vfs: avoid "attempt to access beyond end of device" warnings

The block device access simplification that avoided accessing the (racy)
block size information (commit bbec0270bdd8: "blkdev_max_block: make
private to fs/buffer.c") no longer checks the maximum block size in the
block mapping path.

That was _almost_ as simple as just removing the code entirely, because
the readers and writers all check the size of the device anyway, so
under normal circumstances it "just worked".

However, the block size may be such that the end of the device may
straddle one single buffer_head.  At which point we may still want to
access the end of the device, but the buffer we use to access it
partially extends past the end.

The 'bd_set_size()' function intentionally sets the block size to avoid
this, but mounting the device - or setting the block size by hand to
some other value - can modify that block size.

So instead, teach 'submit_bh()' about the special case of the buffer
head straddling the end of the device, and turning such an access into a
smaller IO access, avoiding the problem.

This, btw, also means that unlike before, we can now access the whole
device regardless of device block size setting.  So now, even if the
device size is only 512-byte aligned, we can read and write even the
last sector even when having a much bigger block size for accessing the
rest of the device.

So with this, we could now get rid of the 'bd_set_size()' block size
code entirely - resulting in faster IO for the common case - but that
would be a separate patch.

Reported-and-tested-by: Romain Francoise <romain@orebokech.com>
Reporeted-and-tested-by: Meelis Roos <mroos@linux.ee>
Reported-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -2893,6 +2893,55 @@ static void end_bio_bh_io_sync(struct bio *bio, int err)
 	bio_put(bio);
 }
 
+/*
+ * This allows us to do IO even on the odd last sectors
+ * of a device, even if the bh block size is some multiple
+ * of the physical sector size.
+ *
+ * We'll just truncate the bio to the size of the device,
+ * and clear the end of the buffer head manually.
+ *
+ * Truly out-of-range accesses will turn into actual IO
+ * errors, this only handles the "we need to be able to
+ * do IO at the final sector" case.
+ */
+static void guard_bh_eod(int rw, struct bio *bio, struct buffer_head *bh)
+{
+	sector_t maxsector;
+	unsigned bytes;
+
+	maxsector = i_size_read(bio->bi_bdev->bd_inode) >> 9;
+	if (!maxsector)
+		return;
+
+	/*
+	 * If the *whole* IO is past the end of the device,
+	 * let it through, and the IO layer will turn it into
+	 * an EIO.
+	 */
+	if (unlikely(bio->bi_sector >= maxsector))
+		return;
+
+	maxsector -= bio->bi_sector;
+	bytes = bio->bi_size;
+	if (likely((bytes >> 9) <= maxsector))
+		return;
+
+	/* Uhhuh. We've got a bh that straddles the device size! */
+	bytes = maxsector << 9;
+
+	/* Truncate the bio.. */
+	bio->bi_size = bytes;
+	bio->bi_io_vec[0].bv_len = bytes;
+
+	/* ..and clear the end of the buffer for reads */
+	if (rw & READ) {
+		void *kaddr = kmap_atomic(bh->b_page);
+		memset(kaddr + bh_offset(bh) + bytes, 0, bh->b_size - bytes);
+		kunmap_atomic(kaddr);
+	}
+}
+
 int submit_bh(int rw, struct buffer_head * bh)
 {
 	struct bio *bio;
@@ -2929,6 +2978,9 @@ int submit_bh(int rw, struct buffer_head * bh)
 	bio->bi_end_io = end_bio_bh_io_sync;
 	bio->bi_private = bh;
 
+	/* Take care of bh's that straddle the end of the device */
+	guard_bh_eod(rw, bio, bh);
+
 	bio_get(bio);
 	submit_bio(rw, bio);