Re: [PATCH bpf-next v2 2/3] bpf: Fix bpf_throw warning on 32-bit arch

Matthieu Baerts
2023-09-18T17:09:40Z

Hi Kumar,

On 18/09/2023 17:52, Kumar Kartikeya Dwivedi wrote:
> On 32-bit architectures, the pointer width is 32-bit, while we try to
> cast from a u64 down to it, the compiler complains on mismatch in
> integer size. Fix this by first casting to long which should match
> the pointer width on targets supported by Linux.

Thank you for the patch, it fixes the issue on our side!

(Not sure you need a tested by tag but just in case: )

Tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>

> Fixes: ec5290a178b7 ("bpf: Prevent KASAN false positive with bpf_throw")
> Reported-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
> kernel/bpf/helpers.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 7ff2a42f1996..dd1c69ee3375 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2488,7 +2488,7 @@ __bpf_kfunc void bpf_throw(u64 cookie)
> * deeper stack depths than ctx.sp as we do not return from bpf_throw,
> * which skips compiler generated instrumentation to do the same.
> */
> - kasan_unpoison_task_stack_below((void *)ctx.sp);
> + kasan_unpoison_task_stack_below((void *)(long)ctx.sp);
I never know what's the recommended way to fix such issues: casting it
to 'long' or 'unsigned long'? But it looks like both are used in the
kernel and 'long' is more often used than the other one so all good I
suppose.

> ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp);
> WARN(1, "A call to BPF exception callback should never return\n");
> }

Cheers,
Matt
--
Tessares | Belgium | Hybrid Access Solutions