On 2019-10-12, Aleksa Sarai cyphar@cyphar.com wrote:
On 2019-10-10, Linus Torvalds torvalds@linux-foundation.org wrote:
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.
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(structnameidata *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 beingrelative-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.