crc32: deal with unaligned buffer, tests, fixes #3317

fixing only the (generic) slice-by-8 crc32 implementation,
it is assumed that CPUs supporting CLMUL can also efficiently
and correctly deal with unaligned accesses.

slice-by-8 is used e.g. on ARM cpus and they might not (efficiently)
support unaligned memory access, leading to bus errors or low
performance.

src/borg/algorithms/crc32_slice_by_8.c

@@ -330,12 +330,28 @@ const uint32_t Crc32Lookup[8][256] =
 uint32_t crc32_slice_by_8(const void* data, size_t length, uint32_t previousCrc32)
 {
   uint32_t crc = ~previousCrc32; // same as previousCrc32 ^ 0xFFFFFFFF
-  const uint32_t* current = (const uint32_t*) data;
+
+  const uint32_t* current;
+  const uint8_t* currentChar;
 
   // enabling optimization (at least -O2) automatically unrolls the inner for-loop
   const size_t Unroll = 4;
   const size_t BytesAtOnce = 8 * Unroll;
-  const uint8_t* currentChar;
+
+  currentChar = (const uint8_t*) data;
+
+  // wanted: 32 bit / 4 Byte alignment, compute leading, unaligned bytes length
+  uintptr_t unaligned_length = (4 - (((uintptr_t) currentChar) & 3)) & 3;
+  // process unaligned bytes, if any (standard algorithm)
+  while ((length != 0) && (unaligned_length != 0))
+  {
+    crc = (crc >> 8) ^ Crc32Lookup[0][(crc & 0xFF) ^ *currentChar++];
+    length--;
+    unaligned_length--;
+  }
+
+  // pointer points to 32bit aligned address now
+  current = (const uint32_t*) currentChar;
 
   // process 4x eight bytes at once (Slicing-by-8)
   while (length >= BytesAtOnce)

src/borg/testsuite/checksums.py

@@ -14,12 +14,17 @@ if checksums.have_clmul:
 
 @pytest.mark.parametrize('implementation', crc32_implementations)
 def test_crc32(implementation):
-    # This includes many critical values, like zero length, 3/4/5, 6/7/8 and so on which are near and on
-    # alignment boundaries. This is of course just a sanity check ie. "did it compile all right?".
-    data = os.urandom(256)
+    # This includes many critical values, like misc. length and misc. aligned start addresses.
+    data = os.urandom(300)
+    mv = memoryview(data)
     initial_crc = 0x12345678
-    for i in range(0, 256):
-        d = data[:i]
+    for start in range(0, 4):  # 4B / int32 alignment, head processing
+        for length in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
+                       31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
+                       63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,
+                       127, 128, 129, 130, 131, 132, 133, 134, 135,
+                       255, 256, 257, ]:
+            d = mv[start:start+length]
             assert zlib.crc32(d, initial_crc) == implementation(d, initial_crc)