Subject: Re: [PATCH 1/8] powerpc/uaccess: Add unsafe_copy_from_user

From: "Christopher M. Riedl" <[email-masked]>
Date: Mon, 19 Oct 2020 22:00:34 -0500

On Fri Oct 16, 2020 at 10:17 AM CDT, Christophe Leroy wrote:
>
>
> Le 15/10/2020 à 17:01, Christopher M. Riedl a écrit :
> > Implement raw_copy_from_user_allowed() which assumes that userspace read
> > access is open. Use this new function to implement raw_copy_from_user().
> > Finally, wrap the new function to follow the usual "unsafe_" convention
> > of taking a label argument. The new raw_copy_from_user_allowed() calls
> > __copy_tofrom_user() internally, but this is still safe to call in user
> > access blocks formed with user_*_access_begin()/user_*_access_end()
> > since asm functions are not instrumented for tracing.
>
> Would objtool accept that if it was implemented on powerpc ?
>
> __copy_tofrom_user() is a function which is optimised for larger memory
> copies (using dcbz, etc ...)
> Do we need such an optimisation for unsafe_copy_from_user() ? Or can we
> do a simple loop as done for
> unsafe_copy_to_user() instead ?

I tried using a simple loop based on your unsafe_copy_to_user()
implementation. Similar to the copy_{vsx,fpr}_from_user() results there
is a hit to signal handling performance. The results with the loop are
in the 'unsafe-signal64-copy' column:

	|                      | hash   | radix  |
	| -------------------- | ------ | ------ |
	| linuxppc/next        | 289014 | 158408 |
	| unsafe-signal64      | 298506 | 253053 |
	| unsafe-signal64-copy | 197029 | 177002 |

Similar to the copy_{vsx,fpr}_from_user() patch I don't fully understand
why this performs so badly yet.

Implementation:

	unsafe_copy_from_user(d, s, l, e) \
	do {                                                                   \
	       u8 *_dst = (u8 *)(d);                                           \
	       const u8 __user *_src = (u8 __user*)(s);                                \
	       size_t _len = (l);                                              \
	       int _i;                                                         \
									       \
	       for (_i = 0; _i < (_len & ~(sizeof(long) - 1)); _i += sizeof(long))             \
		       unsafe_get_user(*(long*)(_dst + _i), (long __user *)(_src + _i), e);    \
	       if (IS_ENABLED(CONFIG_PPC64) && (_len & 4)) {                   \
		       unsafe_get_user(*(u32*)(_dst + _i), (u32 __user *)(_src + _i), e);      \
		       _i += 4;                                                \
	       }                                                               \
	       if (_len & 2) {                                                 \
		       unsafe_get_user(*(u16*)(_dst + _i), (u16 __user *)(_src + _i), e);      \
		       _i += 2;                                                \
	       }                                                               \
	       if (_len & 1)                                                   \
		       unsafe_get_user(*(u8*)(_dst + _i), (u8 __user *)(_src + _i), e);        \
	} while (0)

>
> Christophe
>
> > 
> > Signed-off-by: Christopher M. Riedl <[email-masked]>
> > ---
> >   arch/powerpc/include/asm/uaccess.h | 28 +++++++++++++++++++---------
> >   1 file changed, 19 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> > index 26781b044932..66940b4eb692 100644
> > --- a/arch/powerpc/include/asm/uaccess.h
> > +++ b/arch/powerpc/include/asm/uaccess.h
> > @@ -418,38 +418,45 @@ raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
> >   }
> >   #endif /* __powerpc64__ */
> >   
> > -static inline unsigned long raw_copy_from_user(void *to,
> > -		const void __user *from, unsigned long n)
> > +static inline unsigned long
> > +raw_copy_from_user_allowed(void *to, const void __user *from, unsigned long n)
> >   {
> > -	unsigned long ret;
> >   	if (__builtin_constant_p(n) && (n <= 8)) {
> > -		ret = 1;
> > +		unsigned long ret = 1;
> >   
> >   		switch (n) {
> >   		case 1:
> >   			barrier_nospec();
> > -			__get_user_size(*(u8 *)to, from, 1, ret);
> > +			__get_user_size_allowed(*(u8 *)to, from, 1, ret);
> >   			break;
> >   		case 2:
> >   			barrier_nospec();
> > -			__get_user_size(*(u16 *)to, from, 2, ret);
> > +			__get_user_size_allowed(*(u16 *)to, from, 2, ret);
> >   			break;
> >   		case 4:
> >   			barrier_nospec();
> > -			__get_user_size(*(u32 *)to, from, 4, ret);
> > +			__get_user_size_allowed(*(u32 *)to, from, 4, ret);
> >   			break;
> >   		case 8:
> >   			barrier_nospec();
> > -			__get_user_size(*(u64 *)to, from, 8, ret);
> > +			__get_user_size_allowed(*(u64 *)to, from, 8, ret);
> >   			break;
> >   		}
> >   		if (ret == 0)
> >   			return 0;
> >   	}
> >   
> > +	return __copy_tofrom_user((__force void __user *)to, from, n);
> > +}
> > +
> > +static inline unsigned long
> > +raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> > +{
> > +	unsigned long ret;
> > +
> >   	barrier_nospec();
> >   	allow_read_from_user(from, n);
> > -	ret = __copy_tofrom_user((__force void __user *)to, from, n);
> > +	ret = raw_copy_from_user_allowed(to, from, n);
> >   	prevent_read_from_user(from, n);
> >   	return ret;
> >   }
> > @@ -571,6 +578,9 @@ user_write_access_begin(const void __user *ptr, size_t len)
> >   #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
> >   #define unsafe_put_user(x, p, e) __put_user_goto(x, p, e)
> >   
> > +#define unsafe_copy_from_user(d, s, l, e) \
> > +	unsafe_op_wrap(raw_copy_from_user_allowed(d, s, l), e)
> > +
> >   #define unsafe_copy_to_user(d, s, l, e) \
> >   do {									\
> >   	u8 __user *_dst = (u8 __user *)(d);				\
> >