[PATCH] kernel.h: fix new warnings for container_of()

[PATCH] kernel.h: fix new warnings for container_of()

[Arnd Bergmann]

I see new warnings with gcc-7.0.1 with the modified container_of():

fs/f2fs/dir.c: In function 'F2FS_I':
fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'

This seems to happen for all structures that have a zero offset
between the member and the container structure, i.e. idential
pointers.

Reverting to an intermediate pointer avoids the warning, and using
a void pointer instead of the target type should also avoid
regressing on the previous patch again.

Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/kernel.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d043adadcf33..bd6d96cf80b1 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -856,10 +856,11 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
  *
  */
 #define container_of(ptr, type, member) ({				\
+	void *__mptr = (void *)(ptr);					\
 	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))); })
+	((type *)(__mptr - offsetof(type, member))); })
 
 /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
-- 
2.9.0

Re: [PATCH] kernel.h: fix new warnings for container_of()

[Kees Cook]

On Tue, Jun 20, 2017 at 1:09 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> I see new warnings with gcc-7.0.1 with the modified container_of():
>
> fs/f2fs/dir.c: In function 'F2FS_I':
> fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'

This is actually from the randstruct plugin, not native gcc. I'll go
update the message reporting to include a "randstruct: " which should
make this more obvious.

> This seems to happen for all structures that have a zero offset
> between the member and the container structure, i.e. idential
> pointers.
>
> Reverting to an intermediate pointer avoids the warning, and using
> a void pointer instead of the target type should also avoid
> regressing on the previous patch again.
>
> Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Thanks!

I'll take this into the kspp tree so all the fixes are in the same place.

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH] kernel.h: fix new warnings for container_of()

[Kees Cook]

On Tue, Jun 20, 2017 at 3:43 PM, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Jun 20, 2017 at 1:09 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>> I see new warnings with gcc-7.0.1 with the modified container_of():
>>
>> fs/f2fs/dir.c: In function 'F2FS_I':
>> fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'
>
> This is actually from the randstruct plugin, not native gcc. I'll go
> update the message reporting to include a "randstruct: " which should
> make this more obvious.
>
>> This seems to happen for all structures that have a zero offset
>> between the member and the container structure, i.e. idential
>> pointers.
>>
>> Reverting to an intermediate pointer avoids the warning, and using
>> a void pointer instead of the target type should also avoid
>> regressing on the previous patch again.
>>
>> Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> Thanks!
>
> I'll take this into the kspp tree so all the fixes are in the same place.

Oh, nevermind, I see the Fixes is for what's already in -mm. :) And I
see akpm picked this up already. Sorry for the noise!

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH] kernel.h: fix new warnings for container_of()

[Ian Abbott]

On 20/06/17 21:09, Arnd Bergmann wrote:
> I see new warnings with gcc-7.0.1 with the modified container_of():
>
> fs/f2fs/dir.c: In function 'F2FS_I':
> fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types: 'struct f2fs_inode_info' and 'struct inode'

Is that actually a warning, or just informational?  In any case, it 
seems like a good idea to avoid it.

>
> This seems to happen for all structures that have a zero offset
> between the member and the container structure, i.e. idential
> pointers.
>
> Reverting to an intermediate pointer avoids the warning, and using
> a void pointer instead of the target type should also avoid
> regressing on the previous patch again.
>
> Fixes: mmotm ("kernel.h: handle pointers to arrays better in container_of()")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  include/linux/kernel.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d043adadcf33..bd6d96cf80b1 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -856,10 +856,11 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
>   *
>   */
>  #define container_of(ptr, type, member) ({				\
> +	void *__mptr = (void *)(ptr);					\
>  	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))); })
> +	((type *)(__mptr - offsetof(type, member))); })
>
>  /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
>  #ifdef CONFIG_FTRACE_MCOUNT_RECORD
>

Acked-by: Ian Abbott <abbotti@mev.co.uk>

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@mev.co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-

Re: [PATCH] kernel.h: fix new warnings for container_of()

[Arnd Bergmann]

On Wed, Jun 21, 2017 at 11:25 AM, Ian Abbott <abbotti@mev.co.uk> wrote:
> On 20/06/17 21:09, Arnd Bergmann wrote:
>>
>> I see new warnings with gcc-7.0.1 with the modified container_of():
>>
>> fs/f2fs/dir.c: In function 'F2FS_I':
>> fs/f2fs/f2fs.h:1122:385: note: found mismatched ssa struct pointer types:
>> 'struct f2fs_inode_info' and 'struct inode'
>
>
> Is that actually a warning, or just informational?  In any case, it seems
> like a good idea to avoid it.

It's informational, not a warning. I couldn't actually figure out what the
message is trying to tell me here. Maybe the message could also be
improved?

      Arnd