[samuel.thibault@ens-lyon.org: Re: warning: comparison is always false due to limited range of data type]

[samuel.thibault@ens-lyon.org: Re: warning: comparison is always false due to limited range of data type]

[Samuel Thibault]

Hi,

Le lun 09 aoû 2004 à 04:50:04 +0200, Paul Jackson a tapoté sur son clavier :
> Unfortunately, this patch uses the notorious "gcc warning suppression
> by obfuscation" technique.
> 
> --- 2.6.8-rc2-mm2.orig/include/linux/highuid.h	2004-08-04 19:27:48.000000000 -0700
> +++ 2.6.8-rc2-mm2/include/linux/highuid.h	2004-08-08 19:03:47.000000000 -0700
> @@ -44,8 +44,8 @@ extern void __bad_gid(void);
>  #ifdef CONFIG_UID16
>  
>  /* prevent uid mod 65536 effect by returning a default value for high UIDs */
> -#define high2lowuid(uid) ((uid) > 65535 ? (old_uid_t)overflowuid : (old_uid_t)(uid))
> -#define high2lowgid(gid) ((gid) > 65535 ? (old_gid_t)overflowgid : (old_gid_t)(gid))
> +#define high2lowuid(uid) ((uid) & ~0xFFFF ? (old_uid_t)overflowuid : (old_uid_t)(uid))
> +#define high2lowgid(gid) ((gid) & ~0xFFFF ? (old_gid_t)overflowgid : (old_gid_t)(gid))
>  /*
> ...

Here is another approach. This should never warning, and should get
optimized away as needed.

Regards,
Samuel

--- linux-2.6.8.1-orig/include/linux/highuid.h	2003-10-08 22:20:06.000000000 +0200
+++ linux-2.6.8.1-perso/include/linux/highuid.h	2004-08-19 00:47:31.000000000 +0200
@@ -44,8 +44,8 @@
 #ifdef CONFIG_UID16
 
 /* prevent uid mod 65536 effect by returning a default value for high UIDs */
-#define high2lowuid(uid) ((uid) > 65535 ? (old_uid_t)overflowuid : (old_uid_t)(uid))
-#define high2lowgid(gid) ((gid) > 65535 ? (old_gid_t)overflowgid : (old_gid_t)(gid))
+#define high2lowuid(uid) ((sizeof(uid) > 2 ? (uid) : 0) > 65535 ? (old_uid_t)overflowuid : (old_uid_t)(uid))
+#define high2lowgid(gid) ((sizeof(gid) > 2 ? (gid) : 0) > 65535 ? (old_gid_t)overflowgid : (old_gid_t)(gid))
 /*
  * -1 is different in 16 bits than it is in 32 bits
  * these macros are used by chown(), setreuid(), ...,
@@ -89,8 +89,8 @@
  * Since these macros are used in architectures that only need limited
  * 16-bit UID back compatibility, we won't use old_uid_t and old_gid_t
  */
-#define fs_high2lowuid(uid) ((uid) > 65535 ? (uid16_t)fs_overflowuid : (uid16_t)(uid))
-#define fs_high2lowgid(gid) ((gid) > 65535 ? (gid16_t)fs_overflowgid : (gid16_t)(gid))
+#define fs_high2lowuid(uid) ((sizeof(uid) > 2 ? (uid) : 0) > 65535 ? (uid16_t)fs_overflowuid : (uid16_t)(uid))
+#define fs_high2lowgid(gid) ((sizeof(gid) > 2 ? (gid) : 0) > 65535 ? (gid16_t)fs_overflowgid : (gid16_t)(gid))
 
 #define low_16_bits(x)	((x) & 0xFFFF)
 #define high_16_bits(x)	(((x) & 0xFFFF0000) >> 16)

Re: [samuel.thibault@ens-lyon.org: Re: warning: comparison is always false due to limited range of data type]

[Paul Jackson]

Samuel wrote:
> Here is another approach. This should never warning, and should get
> optimized away as needed.

Yours looks like it has a better chance of not being broken by some
future gcc enhancement that can see through the obfuscation in mine.

I didn't actually try it, but yours looks good to me.

Thanks.

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373