Fix: jhash.h: remove out-of-bound reads

jhash.h implements "special" code for valgrind because it reads memory
out-of-bound (and then applies a mask) when reading strings.

Considering that lttng-ust does not use jhash.h in a fast-path, remove
this "optimization" and use the verifiable VALGRIND code instead. This
fixes an ASan splat.

Fixes: #1238
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/liblttng-ust/jhash.h b/liblttng-ust/jhash.h
index fe785bd5a9560572c0442eb0bee2ae6b8100b580..fef55a1f9599c5938348a09ab2f43f8b2d4d7ccf 100644 (file)
--- a/liblttng-ust/jhash.h
+++ b/liblttng-ust/jhash.h
@@ -109,33 +109,13 @@ uint32_t hashlittle(const void *key, size_t length, uint32_t initval)
 
                /*----------------------------- handle the last (probably partial) block */
                /*
-                * "k[2]&0xffffff" actually reads beyond the end of the string, but
-                * then masks off the part it's not allowed to read.    Because the
-                * string is aligned, the masked-off tail is in the same word as the
-                * rest of the string.  Every machine with memory protection I've seen
-                * does it on word boundaries, so is OK with this.      But VALGRIND will
-                * still catch it and complain. The masking trick does make the hash
-                * noticably faster for short strings (like English words).
+                * The original jhash.h reads beyond the end of string, and implements
+                * a special code path for VALGRIND. It seems to make ASan unhappy too
+                * though, so considering that hashing event names is not a fast-path
+                * in lttng-ust, remove the "fast" code entirely and use the slower
+                * but verifiable VALGRIND version of the code which does not issue
+                * out-of-bound reads.
                 */
-#ifndef VALGRIND
-
-               switch (length) {
-               case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
-               case 11: c+=k[2]&0xffffff; b+=k[1]; a+=k[0]; break;
-               case 10: c+=k[2]&0xffff; b+=k[1]; a+=k[0]; break;
-               case 9 : c+=k[2]&0xff; b+=k[1]; a+=k[0]; break;
-               case 8 : b+=k[1]; a+=k[0]; break;
-               case 7 : b+=k[1]&0xffffff; a+=k[0]; break;
-               case 6 : b+=k[1]&0xffff; a+=k[0]; break;
-               case 5 : b+=k[1]&0xff; a+=k[0]; break;
-               case 4 : a+=k[0]; break;
-               case 3 : a+=k[0]&0xffffff; break;
-               case 2 : a+=k[0]&0xffff; break;
-               case 1 : a+=k[0]&0xff; break;
-               case 0 : return c;              /* zero length strings require no mixing */
-               }
-
-#else /* make valgrind happy */
                {
                        const uint8_t *k8;
 
@@ -156,7 +136,6 @@ uint32_t hashlittle(const void *key, size_t length, uint32_t initval)
                        case 0 : return c;
                        }
                }
-#endif /* !valgrind */
 
        } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) {
                const uint16_t *k = (const uint16_t *) key;     /* read 16-bit chunks */