RE: [PATCH bpf v2 2/4] bpf: map_poke_descriptor is being called with an unstable poke_tab[]

John Fastabend wrote:

When populating poke_tab[] of a subprog we call map_poke_track() after
doing bpf_jit_add_poke_descriptor(). But, bpf_jit_add_poke_descriptor()
may, likely will, realloc the poke_tab[] structure and free the old
one. So that prog->aux->poke_tab is not stable. However, the aux pointer
is referenced from bpf_array_aux and poke_tab[] is used to 'track'
prog<->map link. This way when progs are released the entry in the
map is dropped and vice versa when the map is released we don't drop
it too soon if a prog is in the process of calling it.

I wasn't able to trigger any errors here, for example having map_poke_run
run with a poke_tab[] pointer that was free'd from
bpf_jit_add_poke_descriptor(), but it looks possible and at very least
is very fragile.

This patch moves poke_track call out of loop that is calling add_poke
so that we only ever add stable aux->poke_tab pointers to the map's
bpf_array_aux struct. Further, we need this in the next patch to fix
a real bug where progs are not 'untracked'.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
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Needs a fixes tag,

Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")