lib/mpi: mpi_write_sgl(): fix skipping of leading zero limbs

Currently, if the number of leading zeros is greater than fits into a
complete limb, mpi_write_sgl() skips them by iterating over them limb-wise.

However, it fails to adjust its internal leading zeros tracking variable,
lzeros, accordingly: it does a

  p -= sizeof(alimb);
  continue;

which should really have been a

  lzeros -= sizeof(alimb);
  continue;

Since lzeros never decreases if its initial value >= sizeof(alimb), nothing
gets copied by mpi_write_sgl() in that case.

Instead of skipping the high order zero limbs within the loop as shown
above, fix the issue by adjusting the copying loop's bounds.

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>

1 files changed, 9 insertions, 12 deletions

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index eb15e7dc7b65..6bb52beb06b0 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -380,7 +380,9 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
         buf_len = sgl->length;
         p2 = sg_virt(sgl);
 
-        for (i = a->nlimbs - 1; i >= 0; i--) {
+        for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
+                        lzeros %= BYTES_PER_MPI_LIMB;
+                i >= 0; i--) {
                 alimb = a->d[i];
                 p = (u8 *)&alimb2;
 #if BYTES_PER_MPI_LIMB == 4
@@ -401,17 +403,12 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 #error please implement for this limb size.
 #endif
                 if (lzeros > 0) {
-                        if (lzeros >= sizeof(alimb)) {
-                                p -= sizeof(alimb);
-                                continue;
-                        } else {
-                                mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
-                                mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
-                                                        + lzeros;
-                                *limb1 = *limb2;
-                                p -= lzeros;
-                                y = lzeros;
-                        }
+                        mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
+                        mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
+                                + lzeros;
+                        *limb1 = *limb2;
+                        p -= lzeros;
+                        y = lzeros;
                         lzeros -= sizeof(alimb);
                 }