On Wed, Aug 26, 2015 at 12:32:03PM +0100, Ard Biesheuvel wrote:
> On 26 August 2015 at 13:12, Will Deacon <will.dea...@arm.com> wrote:
> > On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote:
> >> From: Janet Liu <janet....@spreadtrum.com>
> >>
> >> If process A is running on CPU 0 and do execve syscall and after
> >> sched_exec,
> >> dest_cpu is 0, fpsimd_state.cpu is 0. If at the time Process A get
> >> scheduled
> >> out and after some kernel threads running on CPU 0, process A is back in
> >> CPU 0,
> >> A's fpsimd_state.cpu is current cpu id "0", and per_cpu(fpsimd_last_state)
> >> points A's fpsimd_state, TIF_FOREIGN_FPSTATE will be clear, kernel will not
> >> reload the context during it return to userspace. so set the cpu's
> >> fpsimd_last_state to NULL to avoid this.
> >
> > AFAICT, this is only a problem if one of the kernel threads uses the fpsimd
> > registers, right? However, kernel_neon_begin_partial clobbers
> > fpsimd_last_state, so I'm struggling to see the problem.
> >
>
> I think the problem is real, but it would be better to set the
> fpsimd_state::cpu field to an invalid value like we do in
> fpsimd_flush_task_state()
>
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 44d6f7545505..c56956a16d3f 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -158,6 +158,7 @@ void fpsimd_thread_switch(struct task_struct *next)
> void fpsimd_flush_thread(void)
> {
> memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
> + fpsimd_flush_task_state(current);
> set_thread_flag(TIF_FOREIGN_FPSTATE);
> }
>
> (note the memset erroneously initializes that field to CPU 0)
Aha, I see. So the problem is actually that we get a view on our fpsimd
state before the exec, rather than a view on some kernel state.
> This more accurately reflects the state of the process after forking,
> i.e., that its FPSIMD state has never been loaded into any CPU.
Yup, that's much clearer.
Will
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