linux-kernel - Re: Issues with "x86, um: switch to generic fork/vfork/clone" commit

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Message-ID: <20130120031253.GO4939@ZenIV.linux.org.uk>
Date:	Sun, 20 Jan 2013 03:12:53 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Issues with "x86, um: switch to generic fork/vfork/clone" commit

On Sat, Jan 19, 2013 at 06:38:08AM +0000, Al Viro wrote:
> > [   64.313636] kbd[2563]: segfault at 9fe ip 000009fe sp b758293c
> > error 4 in dash[8048000+18000]
> > 
> > After bisecting, the following commit seems responsible:
> > 1d4b4b2994b5fc208963c0b795291f8c1f18becf (x86, um: switch to generic
> > fork/vfork/clone)
> 
> Er...  Bisect of the guest kernel, I take it?  Could you check if building
> the guest !SMP affects anything?

OK...  I think I understand what's going on.  We need asmlinkage_protect
in sys_clone() ;-/  For what it's worth, I really wonder if we ought to
treat that as syscall wrappers - i.e. have SYSCALL_DEFINEx on i386 add
a wrapper that would do asmlinkage_protect itself.  IMO it's the same kind
of thing as argument normalization handled by syscall wrappers - we make
sure that C function plays well with what asm glue is doing and expecting.

Anyway, the following seems to fix the problem here (and yes, I could reproduce
it with your config); could you verify that it fixes things on your setup?
If it does, this sucker should go into mainline and -stable...

diff --git a/kernel/fork.c b/kernel/fork.c
index a31b823..e05cff2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1660,8 +1660,10 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 		 int, tls_val)
 #endif
 {
-	return do_fork(clone_flags, newsp, 0,
-		parent_tidptr, child_tidptr);
+	long ret = do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr);
+	asmlinkage_protect(5, ret, clone_flags, newsp,
+			parent_tidptr, child_tidptr, tls_val);
+	return ret;
 }
 #endif
 
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