Messages in this thread Patch in this message | | | Date | Sat, 1 Oct 2005 19:39:47 +0200 | From | Willy Tarreau <> | Subject | Re: [PATCH 1/3] 2.6.14-rc2-mm1: fixes for overflow msec_to_jiffies() |
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Hi Andrew,
On Thu, Sep 29, 2005 at 12:52:07PM -0700, Andrew Morton wrote: > > On Thu, Sep 29, 2005 at 02:43:12AM -0700, Andrew Morton wrote: > > > Willy Tarreau <willy@w.ods.org> wrote: > > > > > > > > +#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ) > > > > +# define MAX_MSEC_OFFSET \ > > > > + (ULONG_MAX - (MSEC_PER_SEC / HZ) + 1) > > > > > > That generates numbers which don't fit into unsigned ints, yielding vast > > > numbers of > > > > > > include/linux/jiffies.h: In function `msecs_to_jiffies': > > > include/linux/jiffies.h:310: warning: comparison is always false due to limited range of data type > > > include/linux/jiffies.h: In function `usecs_to_jiffies': > > > include/linux/jiffies.h:323: warning: comparison is always false due to limited range of data type > > > > > This was a ppc64 build, gcc-3.3.3, CONFIG_HZ=250 > > Look a the value which MAX_MSEC_OFFSET will take (it's 2^63 minus a bit). > Comparing that to an unsigned int will generate the always-true or > always-false warning.
OK, this was not trivial because gcc is smart enough to detect that even after a cast, the comparison is still always false. So I had to cut the comparison in two parts : one which tests whether we need to compare, and one which does the test on the unsigned int part if necessary. It has shut the warnings on my alpha (HZ=1024) and on my ultrasparc (HZ=250). I've noticed that the code in previous patch could still overflow in the cases where a multiply was used first, because the common type was still int. My test case in user-space used MSEC_PER_SEC = 1000UL so it did not happen. I've fixed this too.
I've also checked the code produced on x86 (because alpha code is unreadable to me), and it resumes to this :
- HZ=1000 : no code generated - HZ=250 : comparison, add, right shift - HZ=100 : comparison, add, divide - HZ=1024 : comparison, left shift, add, mul, right shift
I tried to compile on ppc64, but unfortunately, the kernel does not build there because sizeof(long) == 4 ! I guess this is because gcc's target is powerpc-linux-gnu. I'm trying to recompile it with powerpc64-linux-gnu. Could you send me the output of gcc -v on your ppc64, please ?
I've also added missing parenthesis in the #defines which might have caused trouble to external users of MAX_?SEC_OFFSET (none at the moment).
Here's the new patch, I've build everything on 2.6.14-rc2-mm1 but verified that this code was not touched in -mm2 and the patch still applies.
Could you please retest it on your ppc64 and apply it if you're OK with it ?
Thanks, Willy
Signed-off-by: Willy Tarreau <willy@w.ods.org>
--- linux-2.6.14-rc2-mm1/include/linux/jiffies.h Thu Sep 29 23:04:49 2005 +++ linux-2.6.14-rc2-mm1-jiffies2/include/linux/jiffies.h Sat Oct 1 19:12:13 2005 @@ -246,6 +246,37 @@ #endif + +/* + * We define MAX_MSEC_OFFSET and MAX_USEC_OFFSET as maximal values that can be + * accepted by msecs_to_jiffies() and usec_to_jiffies() respectively, without + * risking a multiply overflow. Those functions return MAX_JIFFY_OFFSET for + * arguments above those values. + */ + +#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ) +# define MAX_MSEC_OFFSET \ + (ULONG_MAX - (MSEC_PER_SEC / HZ) + 1) +#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC) +# define MAX_MSEC_OFFSET \ + (ULONG_MAX / (HZ / MSEC_PER_SEC)) +#else +# define MAX_MSEC_OFFSET \ + ((ULONG_MAX - (MSEC_PER_SEC - 1)) / HZ) +#endif + +#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ) +# define MAX_USEC_OFFSET \ + (ULONG_MAX - (USEC_PER_SEC / HZ) + 1) +#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC) +# define MAX_USEC_OFFSET \ + (ULONG_MAX / (HZ / USEC_PER_SEC)) +#else +# define MAX_USEC_OFFSET \ + ((ULONG_MAX - (USEC_PER_SEC - 1)) / HZ) +#endif + + /* * Convert jiffies to milliseconds and back. * @@ -276,27 +307,29 @@ static inline unsigned long msecs_to_jiffies(const unsigned int m) { - if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET)) + if (MAX_MSEC_OFFSET < UINT_MAX && m > (unsigned int)MAX_MSEC_OFFSET) return MAX_JIFFY_OFFSET; #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ) - return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ); + return ((unsigned long)m + (MSEC_PER_SEC / HZ) - 1) / + (MSEC_PER_SEC / HZ); #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC) - return m * (HZ / MSEC_PER_SEC); + return (unsigned long)m * (HZ / MSEC_PER_SEC); #else - return (m * HZ + MSEC_PER_SEC - 1) / MSEC_PER_SEC; + return ((unsigned long)m * HZ + MSEC_PER_SEC - 1) / MSEC_PER_SEC; #endif } static inline unsigned long usecs_to_jiffies(const unsigned int u) { - if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET)) + if (MAX_USEC_OFFSET < UINT_MAX && u > (unsigned int)MAX_USEC_OFFSET) return MAX_JIFFY_OFFSET; #if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ) - return (u + (USEC_PER_SEC / HZ) - 1) / (USEC_PER_SEC / HZ); + return ((unsigned long)u + (USEC_PER_SEC / HZ) - 1) / + (USEC_PER_SEC / HZ); #elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC) - return u * (HZ / USEC_PER_SEC); + return (unsigned long)u * (HZ / USEC_PER_SEC); #else - return (u * HZ + USEC_PER_SEC - 1) / USEC_PER_SEC; + return ((unsigned long)u * HZ + USEC_PER_SEC - 1) / USEC_PER_SEC; #endif } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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