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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: 2d4d1eea54 ("lib/mpi: Add mpi sgl helpers")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Nicolai Stange 2016-03-22 13:17:27 +01:00 committed by Herbert Xu
parent 60e1b74c22
commit 64c09b0b59
1 changed files with 2 additions and 1 deletions
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3
lib/mpi/mpicoder.c
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@ -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);