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

Linus Torvalds
2019-10-10T17:13:05Z

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