bpf: Re-support uid and gid when mounting bpffs

For a clean, conflict-free revert of the token-related patches in commit
d17aff80 ("Revert BPF token-related functionality"), the bpf fs commit
750e7857 ("bpf: Support uid and gid when mounting bpffs") was undone
temporarily as well.

This patch manually re-adds the functionality from the original one back
in 750e7857, no other functional changes intended.

Testing:

  # mount -t bpf -o uid=65534,gid=65534 bpffs ./foo
  # ls -la . | grep foo
  drwxrwxrwt   2 nobody nogroup          0 Dec 20 13:16 foo
  # mount -t bpf
  bpffs on /root/foo type bpf (rw,relatime,uid=65534,gid=65534)

Also, passing invalid arguments for uid/gid are properly rejected as expected.

Fixes: d17aff80 ("Revert BPF token-related functionality")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Cc: Jie Jiang <jiejiang@chromium.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/bpf/20231220133805.20953-1-daniel@iogearbox.net

kernel/bpf/inode.c

@@ -599,8 +599,15 @@ EXPORT_SYMBOL(bpf_prog_get_type_path);
  */
 static int bpf_show_options(struct seq_file *m, struct dentry *root)
 {
-	umode_t mode = d_inode(root)->i_mode & S_IALLUGO & ~S_ISVTX;
-
+	struct inode *inode = d_inode(root);
+	umode_t mode = inode->i_mode & S_IALLUGO & ~S_ISVTX;
+
+	if (!uid_eq(inode->i_uid, GLOBAL_ROOT_UID))
+		seq_printf(m, ",uid=%u",
+			   from_kuid_munged(&init_user_ns, inode->i_uid));
+	if (!gid_eq(inode->i_gid, GLOBAL_ROOT_GID))
+		seq_printf(m, ",gid=%u",
+			   from_kgid_munged(&init_user_ns, inode->i_gid));
 	if (mode != S_IRWXUGO)
 		seq_printf(m, ",mode=%o", mode);
 	return 0;
@@ -625,15 +632,21 @@ static const struct super_operations bpf_super_ops = {
 };
 
 enum {
+	OPT_UID,
+	OPT_GID,
 	OPT_MODE,
 };
 
 static const struct fs_parameter_spec bpf_fs_parameters[] = {
+	fsparam_u32	("uid",				OPT_UID),
+	fsparam_u32	("gid",				OPT_GID),
 	fsparam_u32oct	("mode",			OPT_MODE),
 	{}
 };
 
 struct bpf_mount_opts {
+	kuid_t uid;
+	kgid_t gid;
 	umode_t mode;
 };
 
@@ -641,6 +654,8 @@ static int bpf_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
 	struct bpf_mount_opts *opts = fc->fs_private;
 	struct fs_parse_result result;
+	kuid_t uid;
+	kgid_t gid;
 	int opt;
 
 	opt = fs_parse(fc, bpf_fs_parameters, param, &result);
@@ -662,12 +677,42 @@ static int bpf_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	}
 
 	switch (opt) {
+	case OPT_UID:
+		uid = make_kuid(current_user_ns(), result.uint_32);
+		if (!uid_valid(uid))
+			goto bad_value;
+
+		/*
+		 * The requested uid must be representable in the
+		 * filesystem's idmapping.
+		 */
+		if (!kuid_has_mapping(fc->user_ns, uid))
+			goto bad_value;
+
+		opts->uid = uid;
+		break;
+	case OPT_GID:
+		gid = make_kgid(current_user_ns(), result.uint_32);
+		if (!gid_valid(gid))
+			goto bad_value;
+
+		/*
+		 * The requested gid must be representable in the
+		 * filesystem's idmapping.
+		 */
+		if (!kgid_has_mapping(fc->user_ns, gid))
+			goto bad_value;
+
+		opts->gid = gid;
+		break;
 	case OPT_MODE:
 		opts->mode = result.uint_32 & S_IALLUGO;
 		break;
 	}
 
 	return 0;
+bad_value:
+	return invalfc(fc, "Bad value for '%s'", param->key);
 }
 
 struct bpf_preload_ops *bpf_preload_ops;
@@ -750,6 +795,8 @@ static int bpf_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_op = &bpf_super_ops;
 
 	inode = sb->s_root->d_inode;
+	inode->i_uid = opts->uid;
+	inode->i_gid = opts->gid;
 	inode->i_op = &bpf_dir_iops;
 	inode->i_mode &= ~S_IALLUGO;
 	populate_bpffs(sb->s_root);
@@ -785,6 +832,8 @@ static int bpf_init_fs_context(struct fs_context *fc)
 		return -ENOMEM;
 
 	opts->mode = S_IRWXUGO;
+	opts->uid = current_fsuid();
+	opts->gid = current_fsgid();
 
 	fc->fs_private = opts;
 	fc->ops = &bpf_context_ops;