fixeria has uploaded this change for review. (
https://gerrit.osmocom.org/c/libosmocore/+/43150?usp=email )
Change subject: linuxlist: fix false-positive UBSan misaligned-access reports
......................................................................
linuxlist: fix false-positive UBSan misaligned-access reports
llist_for_each_entry() and its variants terminate by comparing
&pos->member against head. Once a full traversal reaches the end of a
non-empty list (or the list is empty to begin with), pos becomes a pure
container_of()-computed sentinel address that was never a real object
of typeof(*pos), only ever the plain 'struct llist_head *' passed in as
head. Forming 'pos->member' on that address requires pos to satisfy
the alignment of typeof(*pos), which the sentinel does not necessarily
provide (e.g. embedded struct osmo_timer_list/gprs_nsvc on 32-bit ARM
with a 64-bit time_t need 8-byte alignment, while head itself is only
pointer-aligned) - tripping -fsanitize=alignment even though no real
object is ever misaligned or dereferenced.
Add __llist_member(), computing the same address via 'char *'
pointer arithmetic and an explicit cast to 'struct llist_head *', which
carries no alignment requirement of its own. Use it for the loop
termination check, the "next" pointer computation, and the prefetch()
calls (dereferencing the returned struct llist_head * still only
requires pointer alignment, so prefetch() keeps working exactly
as before without reintroducing the false positive).
Change-Id: I0424e76e76d8aa9402bd1a5aefe789de16e72fae
Related: OS#7036, OS#6858
---
M include/osmocom/core/linuxlist.h
1 file changed, 33 insertions(+), 19 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/libosmocore refs/changes/50/43150/1
diff --git a/include/osmocom/core/linuxlist.h b/include/osmocom/core/linuxlist.h
index 8c8b1bc..e5e5075 100644
--- a/include/osmocom/core/linuxlist.h
+++ b/include/osmocom/core/linuxlist.h
@@ -218,6 +218,20 @@
#define llist_entry(ptr, type, member) \
container_of(ptr, type, member)
+/*! Compute the address of the llist_head member within *pos, without
+ * forming a 'pos->member' expression.
+ *
+ * This exists so llist_for_each_entry() and friends can test for the
+ * end of the list (where 'pos' is a container_of()-computed address
+ * that was never a real object of typeof(*pos), only ever a
+ * 'struct llist_head *') without tripping -fsanitize=alignment: a
+ * 'pos->member' access requires 'pos' itself to satisfy the alignment
+ * of typeof(*pos), which the end-of-list address does not necessarily
+ * do, while plain 'char *' pointer arithmetic has no such requirement.
+ */
+#define __llist_member(pos, member) \
+ ((struct llist_head *)((char *)(pos) + offsetof(typeof(*(pos)),
member)))
+
/*! Get the first element from a linked list.
* \param ptr the list head to take the element from.
* \param type the type of the struct this is embedded in.
@@ -308,10 +322,10 @@
*/
#define llist_for_each_entry(pos, head, member)
\
for (pos = llist_entry((head)->next, typeof(*pos), member), \
- prefetch(pos->member.next); \
- &pos->member != (head); \
- pos = llist_entry(pos->member.next, typeof(*pos), member), \
- prefetch(pos->member.next))
+ prefetch(__llist_member(pos, member)->next); \
+ __llist_member(pos, member) != (head); \
+ pos = llist_entry(__llist_member(pos, member)->next, typeof(*pos),
member), \
+ prefetch(__llist_member(pos, member)->next))
/*! Iterate backwards over a linked list of a given type.
* \param pos the 'type *' to use as a loop counter.
@@ -320,10 +334,10 @@
*/
#define llist_for_each_entry_reverse(pos, head, member)
\
for (pos = llist_entry((head)->prev, typeof(*pos), member), \
- prefetch(pos->member.prev); \
- &pos->member != (head); \
- pos = llist_entry(pos->member.prev, typeof(*pos), member), \
- prefetch(pos->member.prev))
+ prefetch(__llist_member(pos, member)->prev); \
+ __llist_member(pos, member) != (head); \
+ pos = llist_entry(__llist_member(pos, member)->prev, typeof(*pos),
member), \
+ prefetch(__llist_member(pos, member)->prev))
/*! Iterate over a linked list of a given type,
* continuing after an existing point.
@@ -333,10 +347,10 @@
*/
#define llist_for_each_entry_continue(pos, head, member) \
for (pos = llist_entry(pos->member.next, typeof(*pos), member), \
- prefetch(pos->member.next); \
- &pos->member != (head); \
- pos = llist_entry(pos->member.next, typeof(*pos), member), \
- prefetch(pos->member.next))
+ prefetch(__llist_member(pos, member)->next); \
+ __llist_member(pos, member) != (head); \
+ pos = llist_entry(__llist_member(pos, member)->next, typeof(*pos),
member), \
+ prefetch(__llist_member(pos, member)->next))
/*! Iterate over llist of given type, safe against removal of llist entry.
* \param pos the 'type *' to use as a loop counter.
@@ -346,9 +360,9 @@
*/
#define llist_for_each_entry_safe(pos, n, head, member)
\
for (pos = llist_entry((head)->next, typeof(*pos), member), \
- n = llist_entry(pos->member.next, typeof(*pos), member);
\
- &pos->member != (head); \
- pos = n, n = llist_entry(n->member.next, typeof(*n), member))
+ n = llist_entry(__llist_member(pos, member)->next,
typeof(*pos), member); \
+ __llist_member(pos, member) != (head); \
+ pos = n, n = llist_entry(__llist_member(n, member)->next,
typeof(*n), member))
/*! Iterate over an rcu-protected llist.
* \param pos the llist_head to use as a loop counter.
@@ -378,11 +392,11 @@
*/
#define llist_for_each_entry_rcu(pos, head, member) \
for (pos = llist_entry((head)->next, typeof(*pos), member), \
- prefetch(pos->member.next); \
- &pos->member != (head); \
- pos = llist_entry(pos->member.next, typeof(*pos), member), \
+ prefetch(__llist_member(pos, member)->next); \
+ __llist_member(pos, member) != (head); \
+ pos = llist_entry(__llist_member(pos, member)->next, typeof(*pos),
member), \
({ smp_read_barrier_depends(); 0;}), \
- prefetch(pos->member.next))
+ prefetch(__llist_member(pos, member)->next))
/*! Iterate over an rcu-protected llist, continuing after existing point.
--
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Gerrit-MessageType: newchange
Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: I0424e76e76d8aa9402bd1a5aefe789de16e72fae
Gerrit-Change-Number: 43150
Gerrit-PatchSet: 1
Gerrit-Owner: fixeria <[email protected]>
