bpf: Add kfunc bpf_get_dentry_xattr() to read xattr from dentry

This kfunc can be used in LSM hooks with dentry, such as:

security_inode_listxattr
security_inode_permission

and many more.

Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

fs/bpf_fs_kfuncs.c

···
 }
 
 /**
+ * bpf_get_dentry_xattr - get xattr of a dentry
+ * @dentry: dentry to get xattr from
+ * @name__str: name of the xattr
+ * @value_p: output buffer of the xattr value
+ *
+ * Get xattr *name__str* of *dentry* and store the output in *value_ptr*.
+ *
+ * For security reasons, only *name__str* with prefix "user." is allowed.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__str,
+				     struct bpf_dynptr *value_p)
+{
+	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
+	struct inode *inode = d_inode(dentry);
+	u32 value_len;
+	void *value;
+	int ret;
+
+	if (WARN_ON(!inode))
+		return -EINVAL;
+
+	if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
+		return -EPERM;
+
+	value_len = __bpf_dynptr_size(value_ptr);
+	value = __bpf_dynptr_data_rw(value_ptr, value_len);
+	if (!value)
+		return -EINVAL;
+
+	ret = inode_permission(&nop_mnt_idmap, inode, MAY_READ);
+	if (ret)
+		return ret;
+	return __vfs_getxattr(dentry, inode, name__str, value, value_len);
+}
+
+/**
  * bpf_get_file_xattr - get xattr of a file
  * @file: file to get xattr from
  * @name__str: name of the xattr
···
 __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
 				   struct bpf_dynptr *value_p)
 {
-	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
 	struct dentry *dentry;
-	u32 value_len;
-	void *value;
-	int ret;
-
-	if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
-		return -EPERM;
-
-	value_len = __bpf_dynptr_size(value_ptr);
-	value = __bpf_dynptr_data_rw(value_ptr, value_len);
-	if (!value)
-		return -EINVAL;
 
 	dentry = file_dentry(file);
-	ret = inode_permission(&nop_mnt_idmap, dentry->d_inode, MAY_READ);
-	if (ret)
-		return ret;
-	return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len);
+	return bpf_get_dentry_xattr(dentry, name__str, value_p);
 }
 
 __bpf_kfunc_end_defs();
···
 	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)