On 2/24/21 10:30 PM, KP Singh wrote:
I checked with Will about it and learnt that ARM64 does support
patching certain instructions (e.g. branch, brk, nops) using
aarch64_insn_patch_text_nosync, it's used in ftrace:
https://elixir.bootlin.com/linux/latest/source/arch/arm64/kernel/ftrace.c#L24
But one has to tolerate that not all CPUs will execute these
instructions until a context synchronization happens due to an
exception or an ISB instruction. But I think we can start
with the same thing that FTrace does?
Is there any downside or road blocker for a aarch64_insn_patch_text_sync()
variant which would then trigger an explicit isb()? Presumably to perform
this reliably at that point you would end up at aarch64_insn_patch_text()
which needs the brute force stop CPU, right? I guess my noob question is
what happens if, for example, an old JITed BPF prog got freed (RCU) and the
JIT mem got reused for something else in meantime when patching JMP->NOP
via aarch64_insn_patch_text_nosync() and not all CPUs did a context sync,
is such scenario/worry realistic?
On Wed, Feb 24, 2021 at 10:01 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
On 2/24/21 8:54 PM, Luigi Rizzo wrote:
I prepared a BPF version of kstats[1]
https://github.com/luigirizzo/lr-cstats
that uses fentry/fexit hooks to monitor the execution time
of a kernel function.
I hoped to have it working on ARM64 too, but it looks like
arch_prepare_bpf_trampoline() only exists for x86.
Is there any outstanding patch for this function on ARM64,
or any similar function I could look at to implement it myself ?
Not that I'm currently aware of, arm64 support would definitely be great
to have. From x86 side, the underlying arch dependency was basically on
text_poke_bp() to patch instructions on a live kernel. Haven't checked
recently whether an equivalent exists on arm64 yet, but perhaps Will
might know.
[1] kstats is an in-kernel also in the above repo and previously
discussed at https://lwn.net/Articles/813303/
