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.
