Re: [PATCH] net: bridge: mcast: read ngrec once in igmp3/mld2 report

On 20/12/2022 04:48, Joy Gu wrote:

In br_ip4_multicast_igmp3_report() and br_ip6_multicast_mld2_report(),
"ih" or "mld2r" is a pointer into the skb header. It's dereferenced to
get "num", which is used in the for-loop condition that follows.

Compilers are free to not spend a register on "num" and dereference that
pointer every time "num" would be used, i.e. every loop iteration. Which
would be a bug if pskb_may_pull() (called by ip_mc_may_pull() or
ipv6_mc_may_pull() in the loop body) were to change pointers pointing
into the skb header, e.g. by freeing "skb->head".

We can avoid this by using READ_ONCE().

Suggested-by: Joern Engel <redacted>
Signed-off-by: Joy Gu <redacted>
---
 net/bridge/br_multicast.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

I doubt any compiler would do that (partly due to the ntohs()). If you have hit a bug or
seen this with some compiler please provide more details, disassembly of the resulting
code would be best.

Thanks.