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Subject: Re: [PATCH] lib: Reduce user_access_begin() boundaries in
 strncpy_from_user() and strnlen_user()
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-arch <linux-arch@vger.kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux-MM <linux-mm@kvack.org>
References: <70f99f7474826883877e84f93224e937d9c974de.1579767339.git.christophe.leroy@c-s.fr>
 <CAHk-=wgcm5JzyacekDGQ4Ocoe-F5it-7-sbgU8oPnhwnSH3KAA@mail.gmail.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <8de7ba48-7bf4-8d43-5dfc-dacf34f80537@c-s.fr>
Date:   Fri, 24 Jan 2020 07:25:51 +0100
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Le 23/01/2020 à 19:47, Linus Torvalds a écrit :
> On Thu, Jan 23, 2020 at 12:34 AM Christophe Leroy
> <christophe.leroy@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