btrfs: send: handle unexpected data in header buffer in begin_cmd()

BugLink: https://bugs.launchpad.net/bugs/2084005

[ Upstream commit e80e3f732cf53c64b0d811e1581470d67f6c3228 ]

Change BUG_ON to a proper error handling in the unlikely case of seeing
data when the command is started. This is supposed to be reset when the
command is finished (send_cmd, send_encoded_extent).

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Author: kdave

fs/btrfs/send.c

@@ -777,7 +777,12 @@ static int begin_cmd(struct send_ctx *sctx, int cmd)
 	if (WARN_ON(!sctx->send_buf))
 		return -EINVAL;
 
-	BUG_ON(sctx->send_size);
+	if (unlikely(sctx->send_size != 0)) {
+		btrfs_err(sctx->send_root->fs_info,
+			  "send: command header buffer not empty cmd %d offset %llu",
+			  cmd, sctx->send_off);
+		return -EINVAL;
+	}
 
 	sctx->send_size += sizeof(*hdr);
 	hdr = (struct btrfs_cmd_header *)sctx->send_buf;