From 4356d575ef0f39a3e8e0ce0c40d84ce900ac3b61 Mon Sep 17 00:00:00 2001
From: Aleksa Sarai <cyphar@cyphar.com>
Date: Wed, 28 Aug 2024 20:19:43 +1000
Subject: [PATCH] fhandle: expose u64 mount id to name_to_handle_at(2)
Now that we provide a unique 64-bit mount ID interface in statx(2), we
can now provide a race-free way for name_to_handle_at(2) to provide a
file handle and corresponding mount without needing to worry about
racing with /proc/mountinfo parsing or having to open a file just to do
statx(2).
While this is not necessary if you are using AT_EMPTY_PATH and don't
care about an extra statx(2) call, users that pass full paths into
name_to_handle_at(2) need to know which mount the file handle comes from
(to make sure they don't try to open_by_handle_at a file handle from a
different filesystem) and switching to AT_EMPTY_PATH would require
allocating a file for every name_to_handle_at(2) call, turning
err = name_to_handle_at(-EBADF, "/foo/bar/baz", &handle, &mntid,
AT_HANDLE_MNT_ID_UNIQUE);
into
int fd = openat(-EBADF, "/foo/bar/baz", O_PATH | O_CLOEXEC);
err1 = name_to_handle_at(fd, "", &handle, &unused_mntid, AT_EMPTY_PATH);
err2 = statx(fd, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &statxbuf);
mntid = statxbuf.stx_mnt_id;
close(fd);
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/fhandle.c | 29 ++++++++++++++++++++++-------
include/linux/syscalls.h | 2 +-
include/uapi/linux/fcntl.h | 1 +
3 files changed, 24 insertions(+), 8 deletions(-)
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 6e8cea16790ef..8cb665629f4a8 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -16,7 +16,8 @@
static long do_sys_name_to_handle(const struct path *path,
struct file_handle __user *ufh,
- int __user *mnt_id, int fh_flags)
+ void __user *mnt_id, bool unique_mntid,
+ int fh_flags)
{
long retval;
struct file_handle f_handle;
@@ -69,9 +70,19 @@ static long do_sys_name_to_handle(const struct path *path,
} else
retval = 0;
/* copy the mount id */
- if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
- copy_to_user(ufh, handle,
- struct_size(handle, f_handle, handle_bytes)))
+ if (unique_mntid) {
+ if (put_user(real_mount(path->mnt)->mnt_id_unique,
+ (u64 __user *) mnt_id))
+ retval = -EFAULT;
+ } else {
+ if (put_user(real_mount(path->mnt)->mnt_id,
+ (int __user *) mnt_id))
+ retval = -EFAULT;
+ }
+ /* copy the handle */
+ if (retval != -EFAULT &&
+ copy_to_user(ufh, handle,
+ struct_size(handle, f_handle, handle_bytes)))
retval = -EFAULT;
kfree(handle);
return retval;
@@ -83,6 +94,7 @@ static long do_sys_name_to_handle(const struct path *path,
* @name: name that should be converted to handle.
* @handle: resulting file handle
* @mnt_id: mount id of the file system containing the file
+ * (u64 if AT_HANDLE_MNT_ID_UNIQUE, otherwise int)
* @flag: flag value to indicate whether to follow symlink or not
* and whether a decodable file handle is required.
*
@@ -92,7 +104,7 @@ static long do_sys_name_to_handle(const struct path *path,
* value required.
*/
SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
- struct file_handle __user *, handle, int __user *, mnt_id,
+ struct file_handle __user *, handle, void __user *, mnt_id,
int, flag)
{
struct path path;
@@ -100,7 +112,8 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
int fh_flags;
int err;
- if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID))
+ if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
+ AT_HANDLE_MNT_ID_UNIQUE))
return -EINVAL;
lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
@@ -109,7 +122,9 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
lookup_flags |= LOOKUP_EMPTY;
err = user_path_at(dfd, name, lookup_flags, &path);
if (!err) {
- err = do_sys_name_to_handle(&path, handle, mnt_id, fh_flags);
+ err = do_sys_name_to_handle(&path, handle, mnt_id,
+ flag & AT_HANDLE_MNT_ID_UNIQUE,
+ fh_flags);
path_put(&path);
}
return err;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 4bcf6754738d4..5758104921e66 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -870,7 +870,7 @@ asmlinkage long sys_fanotify_mark(int fanotify_fd, unsigned int flags,
#endif
asmlinkage long sys_name_to_handle_at(int dfd, const char __user *name,
struct file_handle __user *handle,
- int __user *mnt_id, int flag);
+ void __user *mnt_id, int flag);
asmlinkage long sys_open_by_handle_at(int mountdirfd,
struct file_handle __user *handle,
int flags);
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 38a6d66d9e887..87e2dec79fea4 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -152,6 +152,7 @@
#define AT_HANDLE_FID 0x200 /* File handle is needed to compare
object identity and may not be
usable with open_by_handle_at(2). */
+#define AT_HANDLE_MNT_ID_UNIQUE 0x001 /* Return the u64 unique mount ID. */
#if defined(__KERNEL__)
#define AT_GETATTR_NOSEC 0x80000000
--
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