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SubjectRe: [patch] avoid kmemcheck warning in epoll
On Mon, 11 Feb 2008, Andrew Morton wrote:

> On Sun, 10 Feb 2008 13:32:01 -0800 (PST)
> Davide Libenzi <davidel@xmailserver.org> wrote:
>
> > Epoll calls rb_set_parent(n, n) to initialize the rb-tree node, but
> > rb_set_parent() accesses node's pointer in its code. This creates a
> > warning in kmemcheck (reported by Vegard Nossum) about an uninitialized
> > memory access. The warning is harmless since the following rb-tree node
> > insert is going to overwrite the node data. In any case I think it's
> > better to not have that happening at all, and fix it by properly
> > initializing the data.
> >
> >
> > Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
> >
> >
> > - Davide
> >
> >
> > ---
> > fs/eventpoll.c | 2 +-
> > include/linux/rbtree.h | 12 ++++++++++++
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > Index: linux-2.6.mod/fs/eventpoll.c
> > ===================================================================
> > --- linux-2.6.mod.orig/fs/eventpoll.c 2008-02-10 12:36:20.000000000 -0800
> > +++ linux-2.6.mod/fs/eventpoll.c 2008-02-10 12:50:41.000000000 -0800
> > @@ -260,7 +260,7 @@
> > /* Special initialization for the RB tree node to detect linkage */
> > static inline void ep_rb_initnode(struct rb_node *n)
> > {
> > - rb_set_parent(n, n);
> > + rb_init_node(n, n);
> > }
> >
> > /* Removes a node from the RB tree and marks it for a fast is-linked check */
> > Index: linux-2.6.mod/include/linux/rbtree.h
> > ===================================================================
> > --- linux-2.6.mod.orig/include/linux/rbtree.h 2008-02-10 12:36:13.000000000 -0800
> > +++ linux-2.6.mod/include/linux/rbtree.h 2008-02-10 12:51:57.000000000 -0800
> > @@ -112,6 +112,18 @@
> > struct rb_node *rb_node;
> > };
> >
> > +/**
> > + * rb_init_node - Initializes the node internal data
> > + *
> > + * @node: Pointer to the RB-Tree node
> > + * @parent: Pointer to the parent node, or NULL
> > + *
> > + */
> > +static inline void rb_init_node(struct rb_node *node, struct rb_node *parent)
> > +{
> > + node->rb_parent_color = (unsigned long) parent;
> > + node->rb_left = node->rb_right = NULL;
> > +}
>
> Is epoll the only rbtree-using code which exhibits this problem? If so,
> what is epoll doing differently from all the others?

Dunno, but I don't think so. Epoll initializes the node to a state so that
later on can check if the file-fd item is inserted or not. And it uses the
"parent" information for that. But rb_set_parent() (that is used in the
current initialization code) uses the data in the node, and this triggers
the uninitialized memory access.
Taking a better look at it, the code (after the latest changes) has no
more point-of-failures after the rb-tree insert, so I can probably avoid
doing the node's parent-initialization and check altogether.
Yeah, let's that ;)

eventpoll.c | 27 +++------------------------
1 file changed, 3 insertions(+), 24 deletions(-)

Andrew, drop that one. I'm gonna send the new one after some test...



- Davide




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