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 <joern@xxxxxxxxxxxxxxx> > Signed-off-by: Joy Gu <jgu@xxxxxxxxxxxxxxx> > --- > 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.
