From: Michael Ellerman <mpe@ellerman•id.au>
To: "Christopher M. Riedl" <cmr@codefail•de>,
Gabriel Paubert <paubert@iram•es>
Cc: David Laight <David.Laight@ACULAB•COM>,
"linuxppc-dev@lists•ozlabs.org" <linuxppc-dev@lists•ozlabs.org>
Subject: Re: [PATCH v4 02/10] powerpc/signal: Add unsafe_copy_{vsx, fpr}_from_user()
Date: Thu, 04 Feb 2021 17:09:24 +1100 [thread overview]
Message-ID: <87mtwkpdnv.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <C8YHSKQ99VC4.M9Y0WOFVUTBQ@geist>
"Christopher M. Riedl" <cmr@codefail•de> writes:
> On Mon Feb 1, 2021 at 10:54 AM CST, Gabriel Paubert wrote:
>> On Mon, Feb 01, 2021 at 09:55:44AM -0600, Christopher M. Riedl wrote:
>> > On Thu Jan 28, 2021 at 4:38 AM CST, David Laight wrote:
>> > > From: Christopher M. Riedl
>> > > > Sent: 28 January 2021 04:04
>> > > >
>> > > > Reuse the "safe" implementation from signal.c except for calling
>> > > > unsafe_copy_from_user() to copy into a local buffer.
>> > > >
>> > > > Signed-off-by: Christopher M. Riedl <cmr@codefail•de>
>> > > > ---
>> > > > arch/powerpc/kernel/signal.h | 33 +++++++++++++++++++++++++++++++++
>> > > > 1 file changed, 33 insertions(+)
>> > > >
>> > > > diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h
>> > > > index 2559a681536e..c18402d625f1 100644
>> > > > --- a/arch/powerpc/kernel/signal.h
>> > > > +++ b/arch/powerpc/kernel/signal.h
>> > > > @@ -53,6 +53,33 @@ unsigned long copy_ckfpr_from_user(struct task_struct *task, void __user *from);
>> > > > &buf[i], label);\
>> > > > } while (0)
>> > > >
>> > > > +#define unsafe_copy_fpr_from_user(task, from, label) do { \
>> > > > + struct task_struct *__t = task; \
>> > > > + u64 __user *__f = (u64 __user *)from; \
>> > > > + u64 buf[ELF_NFPREG]; \
>> > >
>> > > How big is that buffer?
>> > > Isn't is likely to be reasonably large compared to a reasonable
>> > > kernel stack frame.
>> > > Especially since this isn't even a leaf function.
>> > >
>> >
>> > I think Christophe answered this - I don't really have an opinion either
>> > way. What would be a 'reasonable' kernel stack frame for reference?
>>
>> See include/linux/poll.h, where the limit is of the order of 800 bytes
>> and the number of entries in an on stack array is chosen at compile time
>> (different between 32 and 64 bit for example).
>>
>> The values are used in do_sys_poll, which, with almost 1000 bytes of
>> stack footprint, appears close to the top of "make checkstack". In
>> addition do_sys_poll has to call the ->poll function of every file
>> descriptor in its table, so it is not a tail function.
>>
>> This 264 bytes array looks reasonable, but please use 'make checkstack'
>> to verify that the function's total stack usage stays within reason.
>
> Neat, looks like total usage is a bit larger but still reasonable and
> less than half of 800B:
>
> 0xc000000000017e900 __unsafe_restore_sigcontext.constprop.0 [vmlinux]:352
We warn for frames larger than 2KB on 64-bit, see FRAME_WARN in
lib/Kconfig.debug.
So 264 bytes is entirely reasonable IMHO.
cheers
next prev parent reply other threads:[~2021-02-04 6:11 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-28 4:04 [PATCH v4 00/10] Improve signal performance on PPC64 with KUAP Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 01/10] powerpc/uaccess: Add unsafe_copy_from_user Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 02/10] powerpc/signal: Add unsafe_copy_{vsx, fpr}_from_user() Christopher M. Riedl
2021-01-28 10:38 ` David Laight
2021-01-28 12:05 ` Christophe Leroy
2021-02-01 15:55 ` Christopher M. Riedl
2021-02-01 16:15 ` David Laight
2021-02-01 16:55 ` Christopher M. Riedl
2021-02-01 17:37 ` David Laight
2021-02-01 17:43 ` Christopher M. Riedl
2021-02-01 16:54 ` Gabriel Paubert
2021-02-01 20:55 ` Christopher M. Riedl
2021-02-04 6:09 ` Michael Ellerman [this message]
2021-01-28 4:04 ` [PATCH v4 03/10] powerpc/signal64: Move non-inline functions out of setup_sigcontext() Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 04/10] powerpc: Reference param in MSR_TM_ACTIVE() macro Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 05/10] powerpc/signal64: Remove TM ifdefery in middle of if/else block Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 06/10] powerpc/signal64: Replace setup_sigcontext() w/ unsafe_setup_sigcontext() Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 07/10] powerpc/signal64: Replace restore_sigcontext() w/ unsafe_restore_sigcontext() Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 08/10] powerpc/signal64: Rewrite handle_rt_signal64() to minimise uaccess switches Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 09/10] powerpc/signal64: Rewrite rt_sigreturn() " Christopher M. Riedl
2021-01-28 4:04 ` [PATCH v4 10/10] powerpc/signal64: Use __get_user() to copy sigset_t Christopher M. Riedl
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