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.
