Re: [PATCH] arm64: bpf: Fix branch offset in JIT

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Luke, 

On Mon, Sep 14, 2020 at 11:21:58AM -0700, Luke Nelson wrote:
> On Mon, Sep 14, 2020 at 11:08 AM Xi Wang <xi.wang@gmail.com> wrote:
> > I don't think there's some consistent semantics of "offsets" across
> > the JITs of different architectures (maybe it's good to clean that
> > up).  RV64 and RV32 JITs are doing something similar to arm64 with
> > respect to offsets.  CCing Björn and Luke.
> 
> As I understand it, there are two strategies JITs use to keep track of
> the ctx->offset table.
> 
> Some JITs (RV32, RV64, arm32, arm64 currently, x86-32) track the end
> of each instruction (e.g., ctx->offset[i] marks the beginning of
> instruction i + 1).
> This requires care to handle jumps to the first instruction to avoid
> using ctx->offset[-1]. The RV32 and RV64 JITs have special handling
> for this case,
> while the arm32, arm64, and x86-32 JITs appear not to. The arm32 and
> x32 probably need to be fixed for the same reason arm64 does.
> 
> The other strategy is for ctx->offset[i] to track the beginning of
> instruction i. The x86-64 JIT currently works this way.
> This can be easier to use (no need to special case -1) but looks to be
> trickier to construct. This patch changes the arm64 JIT to work this
> way.
> 
> I don't think either strategy is inherently better, both can be
> "correct" as long as the JIT uses ctx->offset in the right way.
> This might be a good opportunity to change the JITs to be consistent
> about this (especially if the arm32, arm64, and x32 JITs all need to
> be fixed anyways).
> Having all JITs agree on the meaning of ctx->offset could help future
> readers debug / understand the code, and could help to someday verify
> the
> ctx->offset construction.
> 
> Any thoughts?

The common strategy does make a lot of sense and yes, both patches will  works 
assuming the ctx->offset ends up being what the JIT engine expects it to be. 
As I mentioned earlier we did consider both, but ended up using the later, 
since as you said, removes the need for handling the special (-1) case.

Cheers
/Ilias

> 
> - Luke