fs: add file and path permissions helpers

Add two simple helpers to check permissions on a file and path
respectively and convert over some callers. It simplifies quite a few
codepaths and also reduces the churn in later patches quite a bit.
Christoph also correctly points out that this makes codepaths (e.g.
ioctls) way easier to follow that would otherwise have to do more
complex argument passing than necessary.

Link: https://lore.kernel.org/r/20210121131959.646623-4-christian.brauner@ubuntu.com
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/init.c

@@ -49,7 +49,7 @@ int __init init_chdir(const char *filename)
 	error = kern_path(filename, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path);
 	if (error)
 		return error;
-	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	error = path_permission(&path, MAY_EXEC | MAY_CHDIR);
 	if (!error)
 		set_fs_pwd(current->fs, &path);
 	path_put(&path);
@@ -64,7 +64,7 @@ int __init init_chroot(const char *filename)
 	error = kern_path(filename, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path);
 	if (error)
 		return error;
-	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	error = path_permission(&path, MAY_EXEC | MAY_CHDIR);
 	if (error)
 		goto dput_and_out;
 	error = -EPERM;
@@ -118,7 +118,7 @@ int __init init_eaccess(const char *filename)
 	error = kern_path(filename, LOOKUP_FOLLOW, &path);
 	if (error)
 		return error;
-	error = inode_permission(d_inode(path.dentry), MAY_ACCESS);
+	error = path_permission(&path, MAY_ACCESS);
 	path_put(&path);
 	return error;
 }

fs/notify/fanotify/fanotify_user.c

@@ -702,7 +702,7 @@ static int fanotify_find_path(int dfd, const char __user *filename,
 	}
 
 	/* you can only watch an inode if you have read permissions on it */
-	ret = inode_permission(path->dentry->d_inode, MAY_READ);
+	ret = path_permission(path, MAY_READ);
 	if (ret) {
 		path_put(path);
 		goto out;

fs/notify/inotify/inotify_user.c

@@ -352,7 +352,7 @@ static int inotify_find_inode(const char __user *dirname, struct path *path,
 	if (error)
 		return error;
 	/* you can only watch an inode if you have read permissions on it */
-	error = inode_permission(path->dentry->d_inode, MAY_READ);
+	error = path_permission(path, MAY_READ);
 	if (error) {
 		path_put(path);
 		return error;

fs/open.c

@@ -492,7 +492,7 @@ SYSCALL_DEFINE1(chdir, const char __user *, filename)
 	if (error)
 		goto out;
 
-	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	error = path_permission(&path, MAY_EXEC | MAY_CHDIR);
 	if (error)
 		goto dput_and_out;
 
@@ -521,7 +521,7 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 	if (!d_can_lookup(f.file->f_path.dentry))
 		goto out_putf;
 
-	error = inode_permission(file_inode(f.file), MAY_EXEC | MAY_CHDIR);
+	error = file_permission(f.file, MAY_EXEC | MAY_CHDIR);
 	if (!error)
 		set_fs_pwd(current->fs, &f.file->f_path);
 out_putf:
@@ -540,7 +540,7 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 	if (error)
 		goto out;
 
-	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	error = path_permission(&path, MAY_EXEC | MAY_CHDIR);
 	if (error)
 		goto dput_and_out;
 

fs/udf/file.c

@@ -183,7 +183,7 @@ long udf_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	long old_block, new_block;
 	int result;
 
-	if (inode_permission(inode, MAY_READ) != 0) {
+	if (file_permission(filp, MAY_READ) != 0) {
 		udf_debug("no permission to access inode %lu\n", inode->i_ino);
 		return -EPERM;
 	}

fs/verity/enable.c

@@ -369,7 +369,7 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 	 * has verity enabled, and to stabilize the data being hashed.
 	 */
 
-	err = inode_permission(inode, MAY_WRITE);
+	err = file_permission(filp, MAY_WRITE);
 	if (err)
 		return err;
 

include/linux/fs.h

@@ -2812,6 +2812,14 @@ static inline int bmap(struct inode *inode,  sector_t *block)
 extern int notify_change(struct dentry *, struct iattr *, struct inode **);
 extern int inode_permission(struct inode *, int);
 extern int generic_permission(struct inode *, int);
+static inline int file_permission(struct file *file, int mask)
+{
+	return inode_permission(file_inode(file), mask);
+}
+static inline int path_permission(const struct path *path, int mask)
+{
+	return inode_permission(d_inode(path->dentry), mask);
+}
 extern int __check_sticky(struct inode *dir, struct inode *inode);
 
 static inline bool execute_ok(struct inode *inode)

kernel/bpf/inode.c

@@ -507,7 +507,7 @@ static void *bpf_obj_do_get(const char __user *pathname,
 		return ERR_PTR(ret);
 
 	inode = d_backing_inode(path.dentry);
-	ret = inode_permission(inode, ACC_MODE(flags));
+	ret = path_permission(&path, ACC_MODE(flags));
 	if (ret)
 		goto out;
 

kernel/sys.c

@@ -1848,7 +1848,7 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	if (!S_ISREG(inode->i_mode) || path_noexec(&exe.file->f_path))
 		goto exit;
 
-	err = inode_permission(inode, MAY_EXEC);
+	err = file_permission(exe.file, MAY_EXEC);
 	if (err)
 		goto exit;
 

mm/madvise.c

@@ -540,7 +540,7 @@ static inline bool can_do_pageout(struct vm_area_struct *vma)
 	 * opens a side channel.
 	 */
 	return inode_owner_or_capable(file_inode(vma->vm_file)) ||
-		inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0;
+	       file_permission(vma->vm_file, MAY_WRITE) == 0;
 }
 
 static long madvise_pageout(struct vm_area_struct *vma,

mm/memcontrol.c

@@ -4899,7 +4899,7 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 
 	/* the process need read permission on control file */
 	/* AV: shouldn't we check that it's been opened for read instead? */
-	ret = inode_permission(file_inode(cfile.file), MAY_READ);
+	ret = file_permission(cfile.file, MAY_READ);
 	if (ret < 0)
 		goto out_put_cfile;
 

mm/mincore.c

@@ -167,7 +167,7 @@ static inline bool can_do_mincore(struct vm_area_struct *vma)
 	 * mappings, which opens a side channel.
 	 */
 	return inode_owner_or_capable(file_inode(vma->vm_file)) ||
-		inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0;
+	       file_permission(vma->vm_file, MAY_WRITE) == 0;
 }
 
 static const struct mm_walk_ops mincore_walk_ops = {

net/unix/af_unix.c

@@ -936,7 +936,7 @@ static struct sock *unix_find_other(struct net *net,
 		if (err)
 			goto fail;
 		inode = d_backing_inode(path.dentry);
-		err = inode_permission(inode, MAY_WRITE);
+		err = path_permission(&path, MAY_WRITE);
 		if (err)
 			goto put_fail;