lib/mpi: mpi_read_raw_data(): fix nbits calculation

commit eef0df6a59537032ab6b708f30b28d9530f8760e upstream.

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>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 4cc6442733f49577f29647984db9f310d06a21c4..fe8b9ffbbb50b68d39c1d1782e92e32c4559ff97 100644 (file)
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -48,7 +48,7 @@ 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);
        else
                nbits = 0;