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 <brauner@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240806230904.71194-3-song@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

fs/bpf_fs_kfuncs.c

@@ -94,26 +94,29 @@ __bpf_kfunc int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz)
 }
 
 /**
- * bpf_get_file_xattr - get xattr of a file
- * @file: file to get xattr from
+ * 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 *file* and store the output in *value_ptr*.
+ * 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_file_xattr(struct file *file, const char *name__str,
-				   struct bpf_dynptr *value_p)
+__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 dentry *dentry;
+	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;
 
@@ -122,11 +125,31 @@ __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
 	if (!value)
 		return -EINVAL;
 
-	dentry = file_dentry(file);
-	ret = inode_permission(&nop_mnt_idmap, dentry->d_inode, MAY_READ);
+	ret = inode_permission(&nop_mnt_idmap, inode, MAY_READ);
 	if (ret)
 		return ret;
-	return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len);
+	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
+ * @value_p: output buffer of the xattr value
+ *
+ * Get xattr *name__str* of *file* 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_file_xattr(struct file *file, const char *name__str,
+				   struct bpf_dynptr *value_p)
+{
+	struct dentry *dentry;
+
+	dentry = file_dentry(file);
+	return bpf_get_dentry_xattr(dentry, name__str, value_p);
 }
 
 __bpf_kfunc_end_defs();
@@ -136,6 +159,7 @@ BTF_ID_FLAGS(func, bpf_get_task_exe_file,
 	     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)