Refactor mpi_write_hlp to not be recursive
Refactor `mpi_write_hlp()` to not be recursive, to fix stack overflows.
Iterate over the `mbedtls_mpi` division of the radix requested,
until it is zero. Each iteration, put the residue in the next LSB
of the output buffer. Fixes
@@ -499,26 +499,38 @@ cleanup:
}
/*
- * Helper to write the digits high-order first
+ * Helper to write the digits high-order first.
*/
-static int mpi_write_hlp( mbedtls_mpi *X, int radix, char **p )
+static int mpi_write_hlp( mbedtls_mpi *X, int radix,
+ char **p, const size_t buflen )
{
int ret;
mbedtls_mpi_uint r;
+ size_t length = 0;
+ char *p_end = *p + buflen;
- if( radix < 2 || radix > 16 )
- return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+ do
+ {
+ if( length >= buflen )
+ {
+ return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+ }
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_int( &r, X, radix ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_div_int( X, NULL, X, radix ) );
+ /*
+ * Write the residue in the current position, as an ASCII character.
+ */
+ if( r < 0xA )
+ *(--p_end) = (char)( '0' + r );
+ else
+ *(--p_end) = (char)( 'A' + ( r - 0xA ) );
- if( mbedtls_mpi_cmp_int( X, 0 ) != 0 )
- MBEDTLS_MPI_CHK( mpi_write_hlp( X, radix, p ) );
+ length++;
+ } while( mbedtls_mpi_cmp_int( X, 0 ) != 0 );
- if( r < 10 )
- *(*p)++ = (char)( r + 0x30 );
- else
- *(*p)++ = (char)( r + 0x37 );
+ memmove( *p, p_end, length );
+ *p += length;
cleanup:
@@ -588,7 +600,7 @@ int mbedtls_mpi_write_string( const mbedtls_mpi *X, int radix,
if( T.s == -1 )
T.s = 1;
- MBEDTLS_MPI_CHK( mpi_write_hlp( &T, radix, &p ) );
+ MBEDTLS_MPI_CHK( mpi_write_hlp( &T, radix, &p, buflen ) );
}
*p++ = '\0';
