so.6... Reading symbols from /home/thiago.bauermann/tmp/glibc-install/lib/ld-linux-aarch64.so.1... [Thread debugging using libthread_db enabled] Using host libthread_db library "/home/thiago.bauermann/tmp/glibc-install/lib64/libthread_db.so.1". __internal_syscall_cancel (a1=a1@entry=0, a2=a2@entry=0, a3=a3@entry=281474087606464, a4=a4@entry=0, a5=a5@entry=0, a6=a6@entry=0, nr=nr@entry=115) at cancellation.c:40

⚠️ warning: 40 cancellation.c: No such file or directory #5 0x0000aaaabcdf0dac in main () at foll-fork-other-thread.c:76 76 sleep_a_bit (); Catchpoint 1 (fork) [New Thread 0xffffacf0f1a0 (LWP 32682)] [Switching to Thread 0xffffacf0f1a0 (LWP 32682)]

Thread 2 "foll-fork-other" hit Catchpoint 1 (forked process 32683), arch_fork (ctid=0xffffacf0f270) at ../sysdeps/unix/sysv/linux/arch-fork.h:41 ⚠️ warning: 41 ../sysdeps/unix/sysv/linux/arch-fork.h: No such file or directory [Switching to thread 1 (Thread 0xffffad0fdf60 (LWP 32660))] #0 __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/aarch64/syscall_cancel.S:50 ⚠️ warning: 50 ../sysdeps/unix/sysv/linux/aarch64/syscall_cancel.S: No such file or directory Breakpoint 2 at 0xaaaabcdf0e0c: file foll-fork-other-thread.c, line 85.

Thread 1 "foll-fork-other" hit Breakpoint 2, main () at foll-fork-other-thread.c:85 85 sleep_a_bit (); /* break here */ [Attaching after Thread 0xffffacf0f1a0 (LWP 32682) fork to child process 32683] [New inferior 2 (process 32683)] [Detaching after fork from parent process 32660] [Inferior 1 (process 32660) detached] [Thread debugging using libthread_db enabled] Using host libthread_db library "/home/thiago.bauermann/tmp/glibc-install/lib64/libthread_db.so.1". ⚠️ warning: Not resuming: switched threads before following fork child. [Switching to Thread 0xffffacf0f1a0 (LWP 32683)] arch_fork (ctid=0xffffacf0f270) at ../sysdeps/unix/sysv/linux/arch-fork.h:41 ⚠️ warning: 41 ../sysdeps/unix/sysv/linux/arch-fork.h: No such file or directory #0 arch_fork (ctid=0xffffacf0f270) at ../sysdeps/unix/sysv/linux/arch-fork.h:41 #1 __GI__Fork () at ../sysdeps/nptl/_Fork.c:33 #2 0x0000000000000000 in ?? () Backtrace stopped: previous frame identical to this frame (corrupt stack?) (gdb)

The "corrupt stack" error above is the detected regression.

I reproduced the problem also on today's trunk (commit 013f5167b9c0). Commit b4b713bd8921 (the parent of your commit) doesn't reproduce the problem. Here is the backtrace from it:

#0 arch_fork (ctid=0xffffa806f270) at ../sysdeps/unix/sysv/linux/arch-fork.h:43 #1 __GI__Fork () at ../sysdeps/nptl/_Fork.c:33 #2 0x0000ffffa81297ec in __libc_fork () at fork.c:75 #3 0x0000aaaacaf20bc0 in gdb_forker_thread (arg=0x0) at foll-fork-other-thread.c:35 #4 0x0000ffffa80f09cc in start_thread (arg=0x0) at pthread_create.c:448 #5 0x0000ffffa814f94c in thread_start () at ../sysdeps/unix/sysv/linux/aarch64/clone3.S:72

Looking at frame #1 from trunk shows:

(gdb) info frame 1 Stack frame at 0xffffacf0e7a0: pc = 0xffffacfc52b8 in __GI__Fork (../sysdeps/nptl/_Fork.c:33); saved pc = 0x0 called by frame at 0xffffacf0e7a0, caller of frame at 0xffffacf0e7a0 source language c. Arglist at 0xffffacf0e770, args: Locals at 0xffffacf0e770, Previous frame's sp is 0xffffacf0e7a0 Saved registers: x19 at 0xffffacf0e780, x20 at 0xffffacf0e788, x29 at 0xffffacf0e770

Notice how it says "saved pc = 0x0". That doesn't look good.

Compare with frame 1 from the parent commit b4b713bd8921, which has a good backtrace:

(gdb) info frame 1 Stack frame at 0xffffa806e7a0: pc = 0xffffa812526c in __GI__Fork (../sysdeps/nptl/_Fork.c:33); saved pc = 0xffffa81297ec called by frame at 0xffffa806e8d0, caller of frame at 0xffffa806e7a0 source language c. Arglist at 0xffffa806e770, args: Locals at 0xffffa806e770, Previous frame's sp is 0xffffa806e7a0 Saved registers: x19 at 0xffffa806e780, x20 at 0xffffa806e788, x29 at 0xffffa806e770, x30 at 0xffffa806e778

Notice that it has a valid "saved pc". Also interesting is the list of saved registers, which includes x30 (aka the link register).