Re: [PATCH] kvm: Don't assume accel_ioctl_end() preserves @errno

[Markus Armbruster <armbru@redhat.com>]

Philippe Mathieu-Daudé <philmd@linaro.org> writes:

> On 25/11/25 10:03, Markus Armbruster wrote:
>> Markus Armbruster <armbru@redhat.com> writes:
>> 
>>> Retrieve the @errno set by ioctl() before we call accel_ioctl_end()
>>> instead of afterwards, so it works whether accel_ioctl_end() preserves
>>> @errno or not.
>>>
>>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> 
>> I did not check whether the assumption holds or not.
>
> Indeed, on Linux the futex syscall is called via qemu_event_set.

So ...

>>  If it doesn't,
>> then this needs
>> 
>>    Fixes: a27dd2de68f3 (KVM: keep track of running ioctls)

... we definitely want this tag, and the commit message should be
clarified as well.  Here's my try:

    kvm: Fix kvm_vm_ioctl() and kvm_device_ioctl() return value

    These functions wrap ioctl().  When ioctl() fails, it sets @errno.
    The wrappers then return that @errno negated.

    Except they call accel_ioctl_end() between calling ioctl() and reading
    @errno.  accel_ioctl_end() can clobber @errno, e.g. when a futex()
    system call fails.  Seems unlikely, but it's a bug all the same.

    Fix by retrieving @errno before calling accel_ioctl_end().

    Fixes: a27dd2de68f3 (KVM: keep track of running ioctls)

> LGTM.
>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Thanks!