lib/mpi: only require buffers as big as needed for the integer

Since mpi_write_to_sgl and mpi_read_buffer explicitly left-align the
integers being written it makes no sense to require a buffer big enough for
the number + the leading zero bytes which are not written.  The error
returned also doesn't convey any information.  So instead require only the
size needed and return -EOVERFLOW to signal when buffer too short.

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 17 insertions, 4 deletions

diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 3db76b8c1115..ec533a6c77b5 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -135,7 +135,9 @@ EXPORT_SYMBOL_GPL(mpi_read_from_buffer);
  * @buf:        bufer to which the output will be written to. Needs to be at
  *              leaset mpi_get_size(a) long.
  * @buf_len:    size of the buf.
- * @nbytes:     receives the actual length of the data written.
+ * @nbytes:     receives the actual length of the data written on success and
+ *              the data to-be-written on -EOVERFLOW in case buf_len was too
+ *              small.
  * @sign:       if not NULL, it will be set to the sign of a.
  *
  * Return:      0 on success or error code in case of error
@@ -148,7 +150,7 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
         unsigned int n = mpi_get_size(a);
         int i, lzeros = 0;
 
-        if (buf_len < n || !buf || !nbytes)
+        if (!buf || !nbytes)
                 return -EINVAL;
 
         if (sign)
@@ -163,6 +165,11 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
                         break;
         }
 
+        if (buf_len < n - lzeros) {
+                *nbytes = n - lzeros;
+                return -EOVERFLOW;
+        }
+
         p = buf;
         *nbytes = n - lzeros;
 
@@ -332,7 +339,8 @@ EXPORT_SYMBOL_GPL(mpi_set_buffer);
  * @nbytes:     in/out param - it has the be set to the maximum number of
  *              bytes that can be written to sgl. This has to be at least
  *              the size of the integer a. On return it receives the actual
- *              length of the data written.
+ *              length of the data written on success or the data that would
+ *              be written if buffer was too small.
  * @sign:       if not NULL, it will be set to the sign of a.
  *
  * Return:      0 on success or error code in case of error
@@ -345,7 +353,7 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
         unsigned int n = mpi_get_size(a);
         int i, x, y = 0, lzeros = 0, buf_len;
 
-        if (!nbytes || *nbytes < n)
+        if (!nbytes)
                 return -EINVAL;
 
         if (sign)
@@ -360,6 +368,11 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
                         break;
         }
 
+        if (*nbytes < n - lzeros) {
+                *nbytes = n - lzeros;
+                return -EOVERFLOW;
+        }
+
         *nbytes = n - lzeros;
         buf_len = sgl->length;
         p2 = sg_virt(sgl);