From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michel Lespinasse Subject: Re: [PATCH v2 01/11] rbtree: Fix rbtree_postorder_for_each_entry_safe() iterator Date: Thu, 7 Nov 2013 03:51:04 -0800 Message-ID: References: <1383788572-25938-1-git-send-email-cody@linux.vnet.ibm.com> <1383788572-25938-2-git-send-email-cody@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: Andrew Morton , EXT4 , Jan Kara , rostedt@goodmis.org, Seth Jennings , LKML To: Cody P Schafer Return-path: In-Reply-To: <1383788572-25938-2-git-send-email-cody@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, Nov 6, 2013 at 5:42 PM, Cody P Schafer wrote: > From: Jan Kara > > The iterator rbtree_postorder_for_each_entry_safe() relies on pointer > underflow behavior when testing for loop termination. In particular > it expects that > &rb_entry(NULL, type, field)->field > is NULL. But the result of this expression is not defined by a C standard > and some gcc versions (e.g. 4.3.4) assume the above expression can never > be equal to NULL. The net result is an oops because the iteration is not > properly terminated. > > Fix the problem by modifying the iterator to avoid pointer underflows. > > Signed-off-by: Jan Kara > Signed-off-by: Cody P Schafer > --- > include/linux/rbtree.h | 16 +++++++++------- > 1 file changed, 9 insertions(+), 7 deletions(-) > > diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h > index aa870a4..57e75ae 100644 > --- a/include/linux/rbtree.h > +++ b/include/linux/rbtree.h > @@ -85,6 +85,11 @@ static inline void rb_link_node(struct rb_node * node, struct rb_node * parent, > *rb_link = node; > } > > +#define rb_entry_safe(ptr, type, member) \ > + ({ typeof(ptr) ____ptr = (ptr); \ > + ____ptr ? rb_entry(____ptr, type, member) : NULL; \ > + }) > + > /** > * rbtree_postorder_for_each_entry_safe - iterate over rb_root in post order of > * given type safe against removal of rb_node entry > @@ -95,12 +100,9 @@ static inline void rb_link_node(struct rb_node * node, struct rb_node * parent, > * @field: the name of the rb_node field within 'type'. > */ > #define rbtree_postorder_for_each_entry_safe(pos, n, root, field) \ > - for (pos = rb_entry(rb_first_postorder(root), typeof(*pos), field),\ > - n = rb_entry(rb_next_postorder(&pos->field), \ > - typeof(*pos), field); \ > - &pos->field; \ > - pos = n, \ > - n = rb_entry(rb_next_postorder(&pos->field), \ > - typeof(*pos), field)) > + for (pos = rb_entry_safe(rb_first_postorder(root), typeof(*pos), field); \ > + pos && ({ n = rb_entry_safe(rb_next_postorder(&pos->field), \ > + typeof(*pos), field); 1; }); \ > + pos = n) > > #endif /* _LINUX_RBTREE_H */ > -- > 1.8.4.2 Well, this really isn't pretty, and I'm not sure that rbtree_postorder_for_each_entry_safe() is a good idea in the first place. Note that we have never had or needed such a macro for the common case of in-order iteration; why would we need it for the less-common case of postorder iteration ? I think it's just as well to have clients write something like struct rb_node *rb_node = rb_first_postorder(root); while (rb_node) { struct rb_node *rb_next_node = rb_next_postorder(rb_node); struct mystruct node = rb_entry(rb_node, struct mystruct, mystruct_rb_field); .... do whatever, possibly destroying node ... rb_node = rb_next_node; } That said, there is some precedent for this kind of API in hlist_for_each_entry_safe, so I guess that's acceptable if there will be enough users of this macro - but it seems very strange to me that we would need it for the postorder traversal while we don't for the in-order traversal. I would prefer keeping rbtree.h minimal if that is possible. Thanks, -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.