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Subject[PATCH] skbuff: Extract list pointers to silence compiler warnings
Date
Under both -Warray-bounds and the object_size sanitizer, the compiler is
upset about accessing prev/next of sk_buff when the object it thinks it
is coming from is sk_buff_head. The warning is a false positive due to
the compiler taking a conservative approach, opting to warn at casting
time rather than access time.

However, in support of enabling -Warray-bounds globally (which has
found many real bugs), arrange things for sk_buff so that the compiler
can unambiguously see that there is no intention to access anything
except prev/next. Introduce and cast to a separate struct sk_buff_list,
which contains _only_ the first two fields, silencing the warnings:

In file included from ./include/net/net_namespace.h:39,
from ./include/linux/netdevice.h:37,
from net/core/netpoll.c:17:
net/core/netpoll.c: In function 'refill_skbs':
./include/linux/skbuff.h:2086:9: warning: array subscript 'struct sk_buff[0]' is partly outside array bounds of 'struct sk_buff_head[1]' [-Warray-bounds]
2086 | __skb_insert(newsk, next->prev, next, list);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/netpoll.c:49:28: note: while referencing 'skb_pool'
49 | static struct sk_buff_head skb_pool;
| ^~~~~~~~

This change results in no executable instruction differences.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/skbuff.h | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index eae4bd3237a4..ec71b45b62b0 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -292,9 +292,11 @@ struct tc_skb_ext {
#endif

struct sk_buff_head {
- /* These two members must be first. */
- struct sk_buff *next;
- struct sk_buff *prev;
+ /* These two members must be first to match sk_buff. */
+ struct_group_tagged(sk_buff_list, list,
+ struct sk_buff *next;
+ struct sk_buff *prev;
+ );

__u32 qlen;
spinlock_t lock;
@@ -730,7 +732,7 @@ typedef unsigned char *sk_buff_data_t;
struct sk_buff {
union {
struct {
- /* These two members must be first. */
+ /* These two members must be first to match sk_buff_head. */
struct sk_buff *next;
struct sk_buff *prev;

@@ -1976,8 +1978,8 @@ static inline void __skb_insert(struct sk_buff *newsk,
*/
WRITE_ONCE(newsk->next, next);
WRITE_ONCE(newsk->prev, prev);
- WRITE_ONCE(next->prev, newsk);
- WRITE_ONCE(prev->next, newsk);
+ WRITE_ONCE(((struct sk_buff_list *)next)->prev, newsk);
+ WRITE_ONCE(((struct sk_buff_list *)prev)->next, newsk);
WRITE_ONCE(list->qlen, list->qlen + 1);
}

@@ -2073,7 +2075,7 @@ static inline void __skb_queue_after(struct sk_buff_head *list,
struct sk_buff *prev,
struct sk_buff *newsk)
{
- __skb_insert(newsk, prev, prev->next, list);
+ __skb_insert(newsk, prev, ((struct sk_buff_list *)prev)->next, list);
}

void skb_append(struct sk_buff *old, struct sk_buff *newsk,
@@ -2083,7 +2085,7 @@ static inline void __skb_queue_before(struct sk_buff_head *list,
struct sk_buff *next,
struct sk_buff *newsk)
{
- __skb_insert(newsk, next->prev, next, list);
+ __skb_insert(newsk, ((struct sk_buff_list *)next)->prev, next, list);
}

/**
--
2.30.2
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