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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Oct 2019 10:07:32 -0700
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Subject: Re: [PATCH v14 2/6] namei: LOOKUP_IN_ROOT: chroot-like path resolution
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
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On Wed, Oct 9, 2019 at 10:42 PM Aleksa Sarai <cyphar@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.

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.

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?

             Linus