linux-kernel - Re: [PATCH] um: Fix pointer cast

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Date:	Tue, 22 Dec 2015 22:30:16 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mickaël Salaün <mic@...ikod.net>
Cc:	linux-kernel@...r.kernel.org, Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH] um: Fix pointer cast

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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