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Prevent clever optimization to prematurely quit loop in safe memcmp

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The previous version of `ssl_safer_memcmp` did not qualify the
pointers to the arrays to be compared as volatile, theoretically
opening the possibility for the compiler to notice that the loop
operation `diff |= A[i] ^ B[i]` is pointless if `diff = -1`. This
commit changes this. It also declares the stack variable `diff` as
volatile, to force read and write in every loop; omitting that, the
compiler would still be allowed to get away with reading `A[i]` and
`B[i]` but not doing the XOR and not updating `diff`.
This commit is contained in:
Hanno Becker 2017-06-26 13:42:44 +01:00
parent 8a2855ee3c
commit d1cf6d68cc
1 changed files with 3 additions and 3 deletions
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6
include/polarssl/ssl.h
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@ -2053,9 +2053,9 @@ int ssl_check_cert_usage( const x509_crt *cert,
static inline int safer_memcmp( const void *a, const void *b, size_t n )
{
size_t i;
const unsigned char *A = (const unsigned char *) a;
const unsigned char *B = (const unsigned char *) b;
unsigned char diff = 0;
volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b;
volatile unsigned char diff = 0;
for( i = 0; i < n; i++ )
diff |= A[i] ^ B[i];