From b4fef22c2fb97fa204f0c99c7c7f1c6b422ef0aa Mon Sep 17 00:00:00 2001 From: Aleksa Sarai Date: Wed, 28 Aug 2024 20:19:42 +1000 Subject: [PATCH 1/2] uapi: explain how per-syscall AT_* flags should be allocated Unfortunately, the way we have gone about adding new AT_* flags has been a little messy. In the beginning, all of the AT_* flags had generic meanings and so it made sense to share the flag bits indiscriminately. However, we inevitably ran into syscalls that needed their own syscall-specific flags. Due to the lack of a planned out policy, we ended up with the following situations: * Existing syscalls adding new features tended to use new AT_* bits, with some effort taken to try to re-use bits for flags that were so obviously syscall specific that they only make sense for a single syscall (such as the AT_EACCESS/AT_REMOVEDIR/AT_HANDLE_FID triplet). Given the constraints of bitflags, this works well in practice, but ideally (to avoid future confusion) we would plan ahead and define a set of "per-syscall bits" ahead of time so that when allocating new bits we don't end up with a complete mish-mash of which bits are supposed to be per-syscall and which aren't. * New syscalls dealt with this in several ways: - Some syscalls (like renameat2(2), move_mount(2), fsopen(2), and fspick(2)) created their separate own flag spaces that have no overlap with the AT_* flags. Most of these ended up allocating their bits sequentually. In the case of move_mount(2) and fspick(2), several flags have identical meanings to AT_* flags but were allocated in their own flag space. This makes sense for syscalls that will never share AT_* flags, but for some syscalls this leads to duplication with AT_* flags in a way that could cause confusion (if renameat2(2) grew a RENAME_EMPTY_PATH it seems likely that users could mistake it for AT_EMPTY_PATH since it is an *at(2) syscall). - Some syscalls unfortunately ended up both creating their own flag space while also using bits from other flag spaces. The most obvious example is open_tree(2), where the standard usage ends up using flags from *THREE* separate flag spaces: open_tree(AT_FDCWD, "/foo", OPEN_TREE_CLONE|O_CLOEXEC|AT_RECURSIVE); (Note that O_CLOEXEC is also platform-specific, so several future OPEN_TREE_* bits are also made unusable in one fell swoop.) It's not entirely clear to me what the "right" choice is for new syscalls. Just saying that all future VFS syscalls should use AT_* flags doesn't seem practical. openat2(2) has RESOLVE_* flags (many of which don't make much sense to burn generic AT_* flags for) and move_mount(2) has separate AT_*-like flags for both the source and target so separate flags are needed anyway (though it seems possible that renameat2(2) could grow *_EMPTY_PATH flags at some point, and it's a bit of a shame they can't be reused). But at least for syscalls that _do_ choose to use AT_* flags, we should explicitly state the policy that 0x2ff is currently intended for per-syscall flags and that new flags should err on the side of overlapping with existing flag bits (so we can extend the scope of generic flags in the future if necessary). And add AT_* aliases for the RENAME_* flags to further cement that renameat2(2) is an *at(2) flag, just with its own per-syscall flags. Suggested-by: Amir Goldstein Reviewed-by: Jeff Layton Reviewed-by: Josef Bacik Signed-off-by: Aleksa Sarai Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-1-10c2c4c16708@cyphar.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- include/uapi/linux/fcntl.h | 80 ++++++++++++++++++++++++++------------ 1 file changed, 56 insertions(+), 24 deletions(-) diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h index e55a3314bcb0..38a6d66d9e88 100644 --- a/include/uapi/linux/fcntl.h +++ b/include/uapi/linux/fcntl.h @@ -90,37 +90,69 @@ #define DN_ATTRIB 0x00000020 /* File changed attibutes */ #define DN_MULTISHOT 0x80000000 /* Don't remove notifier */ +#define AT_FDCWD -100 /* Special value for dirfd used to + indicate openat should use the + current working directory. */ + + +/* Generic flags for the *at(2) family of syscalls. */ + +/* Reserved for per-syscall flags 0xff. */ +#define AT_SYMLINK_NOFOLLOW 0x100 /* Do not follow symbolic + links. */ +/* Reserved for per-syscall flags 0x200 */ +#define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */ +#define AT_NO_AUTOMOUNT 0x800 /* Suppress terminal automount + traversal. */ +#define AT_EMPTY_PATH 0x1000 /* Allow empty relative + pathname to operate on dirfd + directly. */ /* - * The constants AT_REMOVEDIR and AT_EACCESS have the same value. AT_EACCESS is - * meaningful only to faccessat, while AT_REMOVEDIR is meaningful only to - * unlinkat. The two functions do completely different things and therefore, - * the flags can be allowed to overlap. For example, passing AT_REMOVEDIR to - * faccessat would be undefined behavior and thus treating it equivalent to - * AT_EACCESS is valid undefined behavior. + * These flags are currently statx(2)-specific, but they could be made generic + * in the future and so they should not be used for other per-syscall flags. */ -#define AT_FDCWD -100 /* Special value used to indicate - openat should use the current - working directory. */ -#define AT_SYMLINK_NOFOLLOW 0x100 /* Do not follow symbolic links. */ +#define AT_STATX_SYNC_TYPE 0x6000 /* Type of synchronisation required from statx() */ +#define AT_STATX_SYNC_AS_STAT 0x0000 /* - Do whatever stat() does */ +#define AT_STATX_FORCE_SYNC 0x2000 /* - Force the attributes to be sync'd with the server */ +#define AT_STATX_DONT_SYNC 0x4000 /* - Don't sync attributes with the server */ + +#define AT_RECURSIVE 0x8000 /* Apply to the entire subtree */ + +/* + * Per-syscall flags for the *at(2) family of syscalls. + * + * These are flags that are so syscall-specific that a user passing these flags + * to the wrong syscall is so "clearly wrong" that we can safely call such + * usage "undefined behaviour". + * + * For example, the constants AT_REMOVEDIR and AT_EACCESS have the same value. + * AT_EACCESS is meaningful only to faccessat, while AT_REMOVEDIR is meaningful + * only to unlinkat. The two functions do completely different things and + * therefore, the flags can be allowed to overlap. For example, passing + * AT_REMOVEDIR to faccessat would be undefined behavior and thus treating it + * equivalent to AT_EACCESS is valid undefined behavior. + * + * Note for implementers: When picking a new per-syscall AT_* flag, try to + * reuse already existing flags first. This leaves us with as many unused bits + * as possible, so we can use them for generic bits in the future if necessary. + */ + +/* Flags for renameat2(2) (must match legacy RENAME_* flags). */ +#define AT_RENAME_NOREPLACE 0x0001 +#define AT_RENAME_EXCHANGE 0x0002 +#define AT_RENAME_WHITEOUT 0x0004 + +/* Flag for faccessat(2). */ #define AT_EACCESS 0x200 /* Test access permitted for effective IDs, not real IDs. */ +/* Flag for unlinkat(2). */ #define AT_REMOVEDIR 0x200 /* Remove directory instead of unlinking file. */ -#define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */ -#define AT_NO_AUTOMOUNT 0x800 /* Suppress terminal automount traversal */ -#define AT_EMPTY_PATH 0x1000 /* Allow empty relative pathname */ +/* Flags for name_to_handle_at(2). */ +#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_STATX_SYNC_TYPE 0x6000 /* Type of synchronisation required from statx() */ -#define AT_STATX_SYNC_AS_STAT 0x0000 /* - Do whatever stat() does */ -#define AT_STATX_FORCE_SYNC 0x2000 /* - Force the attributes to be sync'd with the server */ -#define AT_STATX_DONT_SYNC 0x4000 /* - Don't sync attributes with the server */ - -#define AT_RECURSIVE 0x8000 /* Apply to the entire subtree */ - -/* Flags for name_to_handle_at(2). We reuse AT_ flag space to save bits... */ -#define AT_HANDLE_FID AT_REMOVEDIR /* file handle is needed to - compare object identity and may not - be usable to open_by_handle_at(2) */ #if defined(__KERNEL__) #define AT_GETATTR_NOSEC 0x80000000 #endif From 4356d575ef0f39a3e8e0ce0c40d84ce900ac3b61 Mon Sep 17 00:00:00 2001 From: Aleksa Sarai Date: Wed, 28 Aug 2024 20:19:43 +1000 Subject: [PATCH 2/2] 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 Signed-off-by: Aleksa Sarai Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com Reviewed-by: Jan Kara Reviewed-by: Josef Bacik Signed-off-by: Christian Brauner --- 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 6e8cea16790e..8cb665629f4a 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 4bcf6754738d..5758104921e6 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 38a6d66d9e88..87e2dec79fea 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