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Bug#1087809: Bug#1086028: I've reproduced the bug in QEMU

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  Bug#1087809: Bug#1086028: I've reproduced the bug in QEMU Salvatore Bonaccorso <carnil@debian.org> - 2025-02-25 17:40 +0100
    Bug#1086028: I've reproduced the bug in QEMU Sergei Golovan <sgolovan@debian.org> - 2025-02-26 14:30 +0100

#86138 — Bug#1087809: Bug#1086028: I've reproduced the bug in QEMU

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Hi Sergei,

On Thu, Feb 13, 2025 at 01:35:13PM +0300, Sergei Golovan wrote:
> tag 1086028 + patch
> tag 1087809 + patch
> tag 1093200 + patch
> thanks
> 
> Hi!
> 
> I've finally managed to reproduce this EFAULT in QEMU (using an
> Erlang-based script which is shipped in the wings3d source package):
> 
> 1) I've installed Debian bookworm for mips64el in qemu-system-mips64el
> virtual machine (version from unstable), and upgraded it to the
> current unstable (machine is loongson3-virt, cpu is Loongson-3A4000).
> 2) I have to enable SMP in qemu and use -rtc clock=rt (otherwise the
> virtual machine won't boot, with clock=rt sometimes it boots,
> sometimes it hangs). The full QEMU command line is:
> 
> qemu-system-mips64el -machine loongson3-virt -m 4g -cpu Loongson-3A4000 \
>             -smp 2,sockets=2,cores=1,threads=1,maxcpus=2 \
>             -kernel vmlinuz-loongson-3  \
>             -rtc clock=rt \
>             -initrd initrd.img-loongson-3 -drive
> if=none,file=hda1.bin,id=hd,format=raw  \
>             -net nic -net tap,ifname=tap0,script=/bin/true \
>             -device virtio-blk-pci,drive=hd -append "root=/dev/vda1
> console=ttyS0" \
>             -nographic
> 
> Here kernel and initrd can be either stock 6.1.123-1 version or
> 6.1.123-1 with the attached patch. Unfortunately, QEMU can't boot for
> me using the newest 6.12.12-1 kernel (it complains that it can't
> uncompress initrd, I don't know why).
> 
> 4) I've install the build dependencies of wings3d (basically, only
> erlang-base is necessary)
> 5) I've extracted the wings3d source package (from stable:
> https://packages.debian.org/source/stable/wings3d)
> 6) I've added the following line as the second line to
> wings3d-2.2.9/intl_tools/gen_char_hrl
> 
> %%! +S 4:4 +SDcpu 4:4 +c false
> 
> (The first two options enable multiple threads, the last one allows
> some workaround for the case when monotonic clock jumps backwards,
> which appears to be the case for QEMU with SMP enabled).
> 7) I've run this gen_char_hrl in a loop until it fails.
> 
> The result is that with the stock 6.1.123-1 kernel approximately in 1%
> cases the script aborts with message:
> 
> signal-dispatcher thread got unexpected error: efault (14)
> 
> which is exactly the error that prevents Erlang (and many Erlang-based
> packages) from building on mips64el.
> 
> On the other hand, with the patched kernel the script loop is still
> running for more than 24 hours (a few thousands runs) without
> aborting. So I'm now fairly confident that the patch fixes the bug.
> 
> I'm not sure if there's no adverse effects caused by the patch, so
> it'd be better to try it on real hardware as well.
> 
> The patch is derived from the thread [1]. It reverses commit [2] with
> an additional change, which is necessary because of changes in
> expand_stack() introduced in commit [3].
> 
> [1] https://lore.kernel.org/all/mvmplxraqmd.fsf@suse.de/T/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4bce37a68ff884e821a02a731897a8119e0c37b7
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8d7071af890768438c14db6172cc8f9f4d04e184

Can you test the two patched attached instead please?

Regards,
Salvatore

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#86156 — Bug#1086028: I've reproduced the bug in QEMU

Hi Salvatore,

On Tue, Feb 25, 2025 at 7:36 PM Salvatore Bonaccorso <carnil@debian.org> wrote:
>
> Hi Sergei,
>
> Can you test the two patched attached instead please?

I confirm that these two patches indeed fix the bug. I've applied them
to the 6.1.128-1 kernel and used erlc (Erlang compiler) to reproduce
the bug in QEMU. So far:

1) Without the patches erlc emits efault (14) once in a 30-40 runs;
2) With the patches erlc did not emit any efaults in a few thousand runs.

So I think the patches are good.
Cheers!
-- 
Sergei Golovan

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