lib/mpi: mpi_read_raw_from_sgl(): fix nbits calculation

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

  nbits = nbytes * 8;

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

  nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros));

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: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers")
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 27703aad287abd09da49fafba5a3f45bd56c61ee..24a015513b40aee68752557d4284858ad83ecc4d 100644 (file)
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -460,7 +460,8 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
        }
 
        if (nbytes > 0)
-               nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros));
+               nbits -= count_leading_zeros(*(u8 *)(sg_virt(sgl) + lzeros)) -
+                       (BITS_PER_LONG - 8);
 
        nlimbs = DIV_ROUND_UP(nbytes, BYTES_PER_MPI_LIMB);
        val = mpi_alloc(nlimbs);