Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page

[Tejun Heo]

Hello,

> It should do set_pte_at() unconditionally.

I think two concerns are tangled here.

1. The racing write. set_pte_at() and the scratch installer's
ptep_try_set() hit the same PTE with no common lock. On x86-64 and arm64
set_pte_at() is a single atomic store, so it can't tear against the
cmpxchg, but a plain store racing a cmpxchg isn't atomic in general.
David, is that the worry - an arch where set_pte_at() is split and could
tear - or something else?

2. The SEGV. It's a BPF program failure propagating out as a SEGV. Maybe
not ideal, but as long as we surface the BPF error properly, it doesn't
necessarily seem broken to me.

Thanks.

--
tejun