Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags

Al Viro
2019-07-12T13:26:21Z

On Fri, Jul 12, 2019 at 01:55:52PM +0100, Al Viro wrote:
> On Fri, Jul 12, 2019 at 01:39:24PM +0100, Al Viro wrote:
> > On Fri, Jul 12, 2019 at 08:57:45PM +1000, Aleksa Sarai wrote:
> >
> > > > > @@ -2350,9 +2400,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > > > > s = ERR_PTR(error);
> > > > > return s;
> > > > > }
> > > > > - error = dirfd_path_init(nd);
> > > > > - if (unlikely(error))
> > > > > - return ERR_PTR(error);
> > > > > + if (likely(!nd->path.mnt)) {
> > > >
> > > > Is that a weird way of saying "if we hadn't already called dirfd_path_init()"?
> > >
> > > Yes. I did it to be more consistent with the other "have we got the
> > > root" checks elsewhere. Is there another way you'd prefer I do it?
> >
> > "Have we got the root" checks are inevitable evil; here you are making the
> > control flow in a single function hard to follow.
> >
> > I *think* what you are doing is
> > absolute pathname, no LOOKUP_BENEATH:
> > set_root
> > error = nd_jump_root(nd)
> > else
> > error = dirfd_path_init(nd)
> > return unlikely(error) ? ERR_PTR(error) : s;
> > which should be a lot easier to follow (not to mention shorter), but I might
> > be missing something in all of that.
>
> PS: if that's what's going on, I would be tempted to turn the entire
> path_init() part into this:
> if (flags & LOOKUP_BENEATH)
> while (*s == '/')
> s++;
> in the very beginning (plus the handling of nd_jump_root() prototype
> change, but that belongs with nd_jump_root() change itself, obviously).
> Again, I might be missing something here...

Argh... I am, at that - you have setting path->root (and grabbing it)
in LOOKUP_BENEATH cases and you do it after dirfd_path_init(). So
how about
if (flags & LOOKUP_BENEATH)
while (*s == '/')
s++;
before the whole thing and
if (*s == '/') { /* can happen only without LOOKUP_BENEATH */
set_root(nd);
error = nd_jump_root(nd);
if (unlikely(error))
return ERR_PTR(error);
} else if (nd->dfd == AT_FDCWD) {
if (flags & LOOKUP_RCU) {
struct fs_struct *fs = current->fs;
unsigned seq;

do {
seq = read_seqcount_begin(&fs->seq);
nd->path = fs->pwd;
nd->inode = nd->path.dentry->d_inode;
nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
} while (read_seqcount_retry(&fs->seq, seq));
} else {
get_fs_pwd(current->fs, &nd->path);
nd->inode = nd->path.dentry->d_inode;
}
} else {
/* Caller must check execute permissions on the starting path component */
struct fd f = fdget_raw(nd->dfd);
struct dentry *dentry;

if (!f.file)
return ERR_PTR(-EBADF);

dentry = f.file->f_path.dentry;

if (*s && unlikely(!d_can_lookup(dentry))) {
fdput(f);
return ERR_PTR(-ENOTDIR);
}

nd->path = f.file->f_path;
if (flags & LOOKUP_RCU) {
nd->inode = nd->path.dentry->d_inode;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
} else {
path_get(&nd->path);
nd->inode = nd->path.dentry->d_inode;
}
fdput(f);
}
if (flags & LOOKUP_BENEATH) {
nd->root = nd->path;
if (!(flags & LOOKUP_RCU))
path_get(&nd->root);
else
nd->root_seq = nd->seq;
}
return s;
replacing the part in the end? Makes for much smaller change; it might
very well still make sense to add dirfd_path_init() as a separate
cleanup (perhaps with the *s == '/' case included), though.