(apologies for the duplicate email, the previous one bounced as it was accidentally using HTML formatting)
If I understand correctly this is run on every context switch so we probably want to make it really fast
> +static int rseq_need_restart(struct task_struct *t, uint32_t cs_flags)
> +{
> + bool need_restart = false;
> + uint32_t flags;
> +
> + /* Get thread flags. */
> + if (__get_user(flags, &t->rseq->flags))
> + return -EFAULT;
> +
> + /* Take into account critical section flags. */
> + flags |= cs_flags;
> +
> + /*
> + * Restart on signal can only be inhibited when restart on
> + * preempt and restart on migrate are inhibited too. Otherwise,
> + * a preempted signal handler could fail to restart the prior
> + * execution context on sigreturn.
> + */
> + if (flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) {
> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
> + return -EINVAL;
> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
> + return -EINVAL;
> + }
How does this error even get to userspace? Is it worth doing this switch on every execution?
> + if (t->rseq_migrate
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
> + need_restart = true;
> + else if (t->rseq_preempt
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
> + need_restart = true;
> + else if (t->rseq_signal
> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL))
> + need_restart = true;
This could potentially be sped up by having the rseq_* fields in t use a single bitmask with the same bit offsets as RSEQ_CS_FLAG_NO_* then using bit operations to check the appropriate overlap.
From 1584079687118467427@xxx Tue Nov 14 21:48:32 +0000 2017
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----- On Nov 14, 2017, at 3:49 PM, Ben Maurer [email protected] wrote:
> (apologies for the duplicate email, the previous one bounced as it was
> accidentally using HTML formatting)
>
> If I understand correctly this is run on every context switch so we probably
> want to make it really fast
Yes, more precisely, it runs on return to user-space, after every context
switch going back to a registered rseq thread.
>
>> +static int rseq_need_restart(struct task_struct *t, uint32_t cs_flags)
>> +{
>> + bool need_restart = false;
>> + uint32_t flags;
>> +
>> + /* Get thread flags. */
>> + if (__get_user(flags, &t->rseq->flags))
>> + return -EFAULT;
>> +
>> + /* Take into account critical section flags. */
>> + flags |= cs_flags;
>> +
>> + /*
>> + * Restart on signal can only be inhibited when restart on
>> + * preempt and restart on migrate are inhibited too. Otherwise,
>> + * a preempted signal handler could fail to restart the prior
>> + * execution context on sigreturn.
>> + */
>> + if (flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) {
>> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
>> + return -EINVAL;
>> + if (!(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
>> + return -EINVAL;
>> + }
>
> How does this error even get to userspace? Is it worth doing this switch on
> every execution?
If we detect this situation, the rseq_need_restart caller will end up
sending a SIGSEGV signal to user-space. Note that the two nested if()
checks are only executing in the unlikely case where the NO_RESTART_ON_SIGNAL
flag is set.
>
>
>> + if (t->rseq_migrate
>> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE))
>> + need_restart = true;
>> + else if (t->rseq_preempt
>> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT))
>> + need_restart = true;
>> + else if (t->rseq_signal
>> + && !(flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL))
>> + need_restart = true;
>
> This could potentially be sped up by having the rseq_* fields in t use a single
> bitmask with the same bit offsets as RSEQ_CS_FLAG_NO_* then using bit
> operations to check the appropriate overlap.
Given that those are not requests impacting the ABI presented to user-space,
I'm tempted to keep these optimizations for the following 4.16 merge window.
Is that ok with you ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
From 1584075547283150793@xxx Tue Nov 14 20:42:44 +0000 2017
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