[PATCH bpf-next v1 0/7] Fix timed may_goto with private stacks

[Siddharth Chintamaneni]

<SNIP>

> I think we can keep this fix local to x86. Would be less churn in the end.
>
> The problem is that x86 switches the BPF frame pointer from RBP to R9 for
> private stacks, but the trampoline still uses RBP. arm64 and powerpc64 keep
> using X25 and R31 respectively for both stack modes, so their trampolines
> already use the right frame pointer.
>
> Instead of changing the generic fixup, can we resolve the pointer in the x86 JIT
> when emitting the call to arch_bpf_timed_may_goto()? We already know whether
> priv_frame_ptr is set there. R10 contains the offset passed through BPF_REG_AX,
> so we can emit:
>
> /* Private stack */
> leaq (%r9, %r10), %r10
>
> /* Normal stack */
> leaq (%rbp, %r10), %r10
>
> Then remove the LEA from the x86 trampoline, as patch 2 already does.
>

make sense! I thought that the remaining arch's have a similar private
stacks approach to x86. I'll give it a try and re-spin the patch.

> This gives bpf_check_timed_may_goto() the same address used by the generated
> loads and stores: the BPF frame pointer plus the counter’s stack offset. For
> normal stacks, we just move the existing calculation into the JIT. For private
> stacks, we use R9 instead of RBP, which fixes the mismatch.
>
> The existing save/restore of R9 around the call should stay. We also leave RBP
> alone, since it is still needed for the native frame chain and unwinding.
>
> I haven’t tested this approach, so it still needs checking with both normal and
> private stacks.
>
> pw-bot: cr
>
> > Siddharth Chintamaneni (7):
> >   bpf: Fix timed may_goto stack pointer for private stacks
> >   bpf, x86: Use resolved pointer for timed may_goto
> >   bpf, arm64: Use resolved pointer for timed may_goto
> >   bpf, powerpc64: Use resolved pointer for timed may_goto
> >   bpf, riscv: Use resolved pointer for timed may_goto
> >   bpf, s390: Use resolved pointer for timed may_goto
> >   selftests/bpf: Test timed may_goto with private stacks
> >
> >  arch/arm64/net/bpf_timed_may_goto.S           | 12 ++------
> >  arch/powerpc/net/bpf_timed_may_goto.S         |  8 ++---
> >  arch/riscv/net/bpf_timed_may_goto.S           | 13 ++++----
> >  arch/s390/net/bpf_jit_comp.c                  |  6 ++--
> >  arch/s390/net/bpf_timed_may_goto.S            |  8 ++---
> >  arch/x86/net/bpf_timed_may_goto.S             |  6 ----
> >  kernel/bpf/fixups.c                           | 19 ++++++------
> >  .../bpf/progs/verifier_bpf_fastcall.c         | 30 ++++++++++---------
> >  .../selftests/bpf/progs/verifier_may_goto_1.c | 17 ++++++-----
> >  .../bpf/progs/verifier_private_stack.c        | 19 ++++++++++++
> >  10 files changed, 75 insertions(+), 63 deletions(-)
> >
> >
> > base-commit: d761934c9483ecde93fe99d8705282f716dfee50
>