Re: [PATCH] slub: fix check_bytes() for slub debugging

[Eric Dumazet]

Le mardi 09 aoÃt 2011 Ã 05:10 +0200, Eric Dumazet a Ãcrit :
> Le dimanche 07 aoÃt 2011 Ã 18:30 +0900, Akinobu Mita a Ãcrit :
> > The check_bytes() function is used by slub debugging.  It returns a pointer
> > to the first unmatching byte for a character in the given memory area.
> > 
> > If the character for matching byte is greater than 0x80, check_bytes()
> > doesn't work.  Becuase 64-bit pattern is generated as below.
> > 
> > 	value64 = value | value << 8 | value << 16 | value << 24;
> > 	value64 = value64 | value64 << 32;
> > 
> > The integer promotions are performed and sign-extended as the type of value
> > is u8.  The upper 32 bits of value64 is 0xffffffff in the first line, and
> > the second line has no effect.
> > 
> > This fixes the 64-bit pattern generation.
> > 
> > Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> > Cc: Matt Mackall <mpm@xxxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > ---
> >  mm/slub.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index eb5a8f9..5695f92 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -701,7 +701,7 @@ static u8 *check_bytes(u8 *start, u8 value, unsigned int bytes)
> >  		return check_bytes8(start, value, bytes);
> >  
> >  	value64 = value | value << 8 | value << 16 | value << 24;
> > -	value64 = value64 | value64 << 32;
> > +	value64 = (value64 & 0xffffffff) | value64 << 32;
> >  	prefix = 8 - ((unsigned long)start) % 8;
> >  
> >  	if (prefix) {
> 
> Still buggy I am afraid. Could we use the following ?
> 
> 
> 	value64 = value;
> 	value64 |= value64 << 8;
> 	value64 |= value64 << 16;
> 	value64 |= value64 << 32;
> 
> 

Well, 'buggy' was not well chosen.

Another possibility would be to use a multiply if arch has a fast
multiplier...


	value64 = value;
#if defined(ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
	value64 *= 0x0101010101010101;
#elif defined(ARCH_HAS_FAST_MULTIPLIER)
	value64 *= 0x01010101;
	value64 |= value64 << 32;
#else
	value64 |= value64 << 8;
	value64 |= value64 << 16;
	value64 |= value64 << 32;
#endif




--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/