On Fri, Nov 4, 2022 at 3:43 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Thu, Nov 3, 2022 at 11:15 AM Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > On Thu, Nov 3, 2022 at 4:23 AM Russell King (Oracle) > > <linux@xxxxxxxxxxxxxxx> wrote: > > > > > > On Thu, Nov 03, 2022 at 05:21:16PM +0800, Yang Jihong 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. > > > > > > Isn't the purpose of this check to ensure that the entire pointer is > > > written, and BPF can't write half of it? > > > > > > > > > > case offsetof(struct __sk_buff, sk): > > > > - if (type == BPF_WRITE || size != sizeof(__u64)) > > > > - return false; > > > > > > Wouldn't "(size != sizeof(struct bpf_sock *) && size != sizeof(__u64))" > > > be more appropriate here, so 32-bit can only write the 32-bit pointer > > > or the full 64-bit value, and 64-bit can only write the 64-bit pointer? > > > Or is there a reason not to? bpf folk? > > > > You're correct. The patch is completely wrong. > > The bug is elsewhere. > > So I looked at this a bit (and replied to the old version of this > patch). What happens in the kernel is that we expect 64-bit load but > rewrite it to 32-bit load on 32-bit architectures (because we just use > sizeof(struct sk_buff, sk) which is 4 bytes on 32-bit arch. > > The problem here is that libbpf adjusts such pointer accesses from > 8-byte read to 4-byte reads for preserve_access_index (because libbpf > sees that pointer is really 4 byte long), which is what we actually > want in the general case. Here the assumption was made before CO-RE > that __sk_buff is a stable (and fake) UAPI and the correct BPF program > will access sk as a 64-bit pointer because BPF-side pointers always > appear as 64-bit. > > But from a correctness standpoint I think it should be fine to enable > both 32- and 64-bit loads for such pointers in __sk_buff for 32-bit > host arch. This will work well with CO-RE and will be correctly > rewritten to 32-bit or 64-bit accesses, depending on host > architecture. > > We should still reject 32-bit load on 64-bit host arch, though. Replied in the other thread as well :) The CO_RE screws up access here. Since it's a load of a pointer the verifier has to see it as a 8-byte load. When CO-RE converts it to 4 byte the verifier won't recognize it as a pointer load anymore. We cannot easily convert 64-bit BPF ISA into 32-bit. It's a massive amount of work.
