This patchset aims to add BPF exceptions supports for riscv64 by
implementing the arch_bpf_stack_walk function and then updating the
prologue and epilogue for BPF JIT on riscv. Also,
bpf_jit_supports_exceptions() returns true now so that the verifier does
not reject programs containing BPF exceptions on riscv64.

On riscv the unwinder used by arch_bpf_stack_walk() is the frame-pointer
unwinder, so exception support is gated on CONFIG_FRAME_POINTER.

In the prologue and epilogue of the RISC-V JIT, I saved the
return address and then the frame-pointer according to [1]. Also,
according to [2], s0 to s11 are callee saved registers, which is why a
new array (rv_exception_csave_regs) has been created to save these
registers which contains all the required registers along with the frame
pointer as well as return address.

The following demonstrates that all the selftests for BPF exceptions pass:
root@virtme-ng:~/bpf# ./test_progs -t exceptions
#114/1   exceptions/exception_throw_always_1:OK
#114/2   exceptions/exception_throw_always_2:OK
#114/3   exceptions/exception_throw_unwind_1:OK
#114/4   exceptions/exception_throw_unwind_2:OK
#114/5   exceptions/exception_throw_default:OK
#114/6   exceptions/exception_throw_default_value:OK
#114/7   exceptions/exception_tail_call:OK
#114/8   exceptions/exception_ext:OK
#114/9   exceptions/exception_ext_mod_cb_runtime:OK
#114/10  exceptions/exception_throw_subprog:OK
#114/11  exceptions/exception_assert_nz_gfunc:OK
#114/12  exceptions/exception_assert_zero_gfunc:OK
#114/13  exceptions/exception_assert_neg_gfunc:OK
#114/14  exceptions/exception_assert_pos_gfunc:OK
#114/15  exceptions/exception_assert_negeq_gfunc:OK
#114/16  exceptions/exception_assert_poseq_gfunc:OK
#114/17  exceptions/exception_assert_nz_gfunc_with:OK
#114/18  exceptions/exception_assert_zero_gfunc_with:OK
#114/19  exceptions/exception_assert_neg_gfunc_with:OK
#114/20  exceptions/exception_assert_pos_gfunc_with:OK
#114/21  exceptions/exception_assert_negeq_gfunc_with:OK
#114/22  exceptions/exception_assert_poseq_gfunc_with:OK
#114/23  exceptions/exception_bad_assert_nz_gfunc:OK
#114/24  exceptions/exception_bad_assert_zero_gfunc:OK
#114/25  exceptions/exception_bad_assert_neg_gfunc:OK
#114/26  exceptions/exception_bad_assert_pos_gfunc:OK
#114/27  exceptions/exception_bad_assert_negeq_gfunc:OK
#114/28  exceptions/exception_bad_assert_poseq_gfunc:OK
#114/29  exceptions/exception_bad_assert_nz_gfunc_with:OK
#114/30  exceptions/exception_bad_assert_zero_gfunc_with:OK
#114/31  exceptions/exception_bad_assert_neg_gfunc_with:OK
#114/32  exceptions/exception_bad_assert_pos_gfunc_with:OK
#114/33  exceptions/exception_bad_assert_negeq_gfunc_with:OK
#114/34  exceptions/exception_bad_assert_poseq_gfunc_with:OK
#114/35  exceptions/exception_assert_range:OK
#114/36  exceptions/exception_assert_range_with:OK
#114/37  exceptions/exception_bad_assert_range:OK
#114/38  exceptions/exception_bad_assert_range_with:OK
#114/39  exceptions/exception_throw_from_void_global:OK
#114/40  exceptions/non-throwing fentry -> exception_cb:OK
#114/41  exceptions/throwing fentry -> exception_cb:OK
#114/42  exceptions/non-throwing fexit -> exception_cb:OK
#114/43  exceptions/throwing fexit -> exception_cb:OK
#114/44  exceptions/throwing extension (with custom cb) -> exception_cb:OK
#114/45  exceptions/throwing extension -> global func in exception_cb:OK
#114/46  exceptions/exception_ext_mod_cb_runtime:OK
#114/47  exceptions/throwing extension (with custom cb) -> global func in exception_cb:OK
#114/48  exceptions/exception_ext:OK
#114/49  exceptions/non-throwing fentry -> non-throwing subprog:OK
#114/50  exceptions/throwing fentry -> non-throwing subprog:OK
