btrfs-progs: receive: remove workaround for setting capabilities

We had a few bugs on the kernel side of send/receive where capabilities
ended up being lost after receiving a send stream. They all stem from the
fact that the kernel used to send all xattrs before issuing the chown
command, and the later clears any existing capabilities in a file or
directory.

Initially a workaround was added to btrfs-progs' receive command, in commit
123a2a0 ("btrfs-progs: receive: restore capabilities after chown"),
and that fixed some instances of the problem. More recently, other instances
of the problem were found, a proper fix for the kernel was made, which fixes
the root problem by making send always emit the setxattr command for setting
capabilities after issuing a chown command. This was done in kernel commit
89efda52e6b693 ("btrfs: send: emit file capabilities after chown"), which
landed in kernel 5.8.

However, the workaround on the receive command now causes us to incorrectly
set a capability on a file that should not have it, because it assumes all
setxattr commands for a file always comes before a chown.

Example reproducer:

  $ cat send-caps.sh
  #!/bin/bash

  DEV1=/dev/sdh
  DEV2=/dev/sdi

  MNT1=/mnt/sdh
  MNT2=/mnt/sdi

  mkfs.btrfs -f $DEV1 > /dev/null
  mkfs.btrfs -f $DEV2 > /dev/null

  mount $DEV1 $MNT1
  mount $DEV2 $MNT2

  touch $MNT1/foo
  touch $MNT1/bar
  setcap cap_net_raw=p $MNT1/foo

  btrfs subvolume snapshot -r $MNT1 $MNT1/snap1

  btrfs send $MNT1/snap1 | btrfs receive $MNT2

  echo
  echo "capabilities on destination filesystem:"
  echo
  getcap $MNT2/snap1/foo
  getcap $MNT2/snap1/bar

  umount $MNT1
  umount $MNT2

When running the test script, we can see that both files foo and bar get
the capability set, when only file foo should have it:

  $ ./send-caps.sh
  Create a readonly snapshot of '/mnt/sdh' in '/mnt/sdh/snap1'
  At subvol /mnt/sdh/snap1
  At subvol snap1

  capabilities on destination filesystem:

  /mnt/sdi/snap1/foo cap_net_raw=p
  /mnt/sdi/snap1/bar cap_net_raw=p

Since the kernel fix was backported to all currently supported stable
releases (5.10.x, 5.4.x, 4.19.x, 4.14.x, 4.9.x and 4.4.x), remove the
workaround from receive. Having such a workaround relying on the order
of commands in a send stream is always troublesome and doomed to break
one day.

A test case for fstests will come soon.

Issue: #85
Issue: #202
Issue: #292
Reported-by: Richard Brown <rbrown@suse.de>
Reviewed-by: Su Yue <l@damenly.su>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

cmds/receive.c

@@ -77,14 +77,6 @@ struct btrfs_receive
 	struct subvol_uuid_search sus;
 
 	int honor_end_cmd;
-
-	/*
-	 * Buffer to store capabilities from security.capabilities xattr,
-	 * usually 20 bytes, but make same room for potentially larger
-	 * encodings. Must be set only once per file, denoted by length > 0.
-	 */
-	char cached_capabilities[64];
-	int cached_capabilities_len;
 };
 
 static int finish_subvol(struct btrfs_receive *rctx)
@@ -825,22 +817,6 @@ static int process_set_xattr(const char *path, const char *name,
 		goto out;
 	}
 
-	if (strcmp("security.capability", name) == 0) {
-		if (bconf.verbose >= 4)
-			fprintf(stderr, "set_xattr: cache capabilities\n");
-		if (rctx->cached_capabilities_len)
-			warning("capabilities set multiple times per file: %s",
-				full_path);
-		if (len > sizeof(rctx->cached_capabilities)) {
-			error("capabilities encoded to %d bytes, buffer too small",
-				len);
-			ret = -E2BIG;
-			goto out;
-		}
-		rctx->cached_capabilities_len = len;
-		memcpy(rctx->cached_capabilities, data, len);
-	}
-
 	if (bconf.verbose >= 3) {
 		fprintf(stderr, "set_xattr %s - name=%s data_len=%d "
 				"data=%.*s\n", path, name, len,
@@ -961,23 +937,6 @@ static int process_chown(const char *path, u64 uid, u64 gid, void *user)
 		error("chown %s failed: %m", path);
 		goto out;
 	}
-
-	if (rctx->cached_capabilities_len) {
-		if (bconf.verbose >= 3)
-			fprintf(stderr, "chown: restore capabilities\n");
-		ret = lsetxattr(full_path, "security.capability",
-				rctx->cached_capabilities,
-				rctx->cached_capabilities_len, 0);
-		memset(rctx->cached_capabilities, 0,
-				sizeof(rctx->cached_capabilities));
-		rctx->cached_capabilities_len = 0;
-		if (ret < 0) {
-			ret = -errno;
-			error("restoring capabilities %s: %m", path);
-			goto out;
-		}
-	}
-
 out:
 	return ret;
 }
@@ -1155,14 +1114,6 @@ static int do_receive(struct btrfs_receive *rctx, const char *tomnt,
 		goto out;
 
 	while (!end) {
-		if (rctx->cached_capabilities_len) {
-			if (bconf.verbose >= 4)
-				fprintf(stderr, "clear cached capabilities\n");
-			memset(rctx->cached_capabilities, 0,
-					sizeof(rctx->cached_capabilities));
-			rctx->cached_capabilities_len = 0;
-		}
-
 		ret = btrfs_read_and_process_send_stream(r_fd, &send_ops,
 							 rctx,
 							 rctx->honor_end_cmd,