From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
Christophe Leroy <christophe.leroy@c-s.fr>,
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: Fri, 3 Jul 2020 22:02:37 +0100 [thread overview]
Message-ID: <20200703210237.GS2786714@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200703013328.GQ2786714@ZenIV.linux.org.uk>
On Fri, Jul 03, 2020 at 02:33:28AM +0100, Al Viro wrote:
> On Thu, Jul 02, 2020 at 02:55:19PM -0700, Linus Torvalds wrote:
>
> > And while XSTATE_OP() is still disgusting, it's
> >
> > (a) slightly less disgusting than it used to be
> >
> > (b) now easily fixable if we do the "exceptions clear AC" thing.
> >
> > so it's an improvement all around.
> >
> > If it works, that is. As mentioned: IT HAS NO TESTING.
>
> What about load_unaligned_zeropad()? Normally the caller doesn't
> want to know about the exception on crossing into an unmapped
> page. Blanket "clear #AC of fixup, don't go through user_access_end()
> in case of exception" would complicate the code that calls that sucker.
Actually, for more serious problem consider arch/x86/lib/copy_user_64.S
In case of an unhandled fault on attempt to read an (unaligned) word,
the damn thing falls back to this:
SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
movl %edx,%ecx
1: rep movsb
2: mov %ecx,%eax
ASM_CLAC
ret
_ASM_EXTABLE_UA(1b, 2b)
SYM_CODE_END(.Lcopy_user_handle_tail)
We could do what alpha, sparc et.al. are doing - have both reads and
writes aligned, with every output word being a mix of two input ones.
But I would expect that to be considerably slower than the current
variants. Sure, we can set AC in .Lcopy_user_handle_tail, but that
doesn't look right.
And while squeezing every byte on a short copy is not a hard requirement,
in situation when the source is one byte before the end of page and
destination is aligned, raw_copy_from_user() really must copy at least
one byte if it's readable.
So I suspect that we need a variant of extable entry that does not
clear AC, at least for these fallbacks.
PS: I'm still going through the _ASM_EXTABLE... users on x86, so there
might be more fun. Will post when I'm done...
next prev parent reply other threads:[~2020-07-03 21:03 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
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 [this message]
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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