Re: [PATCH 0/2] x86/fpu: Allow restoring signal frames with larger xstate_size

[Andrei Vagin]

On Tue, Sep 29, 2026 at 4:26 PM Andrei Vagin <avagin@xxxxxxxxxx> wrote:
>
> When a process is checkpointed on one machine and restored on another
> (e.g., via CRIU during container/process migration), any signal frame on
> its stack carries the xstate layout of the source CPU.
>
> Commit fd14edd82077 ("x86/fpu: Pre-fault only required size of xstate
> buffer") enabled restoring signal frames created on CPUs with a smaller
> xstate_size than the destination host's default. However,
> check_xstate_in_sigframe() still enforces that a signal frame's
> xstate_size must not exceed the current task's fpstate->user_size.
>
> This restriction prevents migrating a process from a CPU with more
> enabled xstate features to a CPU with fewer features, even when the
> process has not actively used any of the unsupported features. Because
> the actual set of active features is recorded in the XSAVE header
> (XSTATE_BV) and validated during restoration, a larger xstate_size is
> safe to restore as long as all active features are present in the
> intersection of frame and task features and the buffer is large enough
> for them.
>
> Cc: Alexander Mikhalitsyn <alexander@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: "Chang S. Bae" <chang.seok.bae@xxxxxxxxx>

Chang,

Could you please review this series? I remember we discussed adding two
arch_prctl commands to get/set which FPU features are actually enabled
for the current process. I have these patches ready and will send them
soon.

Thanks,
Andrei