Re: [PATCH bpf-next v1 13/14] selftests/bpf: Exercise arena arguments on every capable JIT

> diff --git a/tools/testing/selftests/bpf/progs/arena_kfunc.c b/tools/testing/selftests/bpf/progs/arena_kfunc.c
> index bf0d304e0e59c..8ee2bd4d66331 100644
> --- a/tools/testing/selftests/bpf/progs/arena_kfunc.c
> +++ b/tools/testing/selftests/bpf/progs/arena_kfunc.c

[ ... ]

> @@ -174,6 +190,10 @@ int arena_arg_mixed(void *ctx)
>  SEC("syscall")
>  __arch_x86_64
>  __arch_arm64
> +__arch_riscv64
> +__arch_s390x
> +__arch_loongarch
> +__arch_powerpc64
>  __success __retval(0)
>  int arena_arg_unpopulated(void *ctx)
>  {
> @@ -187,7 +207,7 @@ int arena_arg_unpopulated(void *ctx)
>  	 * and returns VM_FAULT_SIGBUS. That condition is diagnosed by
>  	 * bpf_prog_test_run_syscall() as -EFAULT propagated from ctx_access_ok().
>  	 */
> -	val = (u64 *)bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
> +	val = (u64 __arena *)bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
>  	if (!val)
>  		return 3;
>
> @@ -195,9 +215,13 @@ int arena_arg_unpopulated(void *ctx)

Can arena_arg_unpopulated run safely on the newly added architectures?

This test deliberately passes an unpopulated arena address to the kernel
(stash = (u64)val + PAGE_SIZE, then bpf_kfunc_arena_arg_test((u64 *)stash)).
The kernel dereferences this address in bpf_testmod.c:

  old = *val__arena;
  *val__arena = old + 1;

For this to work without crashing, the page fault handler must call
bpf_arena_handle_page_fault(), which installs the per-arena scratch page.

Looking at the fault handler integration, bpf_arena_handle_page_fault() is
only wired up on x86-64 (arch/x86/mm/fault.c) and arm64
(arch/arm64/mm/fault.c). The other architectures use the __weak stub that
always returns false, meaning the fault is never recovered.

Without a scratch page, the fault on an unmapped vmalloc address becomes
an unrecoverable kernel oops. The new JIT capability
(bpf_jit_supports_arena_kfunc_args) only covers rebasing the argument at
the call site, not recovering from unpopulated-page faults.

Should this test remain restricted to x86-64 and arm64, or does the
recovery path need to be implemented on the new architectures first?


[ ... ]

> @@ -221,6 +249,10 @@ int arena_arg_bad_reg(void *ctx)
>  SEC("syscall")
>  __arch_x86_64
>  __arch_arm64
> +__arch_riscv64
> +__arch_s390x
> +__arch_loongarch
> +__arch_powerpc64
>  __failure __msg("arena pointer cannot be a stack argument")
>  int arena_arg_stack(void *ctx)
>  {
> @@ -228,6 +260,10 @@ int arena_arg_stack(void *ctx)
>  	return bpf_kfunc_arena_stack_arg_test(1, 2, 3, 4, 5, (u64 *)1);
>  }

Will arena_arg_stack produce the expected failure message on the new arches?

The test expects the verifier to reject the program with "arena pointer
cannot be a stack argument". However, this message is only reachable after
the verifier checks whether the JIT supports stack arguments.

bpf_kfunc_arena_stack_arg_test() takes 6 parameters, which exceeds
MAX_BPF_FUNC_REG_ARGS (5). In kernel/bpf/verifier.c, gen_kfunc_arg_proto()
checks this first:

  if (nargs > MAX_BPF_FUNC_REG_ARGS && !bpf_jit_supports_stack_args()) {
      verbose(env, "JIT does not support kfunc %s() with %d args\n", ...);
      return -ENOTSUPP;
  }

Only after this gate does the per-argument loop inspect individual
arguments and emit "arena pointer cannot be a stack argument".

bpf_jit_supports_stack_args() returns true only on x86-64 and arm64. On
riscv64, s390, loongarch, and powerpc64, it returns false (the __weak stub
in kernel/bpf/core.c). So the program does fail to load on those arches,
but with "JIT does not support kfunc bpf_kfunc_arena_stack_arg_test() with
6 args" instead of the expected message.

test_loader's validate_msgs() will report this as a test failure since it
does not match the __msg annotation. Should this test stay limited to
x86-64 and arm64?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32538351576