2016-05-26 11:05:49

by Nicolai Stange

Subject: [PATCH 1/2] lib/mpi: mpi_read_raw_data(): purge redundant clearing of nbits

In mpi_read_raw_data(), unsigned nbits is calculated as follows:

nbits = nbytes * 8;

and redundantly cleared later on if nbytes == 0:

if (nbytes > 0)
...
else
nbits = 0;

Purge this redundant clearing for the sake of clarity.

Signed-off-by: Nicolai Stange <[email protected]>
---
Applicable to linux-next-20160525.

lib/mpi/mpicoder.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 747606f..c160393 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -51,8 +51,6 @@ MPI mpi_read_raw_data(const void *xbuffer, size_t nbytes)
}
if (nbytes > 0)
nbits -= count_leading_zeros(buffer[0]);
- else
- nbits = 0;

nlimbs = DIV_ROUND_UP(nbytes, BYTES_PER_MPI_LIMB);
val = mpi_alloc(nlimbs);
--
2.8.2

2016-05-26 11:06:02

by Nicolai Stange

Subject: [PATCH 2/2] lib/mpi: mpi_read_raw_data(): fix nbits calculation

The number of bits, nbits, is calculated in mpi_read_raw_data() as follows:

nbits = nbytes * 8;

Afterwards, the number of leading zero bits of the first byte get
subtracted:

nbits -= count_leading_zeros(buffer[0]);

However, count_leading_zeros() takes an unsigned long and thus,
the u8 gets promoted to an unsigned long.

Thus, the above doesn't subtract the number of leading zeros in the most
significant nonzero input byte from nbits, but the number of leading
zeros of the most significant nonzero input byte promoted to unsigned long,
i.e. BITS_PER_LONG - 8 too many.

Fix this by subtracting

count_leading_zeros(...) - (BITS_PER_LONG - 8)

from nbits only.

Fixes: e1045992949 ("MPILIB: Provide a function to read raw data into an
MPI")
Signed-off-by: Nicolai Stange <[email protected]>
---
Applicable to linux-next-20150525.

lib/mpi/mpicoder.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index c160393..d197210 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -50,7 +50,7 @@ MPI mpi_read_raw_data(const void *xbuffer, size_t nbytes)
return NULL;
}
if (nbytes > 0)
- nbits -= count_leading_zeros(buffer[0]);
+ nbits -= count_leading_zeros(buffer[0]) - (BITS_PER_LONG - 8);

nlimbs = DIV_ROUND_UP(nbytes, BYTES_PER_MPI_LIMB);
val = mpi_alloc(nlimbs);
--
2.8.2

2016-05-31 10:18:59

by Herbert Xu

Subject: Re: [PATCH 1/2] lib/mpi: mpi_read_raw_data(): purge redundant clearing of nbits

On Thu, May 26, 2016 at 01:05:31PM +0200, Nicolai Stange wrote:
> In mpi_read_raw_data(), unsigned nbits is calculated as follows:
>
> nbits = nbytes * 8;
>
> and redundantly cleared later on if nbytes == 0:
>
> if (nbytes > 0)
> ...
> else
> nbits = 0;
>
> Purge this redundant clearing for the sake of clarity.
>
> Signed-off-by: Nicolai Stange <[email protected]>

Both applied.
--
Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt