bpf: Fix potentially incorrect results with bpf_get_local_storage()
Commit b910eaaa ("bpf: Fix NULL pointer dereference in bpf_get_local_storage() helper") fixed a bug for bpf_get_local_storage() helper so different tasks won't mess up with each other's percpu local storage. The percpu data contains 8 slots so it can hold up to 8 contexts (same or different tasks), for 8 different program runs, at the same time. This in general is sufficient. But our internal testing showed the following warning multiple times: [...] warning: WARNING: CPU: 13 PID: 41661 at include/linux/bpf-cgroup.h:193 __cgroup_bpf_run_filter_sock_ops+0x13e/0x180 RIP: 0010:__cgroup_bpf_run_filter_sock_ops+0x13e/0x180 <IRQ> tcp_call_bpf.constprop.99+0x93/0xc0 tcp_conn_request+0x41e/0xa50 ? tcp_rcv_state_process+0x203/0xe00 tcp_rcv_state_process+0x203/0xe00 ? sk_filter_trim_cap+0xbc/0x210 ? tcp_v6_inbound_md5_hash.constprop.41+0x44/0x160 tcp_v6_do_rcv+0x181/0x3e0 tcp_v6_rcv+0xc65/0xcb0 ip6_protocol_deliver_rcu+0xbd/0x450 ip6_input_finish+0x11/0x20 ip6_input+0xb5/0xc0 ip6_sublist_rcv_finish+0x37/0x50 ip6_sublist_rcv+0x1dc/0x270 ipv6_list_rcv+0x113/0x140 __netif_receive_skb_list_core+0x1a0/0x210 netif_receive_skb_list_internal+0x186/0x2a0 gro_normal_list.part.170+0x19/0x40 napi_complete_done+0x65/0x150 mlx5e_napi_poll+0x1ae/0x680 __napi_poll+0x25/0x120 net_rx_action+0x11e/0x280 __do_softirq+0xbb/0x271 irq_exit_rcu+0x97/0xa0 common_interrupt+0x7f/0xa0 </IRQ> asm_common_interrupt+0x1e/0x40 RIP: 0010:bpf_prog_1835a9241238291a_tw_egress+0x5/0xbac ? __cgroup_bpf_run_filter_skb+0x378/0x4e0 ? do_softirq+0x34/0x70 ? ip6_finish_output2+0x266/0x590 ? ip6_finish_output+0x66/0xa0 ? ip6_output+0x6c/0x130 ? ip6_xmit+0x279/0x550 ? ip6_dst_check+0x61/0xd0 [...] Using drgn [0] to dump the percpu buffer contents showed that on this CPU slot 0 is still available, but slots 1-7 are occupied and those tasks in slots 1-7 mostly don't exist any more. So we might have issues in bpf_cgroup_storage_unset(). Further debugging confirmed that there is a bug in bpf_cgroup_storage_unset(). Currently, it tries to unset "current" slot with searching from the start. So the following sequence is possible: 1. A task is running and claims slot 0 2. Running BPF program is done, and it checked slot 0 has the "task" and ready to reset it to NULL (not yet). 3. An interrupt happens, another BPF program runs and it claims slot 1 with the *same* task. 4. The unset() in interrupt context releases slot 0 since it matches "task". 5. Interrupt is done, the task in process context reset slot 0. At the end, slot 1 is not reset and the same process can continue to occupy slots 2-7 and finally, when the above step 1-5 is repeated again, step 3 BPF program won't be able to claim an empty slot and a warning will be issued. To fix the issue, for unset() function, we should traverse from the last slot to the first. This way, the above issue can be avoided. The same reverse traversal should also be done in bpf_get_local_storage() helper itself. Otherwise, incorrect local storage may be returned to BPF program. [0] https://github.com/osandov/drgn Fixes: b910eaaa ("bpf: Fix NULL pointer dereference in bpf_get_local_storage() helper") Signed-off-by:Yonghong Song <yhs@fb.com> Signed-off-by: