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[PATCH] um: Fix pointer cast

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  [PATCH] um: Fix pointer cast Mickaël Salaün <mic@digikod.net> - 2015-12-22 23:10 +0100
    Re: [PATCH] um: Fix pointer cast Al Viro <viro@ZenIV.linux.org.uk> - 2015-12-22 23:40 +0100

#1297092 — [PATCH] um: Fix pointer cast

Fix a pointer cast typo introduced in v4.4-rc5 especially visible for
the i386 subarchitecture where it results in a kernel crash.

Fixes: 8090bfd2bb9a ("um: Fix fpstate handling")

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
---
 arch/x86/um/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/um/signal.c b/arch/x86/um/signal.c
index e5f854ce2d72..cbb541b80358 100644
--- a/arch/x86/um/signal.c
+++ b/arch/x86/um/signal.c
@@ -470,7 +470,7 @@ long sys_sigreturn(void)
 	struct sigcontext __user *sc = &frame->sc;
 	int sig_size = (_NSIG_WORDS - 1) * sizeof(unsigned long);
 
-	if (copy_from_user(&set.sig[0], (void *)sc->oldmask, sizeof(set.sig[0])) ||
+	if (copy_from_user(&set.sig[0], (void *)&sc->oldmask, sizeof(set.sig[0])) ||
 	    copy_from_user(&set.sig[1], frame->extramask, sig_size))
 		goto segfault;
 
-- 
2.6.4

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#1297123

On Tue, Dec 22, 2015 at 09:44:01PM +0100, Mickaël Salaün wrote:
> Fix a pointer cast typo introduced in v4.4-rc5 especially visible for
> the i386 subarchitecture where it results in a kernel crash.

Why the hell bother casting it at all?  _Any_ pointer will quietly convert
to void *, no typecasts needed.  The second argument of copy_from_user
is const void __user *; sc is struct sigcontext __user *sc, so
&sb->oldmask is either __u32 __user * or __u64 __user *, for 32bit and
64bit builds resp.  Either is assignment-compatible with
const void __user *.

Basically, cast is telling the typechecking logics "sod off, I know better".
And here it's not needed at all.  Moreover, the bug you are fixing here is
precisely that this code did *not* know better - if not for that cast,
compiler would've immediately pointed to the problem.

> -	if (copy_from_user(&set.sig[0], (void *)sc->oldmask, sizeof(set.sig[0])) ||
> +	if (copy_from_user(&set.sig[0], (void *)&sc->oldmask, sizeof(set.sig[0])) ||

Please, remove the cast completely.  Simply pass it &sc->oldmask and be
done with that.
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