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[PATCH RESEND v2 13/14] lib/mpi: mpi_read_raw_from_sgl(): sanitize meaning of indices

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Within the byte reading loop in mpi_read_raw_sgl(), there are two
housekeeping indices used, z and x.

At all times, the index z represents the number of output bytes covered
by the input SGEs for which processing has completed so far. This includes
any leading zero bytes within the most significant limb.

The index x changes its meaning after the first outer loop's iteration
though: while processing the first input SGE, it represents

  "number of leading zero bytes in most significant output limb" +
   "current position within current SGE"

For the remaining SGEs OTOH, x corresponds just to

  "current position within current SGE"

After all, it is only the sum of z and x that has any meaning for the
output buffer and thus, the

  "number of leading zero bytes in most significant output limb"

part can be moved away from x into z from the beginning, opening up the
opportunity for cleaner code.

Before the outer loop iterating over the SGEs, don't initialize z with
zero, but with the number of leading zero bytes in the most significant
output limb. For the inner loop iterating over a single SGE's bytes,
get rid of the buf_shift offset to x' bounds and let x run from zero to
sg->length - 1.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 lib/mpi/mpicoder.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 5d02efe..3f114d2 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -477,19 +477,17 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 
 	j = nlimbs - 1;
 	a = 0;
-	z = 0;
-	x = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;
-	x %= BYTES_PER_MPI_LIMB;
+	z = BYTES_PER_MPI_LIMB - nbytes % BYTES_PER_MPI_LIMB;
+	z %= BYTES_PER_MPI_LIMB;
 
 	for_each_sg(sgl, sg, ents, i) {
 		const u8 *buffer = sg_virt(sg) + lzeros;
 		int len = sg->length - lzeros;
-		int buf_shift = x;
 
 		if  (sg_is_last(sg) && (len % BYTES_PER_MPI_LIMB))
 			len += BYTES_PER_MPI_LIMB - (len % BYTES_PER_MPI_LIMB);
 
-		for (; x < len + buf_shift; x++) {
+		for (x = 0; x < len; x++) {
 			a <<= 8;
 			a |= *buffer++;
 			if (((z + x + 1) % BYTES_PER_MPI_LIMB) == 0) {
@@ -498,7 +496,6 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 			}
 		}
 		z += x;
-		x = 0;
 		lzeros = 0;
 	}
 	return val;
-- 
2.7.3

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[PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 05/14] lib/mpi: mpi_write_sgl(): replace open coded endian conversion Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 06/14] lib/mpi: mpi_read_buffer(): optimize skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 12/14] lib/mpi: mpi_read_raw_from_sgl(): fix nbits calculation Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 10/14] lib/mpi: mpi_read_raw_from_sgl(): don't include leading zero SGEs in nbytes Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 07/14] lib/mpi: mpi_read_buffer(): replace open coded endian conversion Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 08/14] lib/mpi: mpi_read_buffer(): fix buffer overflow Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 13/14] lib/mpi: mpi_read_raw_from_sgl(): sanitize meaning of indices Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 14/14] lib/mpi: mpi_read_raw_from_sgl(): fix out-of-bounds buffer access Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 11/14] lib/mpi: mpi_read_raw_from_sgl(): purge redundant clearing of nbits Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:30 +0100
  [PATCH RESEND v2 03/14] lib/mpi: mpi_write_sgl(): purge redundant pointer arithmetic Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 02/14] lib/mpi: mpi_write_sgl(): fix style issue with lzero decrement Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 04/14] lib/mpi: mpi_write_sgl(): fix out-of-bounds stack access Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  [PATCH RESEND v2 01/14] lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs Nicolai Stange <nicstange@gmail.com> - 2016-03-21 14:40 +0100
  Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Tadeusz Struk <tadeusz.struk@intel.com> - 2016-03-22 06:00 +0100
    Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Nicolai Stange <nicstange@gmail.com> - 2016-03-22 08:10 +0100
      Re: [PATCH RESEND v2 00/14] lib/mpi: bug fixes and cleanup Tadeusz Struk <tadeusz.struk@intel.com> - 2016-03-22 15:20 +0100

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