#114/51  exceptions/non-throwing fentry -> throwing subprog:OK
#114/52  exceptions/throwing fentry -> throwing subprog:OK
#114/53  exceptions/non-throwing fexit -> non-throwing subprog:OK
#114/54  exceptions/throwing fexit -> non-throwing subprog:OK
#114/55  exceptions/non-throwing fexit -> throwing subprog:OK
#114/56  exceptions/throwing fexit -> throwing subprog:OK
#114/57  exceptions/non-throwing fmod_ret -> non-throwing subprog:OK
#114/58  exceptions/non-throwing fmod_ret -> non-throwing global subprog:OK
#114/59  exceptions/non-throwing extension -> non-throwing subprog:OK
#114/60  exceptions/non-throwing extension -> throwing subprog:OK
#114/61  exceptions/non-throwing extension -> non-throwing subprog:OK
#114/62  exceptions/non-throwing extension -> throwing global subprog:OK
#114/63  exceptions/throwing extension -> throwing global subprog:OK
#114/64  exceptions/throwing extension -> non-throwing global subprog:OK
#114/65  exceptions/non-throwing extension -> main subprog:OK
#114/66  exceptions/throwing extension -> main subprog:OK
#114/67  exceptions/reject_exception_cb_type_1:OK
#114/68  exceptions/reject_exception_cb_type_2:OK
#114/69  exceptions/reject_exception_cb_type_3:OK
#114/70  exceptions/reject_exception_cb_type_4:OK
#114/71  exceptions/reject_exception_cb_type_5:OK
#114/72  exceptions/reject_async_callback_throw:OK
#114/73  exceptions/reject_with_lock:OK
#114/74  exceptions/reject_subprog_with_lock:OK
#114/75  exceptions/reject_with_rcu_read_lock:OK
#114/76  exceptions/reject_subprog_with_rcu_read_lock:OK
#114/77  exceptions/reject_with_rbtree_add_throw:OK
#114/78  exceptions/reject_with_reference:OK
#114/79  exceptions/reject_global_subprog_throw_with_reference:OK
#114/80  exceptions/reject_with_cb_reference:OK
#114/81  exceptions/reject_with_cb:OK
#114/82  exceptions/reject_with_subprog_reference:OK
#114/83  exceptions/reject_throwing_exception_cb:OK
#114/84  exceptions/reject_exception_cb_call_global_func:OK
#114/85  exceptions/reject_exception_cb_call_static_func:OK
#114/86  exceptions/reject_multiple_exception_cb:OK
#114/87  exceptions/reject_exception_throw_cb:OK
#114/88  exceptions/reject_exception_throw_cb_diff:OK
#114/89  exceptions/reject_subprog_rcu_lock_throw:OK
#114/90  exceptions/reject_subprog_throw_preempt_lock:OK
#114/91  exceptions/reject_subprog_throw_irq_lock:OK
#114/92  exceptions/reject_set_exception_cb_bad_ret1:OK
#114/93  exceptions/reject_set_exception_cb_bad_ret2:OK
#114/94  exceptions/reject_out_of_range_global_throw:OK
#114/95  exceptions/check_assert_eq_int_min:OK
#114/96  exceptions/check_assert_eq_int_max:OK
#114/97  exceptions/check_assert_eq_zero:OK
#114/98  exceptions/check_assert_eq_llong_min:OK
#114/99  exceptions/check_assert_eq_llong_max:OK
#114/100 exceptions/check_assert_lt_pos:OK
#114/101 exceptions/check_assert_lt_zero:OK
#114/102 exceptions/check_assert_lt_neg:OK
#114/103 exceptions/check_assert_le_pos:OK
#114/104 exceptions/check_assert_le_zero:OK
#114/105 exceptions/check_assert_le_neg:OK
#114/106 exceptions/check_assert_gt_pos:OK
#114/107 exceptions/check_assert_gt_zero:OK
#114/108 exceptions/check_assert_gt_neg:OK
#114/109 exceptions/check_assert_ge_pos:OK
#114/110 exceptions/check_assert_ge_zero:OK
#114/111 exceptions/check_assert_ge_neg:OK
#114/112 exceptions/check_assert_range_s64:OK
#114/113 exceptions/check_assert_range_u64:OK
#114/114 exceptions/check_assert_single_range_s64:OK
#114/115 exceptions/check_assert_single_range_u64:OK
#114/116 exceptions/check_assert_generic:OK
#114/117 exceptions/check_assert_with_return:OK
#114     exceptions:OK
Summary: 1/117 PASSED, 0 SKIPPED, 0 FAILED

Since all exception tests pass now, the denylist file has been removed
as it is empty now.

