[PATCH] powerpc/signal64: Don't read sigaction arguments back from user memory

[Christophe Leroy]

Le 14/06/2021 à 07:49, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of June 14, 2021 3:30 pm:
>>
>>
>> Le 14/06/2021 à 03:32, Nicholas Piggin a écrit :
>>> Excerpts from Michael Ellerman's message of June 10, 2021 5:29 pm:
>>>> When delivering a signal to a sigaction style handler (SA_SIGINFO), we
>>>> pass pointers to the siginfo and ucontext via r4 and r5.
>>>>
>>>> Currently we populate the values in those registers by reading the
>>>> pointers out of the sigframe in user memory, even though the values in
>>>> user memory were written by the kernel just prior:
>>>>
>>>>     unsafe_put_user(&frame->info, &frame->pinfo, badframe_block);
>>>>     unsafe_put_user(&frame->uc, &frame->puc, badframe_block);
>>>>     ...
>>>>     if (ksig->ka.sa.sa_flags & SA_SIGINFO) {
>>>>     	err |= get_user(regs->gpr[4], (unsigned long __user *)&frame->pinfo);
>>>>     	err |= get_user(regs->gpr[5], (unsigned long __user *)&frame->puc);
>>>>
>>>> ie. we write &frame->info into frame->pinfo, and then read frame->pinfo
>>>> back into r4, and similarly for &frame->uc.
>>>>
>>>> The code has always been like this, since linux-fullhistory commit
>>>> d4f2d95eca2c ("Forward port of 2.4 ppc64 signal changes.").
>>>>
>>>> There's no reason for us to read the values back from user memory,
>>>> rather than just setting the value in the gpr[4/5] directly. In fact
>>>> reading the value back from user memory opens up the possibility of
>>>> another user thread changing the values before we read them back.
>>>> Although any process doing that would be racing against the kernel
>>>> delivering the signal, and would risk corrupting the stack, so that
>>>> would be a userspace bug.
>>>>
>>>> Note that this is 64-bit only code, so there's no subtlety with the size
>>>> of pointers differing between kernel and user. Also the frame variable
>>>> is not modified to point elsewhere during the function.
>>>>
>>>> In the past reading the values back from user memory was not costly, but
>>>> now that we have KUAP on some CPUs it is, so we'd rather avoid it for
>>>> that reason too.
>>>>
>>>> So change the code to just set the values directly, using the same
>>>> values we have written to the sigframe previously in the function.
>>>>
>>>> Note also that this matches what our 32-bit signal code does.
>>>>
>>>> Using a version of will-it-scale's signal1_threads that sets SA_SIGINFO,
>>>> this results in a ~4% increase in signals per second on a Power9, from
>>>> 229,777 to 239,766.
>>>
>>> Good find, nice improvement. Will make it possible to make the error
>>> handling much nicer too I think.
>>>
>>> Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
>>>
>>> You've moved copy_siginfo_to_user right up to the user access unlock,
>>> could save 2 more KUAP lock/unlocks if we had an unsafe_clear_user. If
>>> we can move the other user access stuff up as well, the stack frame
>>> put_user could use unsafe_put_user as well, saving 1 more. Another few
>>> percent?
>>
>> I'm looking at making an 'unsafe' version of copy_siginfo_to_user().
>> That's straight forward for 'native' signals, but for compat signals that's more tricky.
> 
> Ah nice. Native is most important at the moment.
> 

Finally not so easy. We have a quite efficient clear_user() which uses 'dcbz'. When replacing that 
by a simplistic unsafe_clear_user() on the same model as unsafe_copy_to_user(), performance are 
degradated on 32s. Need to implement it more efficiently.

Christophe