Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0.
From: Jianhua Liu
Date: Wed Aug 26 2015 - 22:44:17 EST
On Wed, Aug 26, 2015 at 7:32 PM, Ard Biesheuvel
> On 26 August 2015 at 13:12, Will Deacon <will.deacon@xxxxxxx> wrote:
>> [adding Ard]
>> On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote:
>>> From: Janet Liu <janet.liu@xxxxxxxxxxxxxx>
>>> 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
> 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);
> (note the memset erroneously initializes that field to CPU 0)
> This more accurately reflects the state of the process after forking,
> i.e., that its FPSIMD state has never been loaded into any CPU.
I agree with you.
Can you send a patch to maintainer as soon as possible to solve this issue?
>> Are you seeing an issue in practice?
>>> Signed-off-by: Janet Liu <janet.liu@xxxxxxxxxxxxxx>
>>> Signed-off-by: Chunyan Zhang <chunyan.zhang@xxxxxxxxxxxxxx>
>>> arch/arm64/kernel/fpsimd.c | 1 +
>>> 1 file changed, 1 insertion(+)
>>> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
>>> index 44d6f75..ec58d94 100644
>>> --- a/arch/arm64/kernel/fpsimd.c
>>> +++ b/arch/arm64/kernel/fpsimd.c
>>> @@ -159,6 +159,7 @@ void fpsimd_flush_thread(void)
>>> memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
>>> + this_cpu_write(fpsimd_last_state, NULL);
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