Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

[Puranjay Mohan]

Hi Andy,


> >
> > I don't think implementing direct calls over call ops is fruitful for
> > RISC-V because once
> > the auipc/jalr can be patched atomically, the direct call trampoline
> > is always reachable.
>
> Yes, the auipc/jalr instruction pair can be patched atomically just as
> long as their size is naturally aligned on. However, we cannot prevent
> 2 instructions from being fetched atomically :P
>
> > Solving the atomic text patching problem would be fun!! I am eager to
> > see how it will be
> > solved.
>
> I have a patch series to solve the atomic code patching issue, which I
> am about to respin [1]. The idea is to solve it with yet another layer
> of indirection. We add a 8-B aligned space at each function entry. The
> space is a pointer to the ftrace entry. During boot, each function
> entry code is updated to perform a load and then take the jump from
> the 8-B space. When ftrace is disabled, we patch the first 4B-aligned
> instruction to a jump so as to skip the ftrace entry.
>
> We are still discussing with Alex to see if we have a better way to do
> it. If not then I'd update the patchset and re-send it. There's a
> pending improvement in the series to reduce complexity. The 8-B
> aligned space can be added before the function entry (just like your
> patch).

I briefly looked at the series and it looks promising. It looks similar to
how BPF programs jump to trampolines on ARM64. If the choice has to
be made about keeping the 8-B aligned space below or above the
function entry and both choices are viable then I would prefer keeping
the 8-B space below the function entry.

I am saying this because when nops are added above the function entry
clang's kcfi[1] doesn't work properly.

Thanks,
Puranjay

[1] https://clang.llvm.org/docs/ControlFlowIntegrity.html#fsanitize-kcfi