linux-kernel - Re: [PATCH v2 23/25] powerpc/signal: Create 'unsafe' versions of copy_[ck][fpr/vsx]_to

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Message-ID: <61bd2283-94bb-edff-5140-8be2b98a02e3@csgroup.eu>
Date:   Tue, 29 Sep 2020 07:33:30 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Christopher M. Riedl" <cmr@...ormatik.wtf>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 23/25] powerpc/signal: Create 'unsafe' versions of
 copy_[ck][fpr/vsx]_to_user()



Le 29/09/2020 à 07:22, Christophe Leroy a écrit :
> 
> 
> Le 29/09/2020 à 04:04, Christopher M. Riedl a écrit :
>> On Tue Aug 18, 2020 at 12:19 PM CDT, Christophe Leroy wrote:
>>> For the non VSX version, that's trivial. Just use unsafe_copy_to_user()
>>> instead of __copy_to_user().
>>>
>>> For the VSX version, remove the intermediate step through a buffer and
>>> use unsafe_put_user() directly. This generates a far smaller code which
>>> is acceptable to inline, see below:
>>>
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>> ---
>>> arch/powerpc/kernel/signal.h | 53 ++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 53 insertions(+)
>>>
>>> diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h
>>> index f610cfafa478..2559a681536e 100644
>>> --- a/arch/powerpc/kernel/signal.h
>>> +++ b/arch/powerpc/kernel/signal.h
>>> @@ -32,7 +32,54 @@ unsigned long copy_fpr_to_user(void __user *to,
>>> struct task_struct *task);
>>> unsigned long copy_ckfpr_to_user(void __user *to, struct task_struct
>>> *task);
>>> unsigned long copy_fpr_from_user(struct task_struct *task, void __user
>>> *from);
>>> unsigned long copy_ckfpr_from_user(struct task_struct *task, void __user
>>> *from);
>>> +
>>> +#define unsafe_copy_fpr_to_user(to, task, label) do { \
>>> + struct task_struct *__t = task; \
>>> + u64 __user *buf = (u64 __user *)to; \
>>> + int i; \
>>> + \
>>> + for (i = 0; i < ELF_NFPREG - 1 ; i++) \
>>> + unsafe_put_user(__t->thread.TS_FPR(i), &buf[i], label); \
>>> + unsafe_put_user(__t->thread.fp_state.fpscr, &buf[i], label); \
>>> +} while (0)
>>> +
>>
>> I've been working on the PPC64 side of this "unsafe" rework using this
>> series as a basis. One question here - I don't really understand what
>> the benefit of re-implementing this logic in macros (similarly for the
>> other copy_* functions below) is?
> 
> Not sure either.
> 
> The whole purpose is to not manage the error through a local var but exclusively use labels.
> However, GCC is probably smart enough to understand it and drop the local var while inlining.
> 
> One important thing however is to make sure we won't end up with an outline function, otherwise you 
> completely loose the benefit of the label stuff. And you get a function call inside a user access, 
> which is what we want to avoid.
> 
>>
>> I am considering  a "__unsafe_copy_*" implementation in signal.c for
>> each (just the original implementation w/ using the "unsafe_" variants
>> of the uaccess stuff) which gets called by the "safe" functions w/ the
>> appropriate "user_*_access_begin/user_*_access_end". Something like
>> (pseudo-ish code):
> 
> Good idea, however ...
> 
>>
>>     /* signal.c */
>>     unsigned long __unsafe_copy_fpr_to_user(...)
>>     {
>>         ...
>>         unsafe_copy_to_user(..., bad);
>>         return 0;
>>     bad:
>>         return 1; /* -EFAULT? */
>>     }
> 
> This __unsafe_copy_fpr_to_user() has to be in signal.h and must be tagged 'static __always_inline' 
> for the reasons explained above.
> 
>>
>>     unsigned long copy_fpr_to_user(...)
>>     {
>>         unsigned long err;
>>         if (!user_write_access_begin(...))
>>             return 1; /* -EFAULT? */
>>
>>         err = __unsafe_copy_fpr_to_user(...);
>>
>>         user_write_access_end();
>>         return err;
>>     }

Also note that at the end (ie when both PPC32 and PPC64 signal code are using "unsafe" versions), 
the "safe" version won't be used anymore and will be dropped.

Christophe