Re: [RFC][PATCH v2 14/34] new step_into() flag: WALK_NOFOLLOW

Al Viro
2020-02-23T22:07:52Z

On Sat, Feb 22, 2020 at 06:14:45PM -0800, Linus Torvalds wrote:
> On Sat, Feb 22, 2020 at 5:20 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > if (likely(!d_is_symlink(path->dentry)) ||
> > - !(flags & WALK_FOLLOW || nd->flags & LOOKUP_FOLLOW)) {
> > + !(flags & WALK_FOLLOW || nd->flags & LOOKUP_FOLLOW) ||
> > + flags & WALK_NOFOLLOW) {
>
> Humor me, and don't mix bitwise ops with logical boolean ops without
> parentheses, ok?
>
> And yes, the old code did it too, so it's not a new thing.
>
> But as it gets even more complex, let's just generally strive for doing
>
> (a & b) || (c & d)
>
> instead of
>
> a & b || c & d
>
> to make it easier to mentally see the grouping.

Can do... FWIW, the only case where the normal "'and' is multiplication,
'or' is addition" doesn't give the right result is
x | y && z
written instead of
x | (y && z)
where you would be better off rewriting the expression anyway.

FWIW, one of the things in the local pile is this:

commit cc1b6724b32de1be108cf6a5f28dbb5aa424b42f
Author: Al Viro <viro@zeniv.linux.org.uk>
Date: Sun Jan 19 12:44:18 2020 -0500

namei: invert the meaning of WALK_FOLLOW

old flags & WALK_FOLLOW <=> new !(flags & WALK_TRAILING)
That's what that flag had really been used for.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 6eb708014d4b..7d938241157f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1786,7 +1786,7 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
return NULL;
}

-enum {WALK_FOLLOW = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};
+enum {WALK_TRAILING = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};

/*
* Do we need to follow links? We _really_ want to be able
@@ -1805,7 +1805,7 @@ static const char *step_into(struct nameidata *nd, int flags,
if (!(flags & WALK_MORE) && nd->depth)
put_link(nd);
if (likely(!d_is_symlink(path.dentry)) ||
- !(flags & WALK_FOLLOW || nd->flags & LOOKUP_FOLLOW) ||
+ (flags & WALK_TRAILING && !(nd->flags & LOOKUP_FOLLOW)) ||
flags & WALK_NOFOLLOW) {
/* not a symlink or should not follow */
path_to_nameidata(&path, nd);
@@ -2157,10 +2157,10 @@ static int link_path_walk(const char *name, struct nameidata *nd)
if (!name)
return 0;
/* last component of nested symlink */
- link = walk_component(nd, WALK_FOLLOW);
+ link = walk_component(nd, 0);
} else {
/* not the last component */
- link = walk_component(nd, WALK_FOLLOW | WALK_MORE);
+ link = walk_component(nd, WALK_MORE);
}
if (unlikely(link)) {
if (IS_ERR(link))
@@ -2288,7 +2288,7 @@ static inline const char *lookup_last(struct nameidata *nd)
nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;

nd->flags &= ~LOOKUP_PARENT;
- link = walk_component(nd, 0);
+ link = walk_component(nd, WALK_TRAILING);
if (link) {
nd->flags |= LOOKUP_PARENT;
nd->stack[0].name = NULL;
@@ -3241,7 +3241,7 @@ static const char *do_last(struct nameidata *nd,
}

finish_lookup:
- link = step_into(nd, 0, dentry, inode, seq);
+ link = step_into(nd, WALK_TRAILING, dentry, inode, seq);
if (unlikely(link)) {
nd->flags |= LOOKUP_PARENT;
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);

and I can simply fold adding extra parens into it. Or I can fold that into
the patch you'd been replying to - I'm still uncertain about the series
containing WALK_TRAILING...