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 <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 37d6a4edb33cbec0ad59cac65c36964fe89fde64..eda34aba017e16026fcea47512792873bc13c8f1 100644 (file)
--- 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);