Re: [PATCH] nvmem: core: Fix msb clearing bits

[Shadab Naseem]

Gentle reminder as it is easily reproducible in 32 bit targets.

Regards

-Shadab

On 2/14/2020 12:46 PM, Mukesh Ojha wrote:
> On 1/31/2020 6:13 PM, Shadab Naseem wrote:
> > When clearing the msb bits of the resultant buffer, it is
> > masked with the modulo of the number of bits needed with
> > respect to the BITS_PER_BYTE. To mask out the buffer,
> > it is passed though GENMASK of the remainder of the bits
> > starting from zeroth bit. This case is valid if nbits is not
> > a multiple of BITS_PER_BYTE and you are actually creating
> > a GENMASK. If the nbits coming is a multiple of BITS_PER_BYTE,
> > it would pass a negative value to the high bit number of
> > GENMASK with zero as the lower bit number.
> >
> > As per the definition of the GENMASK, the higher bit number (h)
> > is right operand for bitwise right shift. If the value of the
> > right operand is negative or is greater or equal to the number
> > of bits in the promoted left operand, the behavior is undefined.
> > So passing a negative value to GENMASK could behave differently
> > across architecture, specifically between 64 and 32 bit.
> > Also, on passing the hard-coded negative value as GENMASK(-1, 0)
> > is giving compiler warning for shift-count-overflow.
> > Hence making a check for clearing the MSB if the nbits are not
> > a multiple of BITS_PER_BYTE.
> >
> > Signed-off-by: Shadab Naseem <snaseem@xxxxxxxxxxxxxx>
> > ---
> > Â drivers/nvmem/core.c | 3 ++-
> > Â 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> > index 1e4a798..23c1547 100644
> > --- a/drivers/nvmem/core.c
> > +++ b/drivers/nvmem/core.c
> > @@ -926,7 +926,8 @@ static void 
> > nvmem_shift_read_buffer_in_place(struct nvmem_cell *cell, void *buf)
> > ÂÂÂÂÂÂÂÂÂ *p-- = 0;
> > Â ÂÂÂÂÂ /* clear msb bits if any leftover in the last byte */
> > -ÂÂÂ *p &= GENMASK((cell->nbits%BITS_PER_BYTE) - 1, 0);
> > +ÂÂÂ if (cell->nbits%BITS_PER_BYTE)
> > +ÂÂÂÂÂÂÂ *p &= GENMASK((cell->nbits%BITS_PER_BYTE) - 1, 0);
>
>
> LGTM..
>
> Reviewed-by: mojha@xxxxxxxxxxxxxx
>
>
> Thanks
> Mukesh
>
> > Â }
> > Â Â static int __nvmem_cell_read(struct nvmem_device *nvmem,