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

[Aleksa Sarai]

On 2019-10-10, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Oct 9, 2019 at 10:42 PM Aleksa Sarai <cyphar@xxxxxxxxxx> 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
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.

> 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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