Changes in v1->v2:
v1: https://lore.kernel.org/bpf/20260621144259.288135-1-varunrmallya@gmail.com/
- resolve the ftrace trampoline address with ftrace_graph_ret_addr()
  inside arch_bpf_stack_walk()
- Remove duplicated code in bpf_jit_build_prologue()

Changes in v2->v3:
v2: https://lore.kernel.org/bpf/20260628081710.113333-1-varunrmallya@gmail.com/
- extend walk_stackframe to use sp/fp similar to how arm64 does it. (Björn Töpel)
- refactor prologue/epilogue into helpers. (Björn Töpel)
- fix comments to make them netdev style.
- save/restore the tail-call counter in the exception prologue/epilogue
- exception_tail_call now passes since bpf2bpf+tailcall mixing landed [3], so
  drop the (now-empty) DENYLIST.riscv64
- (!aux->exception_cb) has been simplified to only one condition (Pu Lehui)

[1]: https://riscv-non-isa.github.io/riscv-elf-psabi-doc/#_frame_pointer_convention
[2]: https://riscv-non-isa.github.io/riscv-elf-psabi-doc/#_integer_register_convention
[3]: https://lore.kernel.org/bpf/bf647816-d292-419c-b79f-fc048122aebc@huaweicloud.com

Varun R Mallya (6):
  riscv: Use arch_stack_walk() for the kernel callchain
  riscv, stacktrace: Pass sp and fp to the walk_stackframe() callback
  riscv, stacktrace: Implement arch_bpf_stack_walk() for BPF
  riscv, bpf: Split prologue and epilogue into helper functions
  riscv, bpf: Add support for BPF exceptions
  riscv, bpf: Remove BPF exceptions from denylist

 arch/riscv/include/asm/stacktrace.h          |   5 +-
 arch/riscv/kernel/perf_callchain.c           |   3 +-
 arch/riscv/kernel/stacktrace.c               |  64 ++++-
 arch/riscv/net/bpf_jit_comp64.c              | 254 ++++++++++++++-----
 tools/testing/selftests/bpf/DENYLIST.riscv64 |   2 -
 5 files changed, 253 insertions(+), 75 deletions(-)
 delete mode 100644 tools/testing/selftests/bpf/DENYLIST.riscv64

-- 
2.55.0

Varun R Mallya <varunrmallya@gmail.com> writes:

> This patchset aims to add BPF exceptions supports for riscv64 by
> implementing the arch_bpf_stack_walk function and then updating the
> prologue and epilogue for BPF JIT on riscv. Also,
> bpf_jit_supports_exceptions() returns true now so that the verifier does
> not reject programs containing BPF exceptions on riscv64.
>
> On riscv the unwinder used by arch_bpf_stack_walk() is the frame-pointer
> unwinder, so exception support is gated on CONFIG_FRAME_POINTER.
>
> In the prologue and epilogue of the RISC-V JIT, I saved the
> return address and then the frame-pointer according to [1]. Also,
> according to [2], s0 to s11 are callee saved registers, which is why a
> new array (rv_exception_csave_regs) has been created to save these
> registers which contains all the required registers along with the frame
> pointer as well as return address.

...

> Since all exception tests pass now, the denylist file has been removed
> as it is empty now.
>
> Changes in v1->v2:
> v1: https://lore.kernel.org/bpf/20260621144259.288135-1-varunrmallya@gmail.com/
> - resolve the ftrace trampoline address with ftrace_graph_ret_addr()
>   inside arch_bpf_stack_walk()
> - Remove duplicated code in bpf_jit_build_prologue()
>
> Changes in v2->v3:
> v2: https://lore.kernel.org/bpf/20260628081710.113333-1-varunrmallya@gmail.com/
> - extend walk_stackframe to use sp/fp similar to how arm64 does it. (Björn Töpel)
> - refactor prologue/epilogue into helpers. (Björn Töpel)
> - fix comments to make them netdev style.
> - save/restore the tail-call counter in the exception prologue/epilogue
> - exception_tail_call now passes since bpf2bpf+tailcall mixing landed [3], so
>   drop the (now-empty) DENYLIST.riscv64
> - (!aux->exception_cb) has been simplified to only one condition (Pu Lehui)

Varun!

Thanks for the rework. The overall structure is much better now, and the
main v2 comments around reusing the stack walker and splitting the
prologue/epilogue have been addressed!

I tried to make sense of Sashiko's reports as well...

Sashiko seems correct about sp being stale after crossing an exception
frame. We restore pc and fp from pt_regs, so sp should be restored from
there as well. Would you agree?

The full pt_regs should also be validated before dereferencing it...?

I *think* the instrumentation concern also looks valid. The new
stack-walk adapter callbacks should not be instrumentable.

I don't think the !CONFIG_FRAME_POINTER report is an exception-support
bug. Exceptions require a valid boundary frame pointer, so the current
gating looks correct.

Sashiko is also correct that deleting DENYLIST.riscv64 breaks the
documented selftest command. Please update the README or retain an empty
file.

Minor wording issue: ra is not callee-saved. Please say “RA and the
callee-saved registers s0-s11.”


Thanks,
Björn