On 13 Jun 2019, at 4:17, Toke Høiland-Jørgensen wrote:
When using the bpf_redirect_map() helper to redirect packets from XDP,
the eBPF
program cannot currently know whether the redirect will succeed, which
makes it
impossible to gracefully handle errors. To properly fix this will
probably
require deeper changes to the way TX resources are allocated, but one
thing that
is fairly straight forward to fix is to allow lookups into devmaps, so
programs
can at least know when a redirect is *guaranteed* to fail because
there is no
entry in the map. Currently, programs work around this by keeping a
shadow map
of another type which indicates whether a map index is valid.
This series contains two changes that are complementary ways to fix
this issue:
- Moving the map lookup into the bpf_redirect_map() helper (and
caching the
result), so the helper can return an error if no value is found in
the map.
This includes a refactoring of the devmap and cpumap code to not
care about
the index on enqueue.
- Allowing regular lookups into devmaps from eBPF programs, using the
read-only
flag to make sure they don't change the values.
The performance impact of the series is negligible, in the sense that
I cannot
measure it because the variance between test runs is higher than the
difference
pre/post series.
Changelog:
v4:
- Fix a few nits from Andrii
- Lose the #defines in bpf.h and just compare the flags argument
directly to
XDP_TX in bpf_xdp_redirect_map().
v3:
- Adopt Jonathan's idea of using the lower two bits of the flag
value as the
return code.
- Always do the lookup, and cache the result for use in
xdp_do_redirect(); to
achieve this, refactor the devmap and cpumap code to get rid the
bitmap for
selecting which devices to flush.
v2:
- For patch 1, make it clear that the change works for any map type.
- For patch 2, just use the new BPF_F_RDONLY_PROG flag to make the
return
value read-only.
Acked-by: Jonathan Lemon <jonathan.le...@gmail.com>
