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authorToke Høiland-Jørgensen <toke@redhat.com>2021-06-24 18:05:55 +0200
committerDaniel Borkmann <daniel@iogearbox.net>2021-06-24 19:41:15 +0200
commit782347b6bcad07ddb574422e01e22c92e05928c8 (patch)
treece258f24b534edad6e5409ed4252529873d1294d /include/linux/filter.h
parent694cea395fded425008e93cd90cfdf7a451674af (diff)
xdp: Add proper __rcu annotations to redirect map entries
XDP_REDIRECT works by a three-step process: the bpf_redirect() and bpf_redirect_map() helpers will lookup the target of the redirect and store it (along with some other metadata) in a per-CPU struct bpf_redirect_info. Next, when the program returns the XDP_REDIRECT return code, the driver will call xdp_do_redirect() which will use the information thus stored to actually enqueue the frame into a bulk queue structure (that differs slightly by map type, but shares the same principle). Finally, before exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will flush all the different bulk queues, thus completing the redirect. Pointers to the map entries will be kept around for this whole sequence of steps, protected by RCU. However, there is no top-level rcu_read_lock() in the core code; instead drivers add their own rcu_read_lock() around the XDP portions of the code, but somewhat inconsistently as Martin discovered[0]. However, things still work because everything happens inside a single NAPI poll sequence, which means it's between a pair of calls to local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could document this intention by using rcu_dereference_check() with rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and lockdep to verify that everything is done correctly. This patch does just that: we add an __rcu annotation to the map entry pointers and remove the various comments explaining the NAPI poll assurance strewn through devmap.c in favour of a longer explanation in filter.c. The goal is to have one coherent documentation of the entire flow, and rely on the RCU annotations as a "standard" way of communicating the flow in the map code (which can additionally be understood by sparse and lockdep). The RCU annotation replacements result in a fairly straight-forward replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE() becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the proper constructs to cast the pointer back and forth between __rcu and __kernel address space (for the benefit of sparse). The one complication is that xskmap has a few constructions where double-pointers are passed back and forth; these simply all gain __rcu annotations, and only the final reference/dereference to the inner-most pointer gets changed. With this, everything can be run through sparse without eliciting complaints, and lockdep can verify correctness even without the use of rcu_read_lock() in the drivers. Subsequent patches will clean these up from the drivers. [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/ [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/ Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20210624160609.292325-6-toke@redhat.com
Diffstat (limited to 'include/linux/filter.h')
-rw-r--r--include/linux/filter.h8
1 files changed, 3 insertions, 5 deletions
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 688856e0b28a..472f97074da0 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -763,11 +763,9 @@ DECLARE_BPF_DISPATCHER(xdp)
static __always_inline u32 bpf_prog_run_xdp(const struct bpf_prog *prog,
struct xdp_buff *xdp)
{
- /* Caller needs to hold rcu_read_lock() (!), otherwise program
- * can be released while still running, or map elements could be
- * freed early while still having concurrent users. XDP fastpath
- * already takes rcu_read_lock() when fetching the program, so
- * it's not necessary here anymore.
+ /* Driver XDP hooks are invoked within a single NAPI poll cycle and thus
+ * under local_bh_disable(), which provides the needed RCU protection
+ * for accessing map entries.
*/
return __BPF_PROG_RUN(prog, xdp, BPF_DISPATCHER_FUNC(xdp));
}