Re: [PATCH 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture

[Alexei Starovoitov]

On Fri, Nov 4, 2022 at 2:56 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Nov 3, 2022 at 1:36 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> >
> > The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
> > This is because bpf_object__relocate modifies the instruction to change memory
> > size to 4 bytes, as shown in the following messages:
> >
> > libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
> >
> > As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
> > unnecessary checks need to be deleted.
> >
> > Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> > ---
> >  net/core/filter.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index bb0136e7a8e4..eab7ce89740c 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -8269,8 +8269,6 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
> >                         return false;
> >                 break;
> >         case offsetof(struct __sk_buff, sk):
> > -               if (type == BPF_WRITE || size != sizeof(__u64))
> > -                       return false;
>
> this probably should be specific to host architecture bitness? I'd
> imagine that size = 4 should be invalid on 64-bit arches (reading half
> of the pointer is bad)

Not quite.
In __sk_buff the field 'sk' is defined as:
__bpf_md_ptr(struct bpf_sock *, sk);
so it's always 64-bit load when bpf prog reads it.
In this case CO_RE shouldn't have been applied to uapi struct __sk_buff.