linux-kernel - kcalloc/kmalloc_array could BUILD_BUG_ON for too-big constant arguments (was Re: [PATCH] [RESEND] aic7xxx: replace kmalloc/memset by kzalloc)

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Date:	Fri, 27 Mar 2015 14:48:24 -0500
From:	Jeff Epler <jepler@...ythonic.net>
To:	Michael Opdenacker <michael.opdenacker@...e-electrons.com>
Cc:	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	Joe Perches <joe@...ches.com>, Hannes Reinecke <hare@...e.de>,
	"JBottomley@...allels.com" <JBottomley@...allels.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: kcalloc/kmalloc_array could BUILD_BUG_ON for too-big constant
 arguments (was Re: [PATCH] [RESEND] aic7xxx: replace kmalloc/memset by
 kzalloc)

The following is a sketch of how a macro kcalloc could BUILD_BUG_ON for
overflows of two compile-time operands, or call "kcalloc_variable" for
nonconstant arguments.  Tested on gcc 4.7.2 only, since it's what I had to
hand.  I didn't do any testing beyond checking that fn2 didn't build, and that
fn1/3 had plausible-looking code on x86_64.

typedef unsigned long size_t;
#define SIZE_MAX (~(size_t)0)
typedef int gfp_t;
extern void *kzalloc(size_t n, gfp_t flags);
extern void *kcalloc_variable(size_t n, size_t size, gfp_t flags);
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

#define kcalloc(n, size, flags) \
	__builtin_choose_expr(__builtin_constant_p((n) | (size)), \
		( \
			BUILD_BUG_ON((n) > SIZE_MAX / (size)), \
			kzalloc((n) * (size), (flags)) \
		), kcalloc_variable((n), (size), (flags)))


void fn1() { kcalloc(3, 3, 0); }
//void fn2() { kcalloc(2, ~(size_t)0, 0); }// compile-time BUILD_BUG_ON
void fn3(int i) { kcalloc(2, i, 0); }

Jeff
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