asan: Fix up asan_redzone_buffer::emit_redzone_byte [PR105396]

On the following testcase, we have in main's frame 3 variables,
some red zone padding, 4 byte d, followed by 12 bytes of red zone padding, then
8 byte b followed by 24 bytes of red zone padding, then 40 bytes c followed
by some red zone padding.
The intended content of shadow memory for that is (note, each byte describes
8 bytes of memory):
f1 f1 f1 f1 04 f2 00 f2 f2 f2 00 00 00 00 00 f3 f3 f3 f3 f3
left red    d  mr b  middle r c              right red zone

f1 is left red zone magic
f2 is middle red zone magic
f3 is right red zone magic
00 when all 8 bytes are accessible
01-07 when only 1 to 7 bytes are accessible followed by inaccessible bytes

The -fdump-rtl-expand-details dump makes it clear that it misbehaves:
Flushing rzbuffer at offset -160 with: f1 f1 f1 f1
Flushing rzbuffer at offset -128 with: 04 f2 00 00
Flushing rzbuffer at offset -128 with: 00 00 00 f2
Flushing rzbuffer at offset -96 with: f2 f2 00 00
Flushing rzbuffer at offset -64 with: 00 00 00 f3
Flushing rzbuffer at offset -32 with: f3 f3 f3 f3
In the end we end up with
f1 f1 f1 f1 00 00 00 f2 f2 f2 00 00 00 00 00 f3 f3 f3 f3 f3
shadow bytes because at offset -128 there are 2 overlapping stores
as asan_redzone_buffer::emit_redzone_byte has flushed the temporary 4 byte
buffer in the middle.

The function is called with an offset and value.  If the passed offset is
consecutive with the prev_offset + buffer size (off == offset), then
we handle it correctly, similarly if the new offset is far enough from the
old one (we then flush whatever was in the buffer and if needed add up to 3
bytes of 00 before actually pushing value.

But what isn't handled correctly is when the offset isn't consecutive to
what has been added last time, but it is in the same 4 byte word of shadow
memory (32 bytes of actual memory), like the above case where
we have consecutive 04 f2 and then skip one shadow memory byte (aka 8 bytes
of real memory) and then want to emit f2.  Emitting that as a store
of little-endian 0x0000f204 followed by a store of 0xf2000000 to the same
address doesn't work, we want to emit 0xf200f204.

The following patch does that by pushing 1 or 2 00 bytes.
Additionally, as a small cleanup, instead of using
      m_shadow_bytes.safe_push (value);
      flush_if_full ();
in all of if, else if and else bodies it sinks those 2 stmts to the end
of function as all do the same thing.

2022-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/105396
	* asan.cc (asan_redzone_buffer::emit_redzone_byte): Handle the case
	where offset is bigger than off but smaller than m_prev_offset + 32
	bits by pushing one or more 0 bytes.  Sink the
	m_shadow_bytes.safe_push (value); flush_if_full (); statements from
	all cases to the end of the function.

	* gcc.dg/asan/pr105396.c: New test.

gcc/asan.cc

@@ -1497,10 +1497,14 @@ asan_redzone_buffer::emit_redzone_byte (HOST_WIDE_INT offset,
   HOST_WIDE_INT off
     = m_prev_offset + ASAN_SHADOW_GRANULARITY * m_shadow_bytes.length ();
   if (off == offset)
+    /* Consecutive shadow memory byte.  */;
+  else if (offset < m_prev_offset + (HOST_WIDE_INT) (ASAN_SHADOW_GRANULARITY
+						     * RZ_BUFFER_SIZE)
+	   && !m_shadow_bytes.is_empty ())
     {
-      /* Consecutive shadow memory byte.  */
-      m_shadow_bytes.safe_push (value);
-      flush_if_full ();
+      /* Shadow memory byte with a small gap.  */
+      for (; off < offset; off += ASAN_SHADOW_GRANULARITY)
+	m_shadow_bytes.safe_push (0);
     }
   else
     {
@@ -1521,9 +1525,9 @@ asan_redzone_buffer::emit_redzone_byte (HOST_WIDE_INT offset,
       m_shadow_mem = adjust_address (m_shadow_mem, VOIDmode,
 				     diff >> ASAN_SHADOW_SHIFT);
       m_prev_offset = offset;
-      m_shadow_bytes.safe_push (value);
-      flush_if_full ();
     }
+  m_shadow_bytes.safe_push (value);
+  flush_if_full ();
 }
 
 /* Emit RTX emission of the content of the buffer.  */

gcc/testsuite/gcc.dg/asan/pr105396.c

@@ -0,0 +1,19 @@
+/* PR sanitizer/105396 */
+/* { dg-do run } */
+/* { dg-skip-if "" { *-*-* } { "*" } { "-O0" } } */
+/* { dg-shouldfail "asan" } */
+
+int
+main ()
+{
+  int a;
+  int *b[1];
+  int c[10];
+  int d[1][1];
+  for (a = 0; a < 1; a++)
+    d[1][a] = 0;
+  return 0;
+}
+
+/* { dg-output "ERROR: AddressSanitizer: stack-buffer-overflow on address.*(\n|\r\n|\r)" } */
+/* { dg-output "WRITE of size.*" } */