[PATCH v14 2/6] namei: LOOKUP_IN_ROOT: chroot-like path resolution

[Aleksa Sarai]

On 2019-10-12, Aleksa Sarai <cyphar at cyphar.com> wrote:
> On 2019-10-10, Linus Torvalds <torvalds at linux-foundation.org> wrote:
> > On Wed, Oct 9, 2019 at 10:42 PM Aleksa Sarai <cyphar at cyphar.com> wrote:
> > >
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -2277,6 +2277,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > >
> > >         nd->m_seq = read_seqbegin(&mount_lock);
> > >
> > > +       /* LOOKUP_IN_ROOT treats absolute paths as being relative-to-dirfd. */
> > > +       if (flags & LOOKUP_IN_ROOT)
> > > +               while (*s == '/')
> > > +                       s++;
> > > +
> > >         /* Figure out the starting path and root (if needed). */
> > >         if (*s == '/') {
> > >                 error = nd_jump_root(nd);
> > 
> > Hmm. Wouldn't this make more sense all inside the if (*s =- '/') test?
> > That way if would be where we check for "should we start at the root",
> > which seems to make more sense conceptually.
> 
> I don't really agree (though I do think that both options are pretty
> ugly). Doing it before the block makes it clear that absolute paths are
> just treated relative-to-dirfd -- doing it inside the block makes it
> look more like "/" is a special-case for nd_jump_root(). And while that

Sorry, I meant "special-case for LOOKUP_IN_ROOT".

> is somewhat true, this is just a side-effect of making the code more
> clean -- my earlier versions reworked the dirfd handling to always grab
> nd->root first if LOOKUP_IS_SCOPED. I switched to this method based on
> Al's review.
> 
> In fairness, I do agree that the lonely while loop looks ugly.

And with the old way I did it (where we grabbed nd->root first) the
semantics were slightly more clear -- stripping leading "/"s doesn't
really look as "clearly obvious" as grabbing nd->root beforehand and
treating "/"s normally. But the code was also needlessly more complex.

> > That test for '/' currently has a "} else if (..)", but that's
> > pointless since it ends with a "return" anyway. So the "else" logic is
> > just noise.
> 
> This depends on the fact that LOOKUP_BENEATH always triggers -EXDEV for
> nd_jump_root() -- if we ever add another "scoped lookup" flag then the
> logic will have to be further reworked.
> 
> (It should be noted that the new version doesn't always end with a
> "return", but you could change it to act that way given the above
> assumption.)
> 
> > And if you get rid of the unnecessary else, moving the LOOKUP_IN_ROOT
> > inside the if-statement works fine.
> > 
> > So this could be something like
> > 
> >     --- a/fs/namei.c
> >     +++ b/fs/namei.c
> >     @@ -2194,11 +2196,19 @@ static const char *path_init(struct
> > nameidata *nd, unsigned flags)
> > 
> >         nd->m_seq = read_seqbegin(&mount_lock);
> >         if (*s == '/') {
> >     -           set_root(nd);
> >     -           if (likely(!nd_jump_root(nd)))
> >     -                   return s;
> >     -           return ERR_PTR(-ECHILD);
> >     -   } else if (nd->dfd == AT_FDCWD) {
> >     +           /* LOOKUP_IN_ROOT treats absolute paths as being
> > relative-to-dirfd. */
> >     +           if (!(flags & LOOKUP_IN_ROOT)) {
> >     +                   set_root(nd);
> >     +                   if (likely(!nd_jump_root(nd)))
> >     +                           return s;
> >     +                   return ERR_PTR(-ECHILD);
> >     +           }
> >     +
> >     +           /* Skip initial '/' for LOOKUP_IN_ROOT */
> >     +           do { s++; } while (*s == '/');
> >     +   }
> >     +
> >     +   if (nd->dfd == AT_FDCWD) {
> >                 if (flags & LOOKUP_RCU) {
> >                         struct fs_struct *fs = current->fs;
> >                         unsigned seq;
> > 
> > instead. The patch ends up slightly bigger (due to the re-indentation)
> > but now it handles all the "start at root" in the same place. Doesn't
> > that make sense?
> 
> It is correct (though I'd need to clean it up a bit to handle
> nd_jump_root() correctly), and if you really would like me to change it
> I will -- but I just don't agree that it's cleaner.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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