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()...
