Re: [PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[Ard Biesheuvel]

On Thu, 1 Oct 2026, at 03:20, Prashant Singh wrote:
> Thanks Ard and Sebastian for the comments.
>
...
>>AFAICT, that would potentially leave KCSAN instrumentation on the reboot
>>path, which might trigger and interfere with the reboot. So instead,
>>I'd like to put this in efi_reboot_required if we can. If it is needed
>>in more places to address an actual KCSAN splat, I don't mind. If it is
>>just to make Sashiko happy, then we shouldn't bother.
>
> Could you please clarify what you mean by efi_reboot_required here? nostatfs
> is only read in efivarfs_statfs(), efivarfs_show_options() and
> efivarfs_init_fs_context(), none of which run on the reboot path, so I'm
> not sure how it would apply.
>

Apologies, I managed to completely confuse myself here. Forget what I said
here, please :-)

> On the annotation itself: an internal review flagged a potential KCSAN
> data race rather than an observed splat -- statfs() can run concurrently
> with a remount updating the flag, so it is a genuine (benign) concurrent
> access. I ran concurrent statfs/remount loops on separate CPUs under
> KCSAN and didn't trigger a report in a bounded run, which could be
> expected given KCSAN samples accesses, so it doesn't disprove the race.
> Since KCSAN only needs one side of the pair marked, data_race() on the
> write covers both readers and the reads stay plain. I'm happy to drop it
> entirely if you'd prefer to keep the benign race unannotated.
>

No, let's keep it as you suggest.

>>Please keep this description _here_ where you have it. Once this is
>>merged, you could send another patch, extending the documentation with
>>the statfs option (I think the workqueue change is in).
>
> Sure -- I'll keep it in Documentation/filesystems/efivarfs.rst for now
> and send a follow-up extending Documentation/core-api/real-time/hardware.rst
> once this is merged.
>
>>You still have the problem that someone reading the variable leads to
>>the same problem but this requires a privileged user. And if I am not
>>mistaken, someone sent patches to have efi-runtime runtime disabled/
>>enabled.
>
> Agreed -- the variable-read path is the same, but needs a privileged
> user unlike the unprivileged statfs()/df trigger.
>

Indeed - this is only about anyone with read permissions on the mount
point being able to trigger this. And looking at your results, the
rate limit we added recently might be a bit too permissive as well.