[PATCH bpf-next v2 0/3] Add BPF Exceptions support for RISC-V

[Varun R Mallya <varunrmallya@gmail.com>]

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
apart from the ones that mix bpf-to-bpf calls and tailcalls pass.
This patch was tested using vmtest.sh to run the selftests.

test_exceptions_success:PASS:exceptions__open 0 nsec
libbpf: prog 'exception_tail_call': BPF program load failed: -EINVAL
libbpf: prog 'exception_tail_call': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; volatile int ret = 0; @ exceptions.c:106
0: (b4) w2 = 0                        ; R2=0
1: (63) *(u32 *)(r10 -4) = r2         ; R2=0 R10=fp0
; ret = exception_tail_call_subprog(ctx); @ exceptions.c:108
2: (85) call pc+4
caller:
 R10=fp0
callee:
 frame1: R1=ctx() R2=0 R10=fp0
7: frame1: R1=ctx() R10=fp0
; int exception_tail_call_subprog(struct __sk_buff *ctx) @ exceptions.c:96
7: (bf) r6 = r1                       ; frame1: R1=ctx() R6=ctx()
; volatile int ret = 10; @ exceptions.c:98
8: (b4) w1 = 10                       ; frame1: R1=10
9: (63) *(u32 *)(r10 -4) = r1         ; frame1: R1=10 R10=fp0 fp-8=mmmm????
; asm volatile("r1 = %[ctx]\n\t" @ bpf_helpers.h:169
10: (18) r7 = 0xff60000080df1800      ; frame1: R7=map_ptr(map=jmp_table,ks=4,vs=4)
12: (bf) r1 = r6                      ; frame1: R1=ctx() R6=ctx()
13: (bf) r2 = r7                      ; frame1: R2=map_ptr(map=jmp_table,ks=4,vs=4) R7=map_ptr(map=jmp_table,ks=4,vs=4)
14: (b7) r3 = 0                       ; frame1: R3=0
15: (85) call bpf_tail_call#12
mixing of tail_calls and bpf-to-bpf calls is not supported
processed 11 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'exception_tail_call': failed to load: -EINVAL
libbpf: failed to load object 'exceptions'
libbpf: failed to load BPF skeleton 'exceptions': -EINVAL
test_exceptions_success:FAIL:exceptions__load unexpected error: -22 (errno 22)
tester_init:PASS:tester_log_buf 0 nsec
process_subtest:PASS:obj_open_mem 0 nsec
process_subtest:PASS:specs_alloc 0 nsec
tester_init:PASS:tester_log_buf 0 nsec
process_subtest:PASS:obj_open_mem 0 nsec
process_subtest:PASS:specs_alloc 0 nsec
#113/1   exceptions/reject_exception_cb_type_1:OK
#113/2   exceptions/reject_exception_cb_type_2:OK
#113/3   exceptions/reject_exception_cb_type_3:OK
#113/4   exceptions/reject_exception_cb_type_4:OK
#113/5   exceptions/reject_exception_cb_type_5:OK
#113/6   exceptions/reject_async_callback_throw:OK
#113/7   exceptions/reject_with_lock:OK
#113/8   exceptions/reject_subprog_with_lock:OK
#113/9   exceptions/reject_with_rcu_read_lock:OK
#113/10  exceptions/reject_subprog_with_rcu_read_lock:OK
#113/11  exceptions/reject_with_rbtree_add_throw:OK
#113/12  exceptions/reject_with_reference:OK
#113/13  exceptions/reject_global_subprog_throw_with_reference:OK
#113/14  exceptions/reject_with_cb_reference:OK
#113/15  exceptions/reject_with_cb:OK
#113/16  exceptions/reject_with_subprog_reference:OK
#113/17  exceptions/reject_throwing_exception_cb:OK
#113/18  exceptions/reject_exception_cb_call_global_func:OK
#113/19  exceptions/reject_exception_cb_call_static_func:OK
#113/20  exceptions/reject_multiple_exception_cb:OK
#113/21  exceptions/reject_exception_throw_cb:OK
#113/22  exceptions/reject_exception_throw_cb_diff:OK
#113/23  exceptions/reject_subprog_rcu_lock_throw:OK
#113/24  exceptions/reject_subprog_throw_preempt_lock:OK
#113/25  exceptions/reject_subprog_throw_irq_lock:OK
#113/26  exceptions/reject_set_exception_cb_bad_ret1:OK
#113/27  exceptions/reject_set_exception_cb_bad_ret2:OK
#113/28  exceptions/reject_out_of_range_global_throw:OK
#113/29  exceptions/check_assert_eq_int_min:OK
#113/30  exceptions/check_assert_eq_int_max:OK
#113/31  exceptions/check_assert_eq_zero:OK
#113/32  exceptions/check_assert_eq_llong_min:OK
#113/33  exceptions/check_assert_eq_llong_max:OK
#113/34  exceptions/check_assert_lt_pos:OK
#113/35  exceptions/check_assert_lt_zero:OK
#113/36  exceptions/check_assert_lt_neg:OK
#113/37  exceptions/check_assert_le_pos:OK
#113/38  exceptions/check_assert_le_zero:OK
#113/39  exceptions/check_assert_le_neg:OK
#113/40  exceptions/check_assert_gt_pos:OK
#113/41  exceptions/check_assert_gt_zero:OK
#113/42  exceptions/check_assert_gt_neg:OK
#113/43  exceptions/check_assert_ge_pos:OK
#113/44  exceptions/check_assert_ge_zero:OK
#113/45  exceptions/check_assert_ge_neg:OK
#113/46  exceptions/check_assert_range_s64:OK
#113/47  exceptions/check_assert_range_u64:OK
#113/48  exceptions/check_assert_single_range_s64:OK
#113/49  exceptions/check_assert_single_range_u64:OK
#113/50  exceptions/check_assert_generic:OK
#113/51  exceptions/check_assert_with_return:OK
#113     exceptions:FAIL

