[PATCH] lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user()

[Christophe Leroy]

Le 23/01/2020 à 19:47, Linus Torvalds a écrit :
> On Thu, Jan 23, 2020 at 12:34 AM Christophe Leroy
> <christophe.leroy at c-s.fr> wrote:
>>
>> The range passed to user_access_begin() by strncpy_from_user() and
>> strnlen_user() starts at 'src' and goes up to the limit of userspace
>> allthough reads will be limited by the 'count' param.
>>
>> On 32 bits powerpc (book3s/32) access has to be granted for each 256Mbytes
>> segment and the cost increases with the number of segments to unlock.
>>
>> Limit the range with 'count' param.
> 
> Ack. I'm tempted to take this for 5.5 too, just so that the
> unquestionably trivial fixes are in that baseline, and the
> infrastructure is ready for any architecture that has issues like
> this.

It would be nice, then the user_access_begin stuff for powerpc could go 
for 5.6 without worring about.

Thanks
Christophe