Re: [PATCH bpf] bpf: fix bpf_dynptr_slice() returning ERR_PTR() on erro

["Toke Høiland-Jørgensen" <toke@kernel.org>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Thu, Nov 2, 2023 at 10:55 AM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>
>> Andrii Nakryiko <andrii@kernel.org> writes:
>>
>> > Let's fix it for real this time. It shouldn't just detect ERR_PTR()
>> > return from bpf_xdp_pointer(), but also turn that into NULL to follow
>> > bpf_dynptr_slice() contract.
>> >
>> > Fixes: 5426700e6841 ("bpf: fix bpf_dynptr_slice() to stop return an ERR_PTR.")
>> > Fixes: 66e3a13e7c2c ("bpf: Add bpf_dynptr_slice and bpf_dynptr_slice_rdwr")
>> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>> > ---
>> >  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 56b0c1f678ee..04049097176c 100644
>> > --- a/kernel/bpf/helpers.c
>> > +++ b/kernel/bpf/helpers.c
>> > @@ -2309,7 +2309,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
>> >       {
>> >               void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);
>> >               if (!IS_ERR_OR_NULL(xdp_ptr))
>> > -                     return xdp_ptr;
>> > +                     return NULL;
>>
>> Erm, the check in the if is inverted - so isn't this 'return xdp_ptr'
>> covering the case where bpf_xdp_pointer() *does* in fact return a valid
>> pointer?
>>
>
> Ah, you are right, I missed the ! part... Ok, then I don't think we
> have an issue, great. Thanks for double checking!
> Perhaps we should add a simple comment "/* we got a valid direct
> pointer, return it */", as this looks like an error-handling case.

Yup, totally agree it's confusing, I had to look at the code three or
four times as well just now, to be sure that it wasn't buggy. Adding a
comment would certainly be useful! :)

-Toke