On 6/21/21 11:39 PM, Toke Høiland-Jørgensen wrote:
Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes:
On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
include/net/xdp_sock.h | 2 +-
kernel/bpf/cpumap.c | 13 +++++++----
kernel/bpf/devmap.c | 49 ++++++++++++++++++------------------------
net/core/filter.c | 28 ++++++++++++++++++++++++
net/xdp/xsk.c | 4 ++--
net/xdp/xsk.h | 4 ++--
net/xdp/xskmap.c | 29 ++++++++++++++-----------
7 files changed, 80 insertions(+), 49 deletions(-)
[...]
__dev_map_entry_free);
diff --git a/net/core/filter.c b/net/core/filter.c
index caa88955562e..0b7db5c70385 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
.arg2_type = ARG_ANYTHING,
};
+/* XDP_REDIRECT works by a three-step process, implemented in the functions
+ * below:
+ *
+ * 1. 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.
+ *
+ * 2. When the program returns the XDP_REDIRECT return code, the driver will
+ * call xdp_do_redirect() which will use the information in struct
+ * bpf_redirect_info to actually enqueue the frame into a map type-specific
+ * bulk queue structure.
+ *
+ * 3. 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, the RCU protection relies on everything happening
+ * inside a single NAPI poll sequence, which means it's between a pair of calls
+ * to local_bh_disable()/local_bh_enable().
+ *
+ * The map entries are marked as __rcu and the map code makes sure to
+ * dereference those pointers with rcu_dereference_check() in a way that works
+ * for both sections that to hold an rcu_read_lock() and sections that are
+ * called from NAPI without a separate rcu_read_lock(). The code below does not
+ * use RCU annotations, but relies on those in the map code.
One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
from cls_bpf_classify() then?
I believe so, yeah. Patch 2 in this series should even make lockdep stop
complaining about it :)
Btw, I was wondering whether we should just get rid of all the WARN_ON_ONCE()s
from those map helpers given in most situations these are not triggered anyway
due to retpoline avoidance where verifier rewrites the calls to jump to the map
backend implementation directly. One alternative could be to have an extension
to the bpf prologue generation under CONFIG_DEBUG_LOCK_ALLOC and call the lockdep
checks from there, but it's probably not worth the effort. (In the trampoline
case we have those __bpf_prog_enter()/__bpf_prog_enter_sleepable() where the
latter in particular has asserts like might_fault(), fwiw.)
I can add a patch removing the rcu_read_lock() from cls_bpf in the next
version.
It would also be great if this scenario in general could be placed
under the Documentation/RCU/whatisRCU.rst as an example, so we could
refer to the official doc on this, too, if Paul is good with this.
I'll take a look and see if I can find a way to fit it in there...
Could you also update the RCU comment in bpf_prog_run_xdp()? Or
alternatively move all the below driver comments in there as a single
location?
/* This code is invoked within a single NAPI poll cycle and thus under
* local_bh_disable(), which provides the needed RCU protection.
*/
Sure, can do. And yeah, I do agree that moving the comment in there
makes more sense than scattering it over all the drivers, even if that
means I have to go back and edit all the drivers again :P
Yeap, all of the above sounds good, thanks!
