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

[Eric Dumazet]

On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet edumazet@xxxxxxxxxx wrote:
>
> > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >>
> >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >> >
> >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
> >> > <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >> > >
> >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@xxxxxxxxx wrote:
> >> > >
> >> > > > From: Eric Dumazet <edumazet@xxxxxxxxxx>
> >> > > >
> >> > > > 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.


>
> 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