linux-kernel - Re: [PATCH] mbcache: LLVMLinux: Remove double calculation from mbcache

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Message-ID: <a54957c1-4825-4265-8126-4565168fe8c6@email.android.com>
Date:	Tue, 29 Apr 2014 18:13:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	behanw@...verseincode.com, viro@...iv.linux.org.uk
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, dwmw2@...radead.org,
	Mark Charlebois <charlebm@...il.com>
Subject: Re: [PATCH] mbcache: LLVMLinux: Remove double calculation from mbcache

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@...verseincode.com wrote:
>From: Mark Charlebois <charlebm@...il.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@...il.com>
>Signed-off-by: Mark Charlebois <charlebm@...il.com>
>Signed-off-by: Behan Webster <behanw@...verseincode.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))
> 

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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