[PATCH] tomoyo: fix clang pointer arithmetic warning

[PATCH] tomoyo: fix clang pointer arithmetic warning

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

clang warns about additions on NULL pointers being undefined in C:

security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
        securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,

Change the code to instead use a cast through uintptr_t to avoid
the warning.

Fixes: 9590837b89aa ("Common functions for TOMOYO Linux.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 security/tomoyo/securityfs_if.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/tomoyo/securityfs_if.c b/security/tomoyo/securityfs_if.c
index 546281c5b233..0a5f00073ef1 100644
--- a/security/tomoyo/securityfs_if.c
+++ b/security/tomoyo/securityfs_if.c
@@ -223,7 +223,7 @@ static const struct file_operations tomoyo_operations = {
 static void __init tomoyo_create_entry(const char *name, const umode_t mode,
 				       struct dentry *parent, const u8 key)
 {
-	securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
+	securityfs_create_file(name, mode, parent, (u8 *)(uintptr_t)key,
 			       &tomoyo_operations);
 }
 
-- 
2.27.0

Re: [PATCH] tomoyo: fix clang pointer arithmetic warning

[Tetsuo Handa]

Thank you for a patch. I have two questions.

On 2020/10/27 6:52, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> clang warns about additions on NULL pointers being undefined in C:
> 
> security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
>         securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
> 
> Change the code to instead use a cast through uintptr_t to avoid
> the warning.
> 

> -	securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
> +	securityfs_create_file(name, mode, parent, (u8 *)(uintptr_t)key,
>  			       &tomoyo_operations);

(1) Does clang warn if "(void *)key" is used instead of "(u8 *)(uintptr_t)key" ?

(2) tomoyo_open() has

	const int key = ((u8 *) file_inode(file)->i_private) - ((u8 *) NULL);

    which decodes the "u8 key" passed to tomoyo_create_entry(). For symmetry,
    I'd like to remove NULL from tomoyo_open() as well. Does clang warn if

	const int key = (u8) (file_inode(file)->i_private);

    is used?

Re: [PATCH] tomoyo: fix clang pointer arithmetic warning

[Arnd Bergmann]

On Wed, Oct 28, 2020 at 2:22 PM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> Thank you for a patch. I have two questions.
>
> On 2020/10/27 6:52, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> >
> > clang warns about additions on NULL pointers being undefined in C:
> >
> > security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
> >         securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
> >
> > Change the code to instead use a cast through uintptr_t to avoid
> > the warning.
> >
>
> > -     securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
> > +     securityfs_create_file(name, mode, parent, (u8 *)(uintptr_t)key,
> >                              &tomoyo_operations);
>
> (1) Does clang warn if "(void *)key" is used instead of "(u8 *)(uintptr_t)key" ?

Yes, both clang and gcc warn when you cast between a pointer and
an integer of a different size.

> (2) tomoyo_open() has
>
>         const int key = ((u8 *) file_inode(file)->i_private) - ((u8 *) NULL);
>
>     which decodes the "u8 key" passed to tomoyo_create_entry(). For symmetry,
>     I'd like to remove NULL from tomoyo_open() as well. Does clang warn if
>
>         const int key = (u8) (file_inode(file)->i_private);
>
>     is used?

Yes, same thing, but

   const int key = (uintptr_t)file_inode(file)->i_private;

works without warnings and seems clearer.

       Arnd

Re: [PATCH] tomoyo: fix clang pointer arithmetic warning

[Tetsuo Handa]

OK. I will apply the following patch to my tree. Thank you.

From: Arnd Bergmann <arnd@arndb.de>
Subject: [PATCH] tomoyo: fix clang pointer arithmetic warning

clang warns about additions on NULL pointers being undefined in C:

security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
        securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,

Change the code to instead use a cast through uintptr_t to avoid
the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 security/tomoyo/securityfs_if.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/security/tomoyo/securityfs_if.c b/security/tomoyo/securityfs_if.c
index 546281c5b233..065f4941c4d8 100644
--- a/security/tomoyo/securityfs_if.c
+++ b/security/tomoyo/securityfs_if.c
@@ -131,8 +131,8 @@ static const struct file_operations tomoyo_self_operations = {
  */
 static int tomoyo_open(struct inode *inode, struct file *file)
 {
-	const int key = ((u8 *) file_inode(file)->i_private)
-		- ((u8 *) NULL);
+	const u8 key = (uintptr_t) file_inode(file)->i_private;
+
 	return tomoyo_open_control(key, file);
 }
 
@@ -223,7 +223,7 @@ static const struct file_operations tomoyo_operations = {
 static void __init tomoyo_create_entry(const char *name, const umode_t mode,
 				       struct dentry *parent, const u8 key)
 {
-	securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
+	securityfs_create_file(name, mode, parent, (void *) (uintptr_t) key,
 			       &tomoyo_operations);
 }
 
-- 
2.18.4

Re: [PATCH] tomoyo: fix clang pointer arithmetic warning

[Nick Desaulniers]

On Mon, Oct 26, 2020 at 2:52 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> clang warns about additions on NULL pointers being undefined in C:
>
> security/tomoyo/securityfs_if.c:226:59: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
>         securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
>
> Change the code to instead use a cast through uintptr_t to avoid
> the warning.
>
> Fixes: 9590837b89aa ("Common functions for TOMOYO Linux.")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Thanks for the patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  security/tomoyo/securityfs_if.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/security/tomoyo/securityfs_if.c b/security/tomoyo/securityfs_if.c
> index 546281c5b233..0a5f00073ef1 100644
> --- a/security/tomoyo/securityfs_if.c
> +++ b/security/tomoyo/securityfs_if.c
> @@ -223,7 +223,7 @@ static const struct file_operations tomoyo_operations = {
>  static void __init tomoyo_create_entry(const char *name, const umode_t mode,
>                                        struct dentry *parent, const u8 key)
>  {
> -       securityfs_create_file(name, mode, parent, ((u8 *) NULL) + key,
> +       securityfs_create_file(name, mode, parent, (u8 *)(uintptr_t)key,
>                                &tomoyo_operations);
>  }
>
> --
> 2.27.0
>


-- 
Thanks,
~Nick Desaulniers