From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: objtool clac/stac handling change..
Date: Wed, 1 Jul 2020 13:36:22 -0700 [thread overview]
Message-ID: <5F1767D0-416A-4BA4-9DFF-E82D1EA3F5EE@amacapital.net> (raw)
In-Reply-To: <CAHk-=wizu7DA7EDrsHQLmkTFBvCRxNyPMHaeMDYMF_U75s9RvQ@mail.gmail.com>
> On Jul 1, 2020, at 12:35 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> On Wed, Jul 1, 2020 at 11:29 AM Andy Lutomirski <luto@kernel.org> wrote:
>>
>> Do we really want the exception handling to do the CLAC? Having
>> unsafe_get_user() do CLAC seems surprising to me, and it will break
>> use cases like:
>>
>> if (!user_access_begin(...)
>> goto out;
>>
>> ret = unsafe_get_user(...);
>>
>> user_access_end();
>>
>> check ret;
>
> That's not how unsafe_get_user() works.
>
> unsafe_get_user() always jumps to the error label, it never returns a
> value. So the code is actually now what you claim above, but
>
> if (!user_access_begin(...)
> goto out;
>
> unsafe_get_user(..., out_fault);
> user_access_end();
> .. this is good, use the value we got..
>
> out_fault:
> user_access_end();
> out:
> return -EFAULT;
Ugh, right. But maybe, with the asm goto magic, we can’t get rid of this. I’ve always disliked the pattern where we enable user access, do a bunch of accesses that branch on error, and finish up. We ought to be able to do it the way I described and get decent code generation too.
If we do this extable change, we end up with a different mess: some exception handlers will clear AC and some won’t. I’m sure objtool can deal with this with some effort, but I’m not convinced it’s worth it.
next prev parent reply other threads:[~2020-07-01 20:36 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-01 18:22 objtool clac/stac handling change Linus Torvalds
2020-07-01 18:29 ` Andy Lutomirski
2020-07-01 19:35 ` Linus Torvalds
2020-07-01 20:36 ` Andy Lutomirski [this message]
2020-07-01 20:51 ` Josh Poimboeuf
2020-07-01 21:02 ` Linus Torvalds
2020-07-02 0:00 ` Josh Poimboeuf
2020-07-02 8:05 ` Peter Zijlstra
2020-07-01 20:51 ` Linus Torvalds
2020-07-02 0:47 ` Andy Lutomirski
2020-07-02 2:30 ` Linus Torvalds
2020-07-02 2:35 ` Linus Torvalds
2020-07-02 3:08 ` Andy Lutomirski
2020-07-01 18:41 ` Al Viro
2020-07-01 19:04 ` Linus Torvalds
2020-07-01 19:59 ` Al Viro
2020-07-01 20:25 ` Linus Torvalds
2020-07-02 13:34 ` Michael Ellerman
2020-07-02 14:01 ` Al Viro
2020-07-02 14:04 ` Al Viro
2020-07-02 15:13 ` Christophe Leroy
2020-07-02 20:13 ` Linus Torvalds
2020-07-03 3:59 ` Michael Ellerman
2020-07-03 3:17 ` Michael Ellerman
2020-07-03 5:27 ` Christophe Leroy
2020-07-02 19:52 ` Linus Torvalds
2020-07-02 20:17 ` Al Viro
2020-07-02 20:32 ` Linus Torvalds
2020-07-02 20:59 ` Al Viro
2020-07-02 21:55 ` Linus Torvalds
2020-07-03 1:33 ` Al Viro
2020-07-03 3:32 ` Linus Torvalds
2020-07-03 21:02 ` Al Viro
2020-07-03 21:10 ` Linus Torvalds
2020-07-03 21:41 ` Andy Lutomirski
2020-07-03 22:25 ` Al Viro
2020-07-03 21:59 ` Al Viro
2020-07-03 22:04 ` Al Viro
2020-07-03 22:12 ` Al Viro
2020-07-04 0:49 ` Al Viro
2020-07-04 1:54 ` Linus Torvalds
2020-07-04 2:30 ` Al Viro
2020-07-04 3:06 ` Linus Torvalds
2020-07-04 2:11 ` Al Viro
2020-07-07 12:35 ` David Laight
2020-07-10 22:37 ` Linus Torvalds
2020-07-13 9:32 ` David Laight
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