All error logs:
test_exceptions_success:PASS:exceptions__open 0 nsec
libbpf: prog 'exception_tail_call': BPF program load failed: -EINVAL
libbpf: prog 'exception_tail_call': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; volatile int ret = 0; @ exceptions.c:106
0: (b4) w2 = 0                        ; R2=0
1: (63) *(u32 *)(r10 -4) = r2         ; R2=0 R10=fp0
; ret = exception_tail_call_subprog(ctx); @ exceptions.c:108
2: (85) call pc+4
caller:
 R10=fp0
callee:
 frame1: R1=ctx() R2=0 R10=fp0
7: frame1: R1=ctx() R10=fp0
; int exception_tail_call_subprog(struct __sk_buff *ctx) @ exceptions.c:96
7: (bf) r6 = r1                       ; frame1: R1=ctx() R6=ctx()
; volatile int ret = 10; @ exceptions.c:98
8: (b4) w1 = 10                       ; frame1: R1=10
9: (63) *(u32 *)(r10 -4) = r1         ; frame1: R1=10 R10=fp0 fp-8=mmmm????
; asm volatile("r1 = %[ctx]\n\t" @ bpf_helpers.h:169
10: (18) r7 = 0xff60000080df1800      ; frame1: R7=map_ptr(map=jmp_table,ks=4,vs=4)
12: (bf) r1 = r6                      ; frame1: R1=ctx() R6=ctx()
13: (bf) r2 = r7                      ; frame1: R2=map_ptr(map=jmp_table,ks=4,vs=4) R7=map_ptr(map=jmp_table,ks=4,vs=4)
14: (b7) r3 = 0                       ; frame1: R3=0
15: (85) call bpf_tail_call#12
mixing of tail_calls and bpf-to-bpf calls is not supported
processed 11 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'exception_tail_call': failed to load: -EINVAL
libbpf: failed to load object 'exceptions'
libbpf: failed to load BPF skeleton 'exceptions': -EINVAL
test_exceptions_success:FAIL:exceptions__load unexpected error: -22 (errno 22)

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()

[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


Varun R Mallya (3):
  riscv: stacktrace: Implement arch_bpf_stack_walk() for BPF
  riscv, bpf: Add support for BPF exceptions
  riscv, bpf: Remove BPF exceptions from BPF CI denylist

 arch/riscv/kernel/stacktrace.c               |  31 +++++
 arch/riscv/net/bpf_jit_comp64.c              | 115 ++++++++++++++++---
 tools/testing/selftests/bpf/DENYLIST.riscv64 |   1 -
 3 files changed, 133 insertions(+), 14 deletions(-)

-- 
2.54.0


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