On Wed, 12 Jul 2023 11:11:08 PDT (-0700), bjorn@xxxxxxxxxx wrote:
Song Shuai <suagrfillet@xxxxxxxxx> 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
