linux-kernel - Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq

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Message-ID: <CANn89iKnQ7KeCo0os0c67GMgEkmrRqhmGhug-xL-Mx5BhR+BkQ@mail.gmail.com>
Date:   Tue, 13 Apr 2021 19:07:10 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.com> wrote:
> > >
> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@...il.com wrote:
> > >
> > > > From: Eric Dumazet <edumazet@...gle.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 ?

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
 }