UBIFS: fix power cut emulation for mtdram

The power cut emulation did not work correctly because we corrupted more than
one max. I/O unit in the buffer and then wrote the entire buffer. This lead to
recovery errors because UBIFS complained about corrupted free space. And this
was easily reproducible on mtdram because max. write size is very small there
(64 bytes), and we could easily have a 1KiB buffer, corrupt 128 bytes there,
and then write the entire buffer.

The fix is to corrupt max. write size bytes at most, and write only up to the
last corrupted max. write size chunk, not the entire buffer.
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

fs/ubifs/debug.c

@@ -2646,18 +2646,16 @@ static int power_cut_emulated(struct ubifs_info *c, int lnum, int write)
 	return 1;
 }
 
-static void cut_data(const void *buf, unsigned int len)
+static int corrupt_data(const struct ubifs_info *c, const void *buf,
+			unsigned int len)
 {
 	unsigned int from, to, i, ffs = chance(1, 2);
 	unsigned char *p = (void *)buf;
 
 	from = random32() % (len + 1);
-	if (chance(1, 2))
-		to = random32() % (len - from + 1);
-	else
-		to = len;
+	/* Corruption may only span one max. write unit */
+	to = min(len, ALIGN(from, c->max_write_size));
 
-	if (from < to)
 	ubifs_warn("filled bytes %u-%u with %s", from, to - 1,
 		   ffs ? "0xFFs" : "random data");
 
@@ -2667,6 +2665,8 @@ static void cut_data(const void *buf, unsigned int len)
 	else
 		for (i = from; i < to; i++)
 			p[i] = random32() % 0x100;
+
+	return to;
 }
 
 int dbg_leb_write(struct ubifs_info *c, int lnum, const void *buf,
@@ -2679,7 +2679,9 @@ int dbg_leb_write(struct ubifs_info *c, int lnum, const void *buf,
 
 	failing = power_cut_emulated(c, lnum, 1);
 	if (failing)
-		cut_data(buf, len);
+		len = corrupt_data(c, buf, len);
+	ubifs_warn("actually write %d bytes to LEB %d:%d (the buffer was corrupted)",
+		   len, lnum, offs);
 	err = ubi_leb_write(c->ubi, lnum, buf, offs, len);
 	if (err)
 		return err;