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[PATCH v5 6/6] kernel.h: handle pointers to arrays better in container_of()

From Ian Abbott <abbotti@mev.co.uk>
Newsgroups linux.kernel
Subject [PATCH v5 6/6] kernel.h: handle pointers to arrays better in container_of()
Date 2017-05-25 14:10 +0200
Message-ID <tKZKy-81K-37@gated-at.bofh.it> (permalink)
References <tKZKx-81K-3@gated-at.bofh.it>
Organization linux.* mail to news gateway

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If the first parameter of container_of() is a pointer to a
non-const-qualified array type (and the third parameter names a
non-const-qualified array member), the local variable __mptr will be
defined with a const-qualified array type.  In ISO C, these types are
incompatible.  They work as expected in GNU C, but some versions will
issue warnings.  For example, GCC 4.9 produces the warning
"initialization from incompatible pointer type".

Here is an example of where the problem occurs:

-------------------------------------------------------
 #include <linux/kernel.h>
 #include <linux/module.h>

MODULE_LICENSE("GPL");

struct st {
	int a;
	char b[16];
};

static int __init example_init(void) {
	struct st t = { .a = 101, .b = "hello" };
	char (*p)[16] = &t.b;
	struct st *x = container_of(p, struct st, b);
	printk(KERN_DEBUG "%p %p\n", (void *)&t, (void *)x);
	return 0;
}

static void __exit example_exit(void) {
}

module_init(example_init);
module_exit(example_exit);
-------------------------------------------------------

Building the module with gcc-4.9 results in these warnings (where '{m}'
is the module source and '{k}' is the kernel source):

-------------------------------------------------------
In file included from {m}/example.c:1:0:
{m}/example.c: In function ‘example_init’:
{k}/include/linux/kernel.h:854:48: warning: initialization from
incompatible pointer type
  const typeof( ((type *)0)->member ) *__mptr = (ptr); \
                                                ^
{m}/example.c:14:17: note: in expansion of macro ‘container_of’
  struct st *x = container_of(p, struct st, b);
                 ^
{k}/include/linux/kernel.h:854:48: warning: (near initialization for
‘x’)
  const typeof( ((type *)0)->member ) *__mptr = (ptr); \
                                                ^
{m}/example.c:14:17: note: in expansion of macro ‘container_of’
  struct st *x = container_of(p, struct st, b);
                 ^
-------------------------------------------------------

Replace the type checking performed by the macro to avoid these
warnings.  Make sure `*(ptr)` either has type compatible with the
member, or has type compatible with `void`, ignoring qualifiers.  Raise
compiler errors if this is not true.  This is stronger than the previous
behaviour, which only resulted in compiler warnings for a type mismatch.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Potapenko <glider@google.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
---
v2: Rebased and altered description to provide an example of when the
compiler warnings occur.  v1 (from 2016-10-10) also modified a
'container_of_safe()' macro that never made it out of "linux-next".
v3: Added back some type checking at the suggestion of Michal
Nazarewicz with some helpful hints by Peter Zijlstra.
v4: No change.
v5: Added Acked-by for Michal Nazarewicz.  Included <linux/build_bug.h>
instead of <linux/bug.h> to avoid a circular dependency that resulted in
build failures when <asm/bug.h> was included before <linux/kernel.h>.
---
 include/linux/kernel.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 13bc08aba704..1c9c11c9f1a8 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -11,6 +11,7 @@
 #include <linux/log2.h>
 #include <linux/typecheck.h>
 #include <linux/printk.h>
+#include <linux/build_bug.h>
 #include <asm/byteorder.h>
 #include <uapi/linux/kernel.h>
 
@@ -850,9 +851,11 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
  * @member:	the name of the member within the struct.
  *
  */
-#define container_of(ptr, type, member) ({			\
-	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
-	(type *)( (char *)__mptr - offsetof(type,member) );})
+#define container_of(ptr, type, member) ({				\
+	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
+			 !__same_type(*(ptr), void),			\
+			 "pointer type mismatch in container_of()");	\
+	((type *)((char *)(ptr) - offsetof(type, member))); })
 
 /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
-- 
2.11.0

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Thread

[PATCH v5 0/6] kernel.h: container_of() pointer checking Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
  [PATCH v5 4/6] linux/bug.h: correct "space required before that '-'" Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
    Re: [PATCH v5 4/6] linux/bug.h: correct "space required before that '-'" Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:10 +0200
    Re: [PATCH v5 4/6] linux/bug.h: correct "space required before that '-'" Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:10 +0200
  [PATCH v5 1/6] asm-generic/bug.h: declare struct pt_regs; before function prototype Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
  [PATCH v5 2/6] linux/bug.h: correct formatting of block comment Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
    Re: [PATCH v5 2/6] linux/bug.h: correct formatting of block comment Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:00 +0200
  [PATCH v5 3/6] linux/bug.h: correct "(foo*)" should be "(foo *)" Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
    Re: [PATCH v5 3/6] linux/bug.h: correct "(foo*)" should be "(foo *)" Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:00 +0200
  [PATCH v5 5/6] bug: split BUILD_BUG stuff out into <linux/build_bug.h> Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
    Re: [PATCH v5 5/6] bug: split BUILD_BUG stuff out into <linux/build_bug.h> Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:10 +0200
    Re: [PATCH v5 5/6] bug: split BUILD_BUG stuff out into <linux/build_bug.h> Kees Cook <keescook@chromium.org> - 2017-05-25 20:40 +0200
  [PATCH v5 6/6] kernel.h: handle pointers to arrays better in container_of() Ian Abbott <abbotti@mev.co.uk> - 2017-05-25 14:10 +0200
    Re: [PATCH v5 6/6] kernel.h: handle pointers to arrays better in container_of() Michal Nazarewicz <mina86@mina86.com> - 2017-05-25 16:10 +0200
    Re: [PATCH v5 6/6] kernel.h: handle pointers to arrays better in container_of() Kees Cook <keescook@chromium.org> - 2017-05-25 20:40 +0200
      Re: [PATCH v5 6/6] kernel.h: handle pointers to arrays better in  container_of() Ian Abbott <abbotti@mev.co.uk> - 2017-05-26 16:00 +0200

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