lib/talloc: Provide multiple-loading detection for libtalloc via rand()

The use of rand() is strongly discrouanged, but here is it ideal, as we just want to select a different
set of random bytes if we are called again within the same process.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Adrian Cochrane <adrianc@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>

diff --git a/lib/talloc/talloc.c b/lib/talloc/talloc.c
index 37a652259034f41e340342002bbe486365de90a8..b35e1c206202c852afe8b266ff44d244b4b5010e 100644 (file)
--- a/lib/talloc/talloc.c
+++ b/lib/talloc/talloc.c
@@ -320,7 +320,22 @@ void talloc_lib_init(void)
         */
        p = (uint8_t *) getauxval(AT_RANDOM);
        if (p) {
-               memcpy(&random_value, p, sizeof(random_value));
+               /*
+                * We get 16 bytes from getauxval.  By calling rand(),
+                * a totally insecure PRNG, but one that will
+                * deterministically have a different value when called
+                * twice, we ensure that if two talloc-like libraries
+                * are somehow loaded in the same address space, that
+                * because we choose different bytes, we will keep the
+                * protection against collision of multiple talloc
+                * libs.
+                *
+                * This protection is important because the effects of
+                * passing a talloc pointer from one to the other may
+                * be very hard to determine.
+                */
+               int offset = rand() % (16 - sizeof(random_value));
+               memcpy(&random_value, p + offset, sizeof(random_value));
        } else
 #endif
        {