linux-kernel - Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs without ERMS

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Message-ID: <CAGudoHFAzfA+PoCYD_XabFAQxow1e32M4Wxf_3AqczKKAtc0Wg@mail.gmail.com>
Date:   Wed, 6 Sep 2023 06:11:02 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, bp@...en8.de
Subject: Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs
 without ERMS

On 9/6/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Tue, 5 Sept 2023 at 13:41, Mateusz Guzik <mjguzik@...il.com> wrote:
>>
>> @@ -312,7 +314,15 @@ int vfs_fstatat(int dfd, const char __user
>> *filename,
>>         struct filename *name;
>>
>>         name = getname_flags(filename,
>> getname_statx_lookup_flags(statx_flags), NULL);
>> -       ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS);
>> +       /*
>> +        * Hack: ugliness below damage controls glibc which reimplemented
>> fstat
>> +        * on top of newfstatat(fd, "", buf, AT_EMPTY_PATH). We still pay
>> for
>> +        * kmalloc and user access, but elide lockref.
>> +        */
>> +       if (name->name[0] == '\0' && flags == AT_EMPTY_PATH && dfd >= 0)
>> +               ret = vfs_fstat(dfd, stat);
>> +       else
>> +               ret = vfs_statx(dfd, name, statx_flags, stat,
>> STATX_BASIC_STATS);
>>         putname(name);
>
> I actually think I might prefer the earlier hacky thing, because it
> avoids the whole nasty pathname allocation thing (ie the __getname()
> dance in getname_flags(), and the addition of the pathname to the
> audit records etc).
>
> I suspect your "there are no real loads that combine AT_EMPTY_PATH
> with a path" comment is true.
>
> So if we have this short-circuit of the code, let's go all hog on it,
> and avoid not just the RCU lookup (with lockref etc), but the pathname
> allocation too.
>

Ok, I'm buggering off the subject.

-- 
Mateusz Guzik <mjguzik gmail.com>