commit d5572fade85243514566f02a1d82e15defdf3191
author Jens Wiklander <jens.wiklander@linaro.org> Mon May 27 16:42:58 2019 +0200
committer Joakim Bech <joakim.bech@linaro.org> Tue May 28 12:58:23 2019 +0200
tree 73c81dbd65cebf949a5be00bc14d406489e93fe1
parent 68df6eb0f2568e39175fc3a3c7ccbbf00e742b31

libmbedtls: mbedtls_mpi_exp_mod(): optimize mempool usage

With W allocated in the mempool instead of the stack it is more important
to utilize the mempool in a stack like way.

With this patch allocation and initialization of W is moved to a point
where all following mempool allocations are free before the function
returns.

This reduces maximum memory consumption of mempool in regression case
8101 for an AArch64 TA in from 17280 to 7640 bytes. Figures for an
AArch32 TA are 12040 to 5288 bytes.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

lib/libmbedtls/mbedtls/library/bignum.c

diff --git a/lib/libmbedtls/mbedtls/library/bignum.c b/lib/libmbedtls/mbedtls/library/bignum.c
index 4c1840a..e947a1c 100644
--- a/lib/libmbedtls/mbedtls/library/bignum.c
+++ b/lib/libmbedtls/mbedtls/library/bignum.c
@@ -1789,7 +1789,7 @@
     size_t bufsize, nbits;
     mbedtls_mpi_uint ei, mm, state;
     mbedtls_mpi RR, T, Apos;
-    mbedtls_mpi *W;
+    mbedtls_mpi *W = NULL;
     const size_t array_size_W = 2 << MBEDTLS_MPI_WINDOW_SIZE;
     int neg;
 
@@ -1804,19 +1804,12 @@
     if( mbedtls_mpi_cmp_int( E, 0 ) < 0 )
         return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 
-    W = mempool_alloc( mbedtls_mpi_mempool,
-                       sizeof( mbedtls_mpi ) * array_size_W );
-    if( W == NULL )
-        return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
-
     /*
      * Init temps and window size
      */
     mbedtls_mpi_montg_init( &mm, N );
     mbedtls_mpi_init_mempool( &RR ); mbedtls_mpi_init_mempool( &T );
     mbedtls_mpi_init_mempool( &Apos );
-    for( i = 0; i < array_size_W; i++ )
-        mbedtls_mpi_init_mempool( W + i );
 
     i = mbedtls_mpi_bitlen( E );
 
@@ -1828,7 +1821,7 @@
 
     j = N->n + 1;
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j ) );
-    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1],  j ) );
+
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T, j * 2 ) );
 
     /*
@@ -1857,6 +1850,16 @@
     else
         memcpy( &RR, _RR, sizeof( mbedtls_mpi ) );
 
+    W = mempool_alloc( mbedtls_mpi_mempool,
+                       sizeof( mbedtls_mpi ) * array_size_W );
+    if( W == NULL ) {
+        ret = MBEDTLS_ERR_MPI_ALLOC_FAILED;
+        goto cleanup;
+    }
+    for( i = 0; i < array_size_W; i++ )
+        mbedtls_mpi_init_mempool( W + i );
+    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1],  j ) );
+
     /*
      * W[1] = A * R^2 * R^-1 mod N = A * R mod N
      */
@@ -1988,8 +1991,9 @@
 
 cleanup:
 
-    for( i = 0; i < array_size_W; i++ )
-        mbedtls_mpi_free( W + i );
+    if( W )
+        for( i = 0; i < array_size_W; i++ )
+            mbedtls_mpi_free( W + i );
     mempool_free( mbedtls_mpi_mempool , W );
 
     mbedtls_mpi_free( &T ); mbedtls_mpi_free( &Apos );