On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers
<[email protected]> wrote:
>
> ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet [email protected] wrote:
>
> > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <[email protected]> wrote:
> >>
> >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <[email protected]> wrote:
> >> >
> >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
> >> > <[email protected]> wrote:
> >> > >
> >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet [email protected] wrote:
> >> > >
> >> > > > From: Eric Dumazet <[email protected]>
> >> > > >
> >> > > > 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
----- On Apr 13, 2021, at 1:33 PM, Eric Dumazet [email protected] wrote:
> On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers
> <[email protected]> wrote:
>>
>> ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet [email protected] wrote:
>>
>> > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <[email protected]> wrote:
>> >>
>> >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <[email protected]> wrote:
>> >> >
>> >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
>> >> > <[email protected]> wrote:
>> >> > >
>> >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet [email protected] wrote:
>> >> > >
>> >> > > > From: Eric Dumazet <[email protected]>
>> >> > > >
>> >> > > > 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
On Tue, Apr 13, 2021 at 8:00 PM Mathieu Desnoyers
<[email protected]> wrote:
>
> 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.
>
Quite frankly V1 was fine, I can't really make it looking better.
> 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