[PATCH V11 0/5] riscv: Optimize function trace

[Palmer Dabbelt]

On Wed, 12 Jul 2023 11:11:08 PDT (-0700), bjorn at kernel.org wrote:
> Song Shuai <suagrfillet at gmail.com> writes:
>
> [...]
>
>> Add WITH_DIRECT_CALLS support [3] (patch 3, 4)
>> ==============================================
>
> We've had some offlist discussions, so here's some input for a wider
> audience! Most importantly, this is for Palmer, so that this series is
> not merged until a proper BPF trampoline fix is in place.
>
> Note that what's currently usable from BPF trampoline *works*. It's
> when this series is added that it breaks.
>
> TL;DR This series adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS, which enables
> fentry/fexit BPF trampoline support. Unfortunately the
> fexit/BPF_TRAMP_F_SKIP_FRAME parts of the RV BPF trampoline breaks
> with this addition, and need to be addressed *prior* merging this
> series. An easy way to reproduce, is just calling any of the kselftest
> tests that uses fexit patching.
>
> The issue is around the nop seld, and how a call is done; The nop sled
> (patchable-function-entry) size changed from 16B to 8B in commit
> 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half"), but
> BPF code still uses the old 16B. So it'll work for BPF programs, but not
> for regular kernel functions.
>
> An example:
>
>   | ffffffff80fa4150 <bpf_fentry_test1>:
>   | ffffffff80fa4150:       0001                    nop
>   | ffffffff80fa4152:       0001                    nop
>   | ffffffff80fa4154:       0001                    nop
>   | ffffffff80fa4156:       0001                    nop
>   | ffffffff80fa4158:       1141                    add     sp,sp,-16
>   | ffffffff80fa415a:       e422                    sd      s0,8(sp)
>   | ffffffff80fa415c:       0800                    add     s0,sp,16
>   | ffffffff80fa415e:       6422                    ld      s0,8(sp)
>   | ffffffff80fa4160:       2505                    addw    a0,a0,1
>   | ffffffff80fa4162:       0141                    add     sp,sp,16
>   | ffffffff80fa4164:       8082                    ret
>
> is patched to:
>
>   | ffffffff80fa4150:  f70c0297                     auipc   t0,-150208512
>   | ffffffff80fa4154:  eb0282e7                     jalr    t0,t0,-336
>
> The return address to bpf_fentry_test1 is stored in t0 at BPF
> trampoline entry. Return to the *parent* is in ra. The trampline has
> to deal with this.
>
> For BPF_TRAMP_F_SKIP_FRAME/CALL_ORIG, the BPF trampoline will skip too
> many bytes, and not correctly handle parent calls.
>
> Further; The BPF trampoline currently has a different way of patching
> the nops for BPF programs, than what ftrace does. That should be changed
> to match what ftrace does (auipc/jalr t0).
>
> To summarize:
>  * Align BPF nop sled with patchable-function-entry: 8B.
>  * Adapt BPF trampoline for 8B nop sleds.
>  * Adapt BPF trampoline t0 return, ra parent scheme.

Thanks for digging into this one, I agree we need to sort out the BPF 
breakages before we merge this.  Sounds like there's a rabbit hole here, 
but hopefully we can get it sorted out.

I've dropped this from patchwork and such, as we'll need at least 
another spin.

> Cheers,
> Björn