Re: [PATCH bpf-next v2 1/5] bpf: Avoid per-CPU private stack in preemptible tracing progs

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] selftests/bpf: raw_tp tests in verifier_private_stack no longer
  test priv stack
--

commit 173ae153d8d266fe351d0833f08f2920b88ec9ce
Author: Mike Bazov <mikeba@sentinelone.com>

bpf: Avoid per-CPU private stack in preemptible tracing progs

This commit disables per-CPU private stacks for tracing, raw_tp, and
tracepoint programs because they run preemptibly and can be entered by a
second task on the same CPU, and updates check_recursion() and the BPF
selftests accordingly.

> diff --git a/tools/testing/selftests/bpf/progs/verifier_private_stack.c b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> index e7f410a74a21b..bebdb0aff23fd 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> @@ -127,6 +127,23 @@ __naked void no_private_stack_kprobe(void)
>  "	::: __clobber_all);
>  }
>  
> +SEC("fentry/bpf_fentry_test9")
> +__description("No private stack, fentry")
> +__success
> +__arch_x86_64
> +__jited("	subq	$0x100, %rsp")
> +__arch_arm64
> +__jited("	sub	sp, sp, #0x100")
> +__naked void no_private_stack_fentry(void)
> +{
> +	asm volatile ("			\
> +	r1 = 42;			\
> +	*(u64 *)(r10 - 256) = r1;	\
> +	r0 = 0;				\
> +	exit;				\
> +"	::: __clobber_all);
> +}
> +
>  SEC("uprobe")
>  __description("No private stack, uprobe")
>  __success

[Severity: Low]
Should no_private_stack_nested() and no_private_stack_stack_args() above
this hunk also be switched from SEC("raw_tp") to a program type that is
still eligible for a private stack, such as SEC("kprobe.multi") or
SEC("perf_event")?

With raw_tp returning NO_PRIV_STACK from bpf_enable_priv_stack(),
check_max_stack_depth() now skips both tests on priv_stack_mode !=
PRIV_STACK_ADAPTIVE before reaching the 64-byte BPF_PRIV_STACK_MIN_SIZE
threshold or the x86-64 si[s].stack_arg_cnt check they were written to
cover:

tools/testing/selftests/bpf/progs/verifier_private_stack.c:
    SEC("raw_tp")
    __description("No private stack")
    ...
    __naked void no_private_stack_nested(void)
    ...
    /* x86-64 has one register for the private stack and for the sixth argument */
    SEC("raw_tp")
    __description("No private stack, stack arguments")
    ...
    __naked void no_private_stack_stack_args(void)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111-bpf-preempt-v2-r2-v2-0-9d8f6b952ad2@sentinelone.com?part=1