[crypto] Remove harmless but technically undefined right shift

Automated reporting tools tend to pick up the right-shift by an
attacker-controllable shift amount as a potential defect, since a
right-shift by greater than the word size is technically undefined
behaviour.

The result of an undefined shift is already ignored by the following
range check on the shift amount, and the separate "unused_mask"
variable exists only to make the code clearer to read.  Sacrifice this
very small improvement in legibility for the sake of reducing future
reporting noise.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/crypto/asn1.c b/src/crypto/asn1.c
index f36962c0936f3b7e7566c17765073af93d62b9c8..9a0e25649b20393703c3df698853319c258523bd 100644 (file)
--- a/src/crypto/asn1.c
+++ b/src/crypto/asn1.c
@@ -385,7 +385,6 @@ int asn1_enter_bits ( struct asn1_cursor *cursor, unsigned int *unused ) {
        } __attribute__ (( packed )) *bit_string;
        const uint8_t *last;
        unsigned int unused_bits;
-       uint8_t unused_mask;
        int rc;
 
        /* Enter bit string */
@@ -405,11 +404,10 @@ int asn1_enter_bits ( struct asn1_cursor *cursor, unsigned int *unused ) {
        unused_bits = bit_string->unused;
 
        /* Check validity of unused bits */
-       unused_mask = ( 0xff >> ( 8 - unused_bits ) );
        last = ( cursor->data + cursor->len - 1 );
        if ( ( unused_bits >= 8 ) ||
             ( ( unused_bits > 0 ) && ( cursor->len == 0 ) ) ||
-            ( ( *last & unused_mask ) != 0 ) ) {
+            ( ( *last & ( 0xffU >> ( 8 - unused_bits ) ) ) != 0 ) ) {
                DBGC ( cursor, "ASN1 %p invalid bit string:\n", cursor );
                DBGC_HDA ( cursor, 0, cursor->data, cursor->len );
                asn1_invalidate_cursor ( cursor );