On Tue, Oct 08, 2024 at 05:27:51AM +0100, Al Viro wrote:
On Tue, Oct 08, 2024 at 05:16:21AM +0100, Al Viro wrote:
Folks, please don't go there. Really. IMO vfs_empty_path() is a wrong API in the first place. Too low-level and racy as well.
See the approach in #work.xattr; I'm going to lift that into fs/namei.c (well, the slow path - everything after "if path is NULL, we are done") and yes, fs/stat.c users get handled better that way.
FWIW, the intermediate (just after that commit) state of those functions is
int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat, int flags) { int ret; int statx_flags = flags | AT_NO_AUTOMOUNT; struct filename *name = getname_maybe_null(filename, flags);
if (!name) return vfs_fstat(dfd, stat); ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS); putname(name); return ret;}
and
SYSCALL_DEFINE5(statx, int, dfd, const char __user *, filename, unsigned, flags, unsigned int, mask, struct statx __user *, buffer) { int ret; unsigned lflags; struct filename *name = getname_maybe_null(filename, flags);
/* * Short-circuit handling of NULL and "" paths. * * For a NULL path we require and accept only the AT_EMPTY_PATH flag * (possibly |'d with AT_STATX flags). * * However, glibc on 32-bit architectures implements fstatat as statx * with the "" pathname and AT_NO_AUTOMOUNT | AT_EMPTY_PATH flags. * Supporting this results in the uglification below. */ lflags = flags & ~(AT_NO_AUTOMOUNT | AT_STATX_SYNC_TYPE); if (!name) return do_statx_fd(dfd, flags & ~AT_NO_AUTOMOUNT, mask, buffer); ret = do_statx(dfd, name, flags, mask, buffer); putname(name); return ret;}
static inline struct filename *getname_maybe_null(const char __user *name, int flags) { if (!(flags & AT_EMPTY_PATH)) return getname(name);
if (!name) return NULL; return __getname_maybe_null(name);}
struct filename *__getname_maybe_null(const char __user *pathname) { struct filename *name; char c;
/* try to save on allocations; loss on um, though */ if (get_user(c, pathname)) return ERR_PTR(-EFAULT); if (!c) return NULL; name = getname_flags(pathname, LOOKUP_EMPTY); if (!IS_ERR(name) && !(name->name[0])) { putname(name); name = NULL; } return name;}
Incidentally, the name 'getname_statx_lookup_flags' is an atrocity: * getname and its variants do not give a fuck for the state of any flags besides AT_EMPTY_PATH * lookups, OTOH, ignore LOOKUP_EMPTY (which shouldn't have been in the LOOKUP_... namespace to start with).
Another fun question: why do we play with setting ->mnt_id, etc. in vfs_statx_path() if vfs_getattr() returns non-zero? Or when we hit it via vfs_statx() from vfs_fstatat()...