Re: 9P change breaks bpftrace running in qemu+9p?

[Tingmao Wang <m@maowtm.org>]

Hi Song,

On 10/20/25 18:40, Song Liu wrote:
> Hi,
> 
> I noticed a new error in the upstream kernel when I run bpftrace with
> vmtest [2]:
> 
> $ vmtest -k arch/x86/boot/bzImage "bpftrace.real -e
> 'fexit:cmdline_proc_show {exit();}'"
> => bzImage
> ===> Booting
> ===> Setting up VM
> ===> Running command
> [    5.610741] id (178) used greatest stack depth: 11592 bytes left
> Parse error: Input/output error
> Command failed with exit code: 2
> [root@(none) /]#
> 
> git bisect points to patch 1/3 of this set [1].
> 
> Any idea what is happening here?

I have attempted to reproduce this but couldn't yet:

	> git log --format=short -1
	commit 211ddde0823f1442e4ad052a2f30f050145ccada (HEAD, tag: v6.18-rc2, origin/master, origin/HEAD)
	Author: Linus Torvalds <torvalds@linux-foundation.org>
	
	    Linux 6.18-rc2
	(0.067s) linux-devbox-2 (mao; ; ?) ~/9pfs
	> vmtest -k arch/x86/boot/bzImage "bpftrace -e 'fexit:cmdline_proc_show {exit();}'"
	=> bzImage
	===> Booting
	===> Setting up VM
	===> Running command
	Attached 1 probe
	^C⏎
	 (130) (14.421s) linux-devbox-2 (mao; ; ?) ~/9pfs
	> vmtest -k arch/x86/boot/bzImage "(sleep 2; cat /proc/cmdline) & bpftrace -e 'fexit:cmdline_proc_show {exit();}'"
	=> bzImage
	===> Booting
	===> Setting up VM
	===> Running command
	Attached 1 probe
	rootfstype=9p rootflags=trans=virtio,cache=mmap,msize=1048576 rw earlyprintk=serial,0,115200 printk.devkmsg=on console=0,115200 loglevel=7 ra...
	
	(5.29s) linux-devbox-2 (mao; ; ?) ~/9pfs
	> qemu-system-x86_64 -version
	QEMU emulator version 10.1.1
	Copyright (c) 2003-2025 Fabrice Bellard and the QEMU Project developers
	(0.011s) linux-devbox-2 (mao; ; ?) ~/9pfs
	> bpftrace --version
	bpftrace v0.24.1

I'm happy to take a further look - is there anything special about your
setup, what version of QEMU / bpftrace are you running?

> 
> Thanks,
> Song
> 
> [1] https://lore.kernel.org/v9fs/cover.1743956147.git.m@maowtm.org/
> [2] https://github.com/danobi/vmtest