| Message ID | 1398820239-1890-1-git-send-email-behanw@converseincode.com |
|---|---|
| State | New |
| Headers | show |
I suspect this will generate FP instructions on x86 which will corrupt user space state. This is thus a critical bug!! On April 29, 2014 6:10:39 PM PDT, behanw@converseincode.com wrote: >From: Mark Charlebois <charlebm@gmail.com> > >The call to __builtin_log2 presumes there is a >double log2(double x) function defined in the kernel. > >The call to hash_log is a call to hash_64 which is >defined in include/linux/hash.h > >static __always_inline u64 hash_64(u64 val, unsigned int bits) > >That means that __builtin_log2(NR_BG_LOCKS) is converting >NR_BG_LOCKS to a double and returning a double and then that >is converted to an unsigned int. > >Using ilog2 is much more appropriate and efficient. > >Another side effect of using __builtin_log2 is that is uses >__aeabi_* functions for ARM that require linking with libgcc.a. > >Author: Mark Charlebois <charlebm@gmail.com> >Signed-off-by: Mark Charlebois <charlebm@gmail.com> >Signed-off-by: Behan Webster <behanw@converseincode.com> >--- > fs/mbcache.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > >diff --git a/fs/mbcache.c b/fs/mbcache.c >index bf166e3..2c0752b 100644 >--- a/fs/mbcache.c >+++ b/fs/mbcache.c >@@ -93,7 +93,7 @@ > > #define MB_CACHE_WRITER ((unsigned short)~0U >> 1) > >-#define MB_CACHE_ENTRY_LOCK_BITS __builtin_log2(NR_BG_LOCKS) >+#define MB_CACHE_ENTRY_LOCK_BITS ilog2(NR_BG_LOCKS) > #define MB_CACHE_ENTRY_LOCK_INDEX(ce) \ > (hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS)) >
On 04/29/2014 06:13 PM, H. Peter Anvin wrote:
> I suspect this will generate FP instructions on x86 which will corrupt user space state. This is thus a critical bug!!
It looks like at least x86-64 gcc does the evaluation at compile time
and there aren't any fp instructions left in the code. However, it
should still be fixed.
-hpa
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diff --git a/fs/mbcache.c b/fs/mbcache.c index bf166e3..2c0752b 100644 --- a/fs/mbcache.c +++ b/fs/mbcache.c @@ -93,7 +93,7 @@ #define MB_CACHE_WRITER ((unsigned short)~0U >> 1) -#define MB_CACHE_ENTRY_LOCK_BITS __builtin_log2(NR_BG_LOCKS) +#define MB_CACHE_ENTRY_LOCK_BITS ilog2(NR_BG_LOCKS) #define MB_CACHE_ENTRY_LOCK_INDEX(ce) \ (hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))