linux-kernel - [PATCH RFC 5/6] exportfs: add permission method

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Message-ID: <20241129-work-pidfs-file_handle-v1-5-87d803a42495@kernel.org>
Date: Fri, 29 Nov 2024 14:38:04 +0100
From: Christian Brauner <brauner@...nel.org>
To: Erin Shepherd <erin.shepherd@....eu>,
	Amir Goldstein <amir73il@...il.com>,
	Jeff Layton <jlayton@...nel.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>,
	Chuck Lever <chuck.lever@...cle.com>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org
Subject: [PATCH RFC 5/6] exportfs: add permission method

This allows filesystems such as pidfs to provide their custom permission
checks.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 fs/fhandle.c             | 21 +++++++--------------
 include/linux/exportfs.h | 17 ++++++++++++++++-
 2 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 031ad5592a0beabcc299436f037ad5fe626332e6..23491094032ec037066a271873ea8ff794616bee 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -187,17 +187,6 @@ static int get_path_from_fd(int fd, struct path *root)
 	return 0;
 }
 
-enum handle_to_path_flags {
-	HANDLE_CHECK_PERMS   = (1 << 0),
-	HANDLE_CHECK_SUBTREE = (1 << 1),
-};
-
-struct handle_to_path_ctx {
-	struct path root;
-	enum handle_to_path_flags flags;
-	unsigned int fh_flags;
-};
-
 static int vfs_dentry_acceptable(void *context, struct dentry *dentry)
 {
 	struct handle_to_path_ctx *ctx = context;
@@ -335,15 +324,19 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 	struct file_handle f_handle;
 	struct file_handle *handle = NULL;
 	struct handle_to_path_ctx ctx = {};
+	const struct export_operations *eops;
 
 	retval = get_path_from_fd(mountdirfd, &ctx.root);
 	if (retval)
 		goto out_err;
 
-	if (!may_decode_fh(&ctx, o_flags)) {
-		retval = -EPERM;
+	eops = ctx.root.mnt->mnt_sb->s_export_op;
+	if (eops && eops->permission)
+		retval = eops->permission(&ctx, o_flags);
+	else
+		retval = may_decode_fh(&ctx, o_flags);
+	if (retval)
 		goto out_path;
-	}
 
 	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle))) {
 		retval = -EFAULT;
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index c69b79b64466d5bc32ffe9b2796a388130fe72d8..a087606ace194ccc9d1250f990ce55627aaf8dc5 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -3,6 +3,7 @@
 #define LINUX_EXPORTFS_H 1
 
 #include <linux/types.h>
+#include <linux/path.h>
 
 struct dentry;
 struct iattr;
@@ -10,7 +11,6 @@ struct inode;
 struct iomap;
 struct super_block;
 struct vfsmount;
-struct path;
 
 /* limit the handle size to NFSv4 handle size now */
 #define MAX_HANDLE_SZ 128
@@ -157,6 +157,17 @@ struct fid {
 	};
 };
 
+enum handle_to_path_flags {
+	HANDLE_CHECK_PERMS   = (1 << 0),
+	HANDLE_CHECK_SUBTREE = (1 << 1),
+};
+
+struct handle_to_path_ctx {
+	struct path root;
+	enum handle_to_path_flags flags;
+	unsigned int fh_flags;
+};
+
 #define EXPORT_FH_CONNECTABLE	0x1 /* Encode file handle with parent */
 #define EXPORT_FH_FID		0x2 /* File handle may be non-decodeable */
 #define EXPORT_FH_DIR_ONLY	0x4 /* Only decode file handle for a directory */
@@ -226,6 +237,9 @@ struct fid {
  *    is also a directory.  In the event that it cannot be found, or storage
  *    space cannot be allocated, a %ERR_PTR should be returned.
  *
+ * permission:
+ *    Allow filesystems to specify a custom permission function.
+ *
  * open:
  *    Allow filesystems to specify a custom open function.
  *
@@ -255,6 +269,7 @@ struct export_operations {
 			  bool write, u32 *device_generation);
 	int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
 			     int nr_iomaps, struct iattr *iattr);
+	int (*permission)(struct handle_to_path_ctx *ctx, unsigned int oflags);
 	struct file * (*open)(struct path *path, unsigned int oflags);
 #define	EXPORT_OP_NOWCC			(0x1) /* don't collect v3 wcc data */
 #define	EXPORT_OP_NOSUBTREECHK		(0x2) /* no subtree checking */

-- 
2.45.2