Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Apr 13, 2021, at 1:33 PM, Eric Dumazet edumazet@google.com wrote:

> On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet edumazet@google.com wrote:
>>
>> > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <edumazet@google.com> wrote:
>> >>
>> >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <edumazet@google.com> wrote:
>> >> >
>> >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
>> >> > <mathieu.desnoyers@efficios.com> wrote:
>> >> > >
>> >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@gmail.com wrote:
>> >> > >
>> >> > > > From: Eric Dumazet <edumazet@google.com>
>> >> > > >
>> >> > > > Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
>> >> > > > update includes") added regressions for our servers.
>> >> > > >
>> >> > > > Using copy_from_user() and clear_user() for 64bit values
>> >> > > > is suboptimal.
>> >> > > >
>> >> > > > We can use faster put_user() and get_user().
>> >> > > >
>> >> > > > 32bit arches can be changed to use the ptr32 field,
>> >> > > > since the padding field must always be zero.
>> >> > > >
>> >> > > > v2: added ideas from Peter and Mathieu about making this
>> >> > > >    generic, since my initial patch was only dealing with
>> >> > > >    64bit arches.
>> >> > >
>> >> > > Ah, now I remember the reason why reading and clearing the entire 64-bit
>> >> > > is important: it's because we don't want to allow user-space processes to
>> >> > > use this change in behavior to figure out whether they are running on a
>> >> > > 32-bit or in a 32-bit compat mode on a 64-bit kernel.
>> >> > >
>> >> > > So although I'm fine with making 64-bit kernels faster, we'll want to keep
>> >> > > updating the entire 64-bit ptr field on 32-bit kernels as well.
>> >> > >
>> >> > > Thanks,
>> >> > >
>> >> >
>> >> > So... back to V1 then ?
>> >>
>> >> Or add more stuff as in :
>> >
>> > diff against v2, WDYT ?
>>
>> I like this approach slightly better, because it moves the preprocessor ifdefs
>> into
>> rseq_get_rseq_cs and clear_rseq_cs, while keeping the same behavior for a 32-bit
>> process running on native 32-bit kernel and as compat task on a 64-bit kernel.
>>
>> That being said, I don't expect anyone to care much about performance of 32-bit
>> kernels, so we could use copy_from_user() on 32-bit kernels to remove
>> special-cases
>> in 32-bit specific code. This would eliminate the 32-bit specific "padding"
>> read, and
>> let the TASK_SIZE comparison handle the check for both 32-bit and 64-bit
>> kernels.
>>
>> As for clear_user(), I wonder whether we could simply keep using it, but change
>> the
>> clear_user() macro to figure out that it can use a faster 8-byte put_user ? I
>> find it
>> odd that performance optimizations which would be relevant elsewhere creep into
>> the
>> rseq code.
> 
> 
> clear_user() is a maze of arch-dependent macros/functions/assembly
> 
> I guess the same could be said from  copy_in_user(), but apparently we removed
> special-casing, like in commit a41e0d754240fe8ca9c4f2070bf67e3b0228aa22
> 
> Definitely it seems odd having to carefully choose between multiple methods.

As long as the ifdefs are localized within clearly identified wrappers in the
rseq code I don't mind doing the special-casing there.

The point which remains is that I don't think we want to optimize for speed
on 32-bit architectures when it adds special-casing and complexity to the 32-bit
build. I suspect there is less and less testing performed on 32-bit architectures
nowadays, and it's good that as much code as possible is shared between 32-bit and
64-bit builds to share the test coverage.

Thanks,

Mathieu

> 
> 
>>
>> Thanks,
>>
>> Mathieu
>>
>> >
>> > diff --git a/kernel/rseq.c b/kernel/rseq.c
>> > index
>> > f2eee3f7f5d330688c81cb2e57d47ca6b843873e..537b1f684efa11069990018ffa3642c209993011
>> > 100644
>> > --- a/kernel/rseq.c
>> > +++ b/kernel/rseq.c
>> > @@ -136,6 +136,10 @@ static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp,
>> > {
>> >        u32 ptr;
>> >
>> > +       if (get_user(ptr, &rseq->rseq_cs.ptr.padding))
>> > +               return -EFAULT;
>> > +       if (ptr)
>> > +               return -EINVAL;
>> >        if (get_user(ptr, &rseq->rseq_cs.ptr.ptr32))
>> >                return -EFAULT;
>> >        *uptrp = (struct rseq_cs __user *)ptr;
>> > @@ -150,8 +154,9 @@ static int rseq_get_rseq_cs(struct task_struct *t,
>> > struct rseq_cs *rseq_cs)
>> >        u32 sig;
>> >        int ret;
>> >
>> > -       if (rseq_get_cs_ptr(&urseq_cs, t->rseq))
>> > -               return -EFAULT;
>> > +       ret = rseq_get_cs_ptr(&urseq_cs, t->rseq);
>> > +       if (ret)
>> > +               return ret;
>> >        if (!urseq_cs) {
>> >                memset(rseq_cs, 0, sizeof(*rseq_cs));
>> >                return 0;
>> > @@ -237,7 +242,8 @@ static int clear_rseq_cs(struct task_struct *t)
>> > #ifdef CONFIG_64BIT
>> >        return put_user(0UL, &t->rseq->rseq_cs.ptr64);
>> > #else
>> > -       return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32);
>> > +       return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32) |
>> > +              put_user(0UL, &t->rseq->rseq_cs.ptr.padding);
>> > #endif
>> >  }
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com