[PATCH] Make sure abs() always works on a signed argument
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From: Michal Januszewski on 28 Jul 2010 08:50 The abs() macro is commonly used to calculate the modulus of a difference of two numbers. With the introduction of "long" inside the definition of abs() in the commit a49c59c042c63b432307c1bbf7dac5a104c786e6, some of the abs() calls in the kernel started returning unexpected values (for instance, see abs() usage in drivers/video/modedb.c and the resulting problems reported in https://bugs.gentoo.org/show_bug.cgi?id=296539). The problem is apparent if the argument of abs() is a difference of two 32-bit integers, at least one of which is unsigned. The result is then assumed to be an unsigned integer, which gets cast to a positive long. The return value of abs() is then this large positive integer, instead of the expected small positive integer representing the modulus of the argument. Example: u32 a = 0, b = 1; u32 c = abs(a - b); 'c' will end up with a value of 0xffffffff instead of the expected 0x1. To fix this problem, explicitly cast the argument of abs() to signed, so that it gets properly expanded to long in case the original argument was an int. Signed-off-by: Michal Januszewski <michalj(a)gmail.com> -- diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 8317ec4..29fd43e 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -158,7 +158,7 @@ extern int _cond_resched(void); #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0) #define abs(x) ({ \ - long __x = (x); \ + long __x = (signed)(x); \ (__x < 0) ? -__x : __x; \ }) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |