Messages in this thread | | | Subject | Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture | From | Yang Jihong <> | Date | Mon, 7 Nov 2022 17:22:04 +0800 |
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Hello,
On 2022/11/5 7:37, Alexei Starovoitov wrote: > On Fri, Nov 4, 2022 at 3:43 PM Andrii Nakryiko > <andrii.nakryiko@gmail.com> wrote: >> >> On Thu, Nov 3, 2022 at 11:15 AM Alexei Starovoitov >> <alexei.starovoitov@gmail.com> wrote: >>> >>> On Thu, Nov 3, 2022 at 4:23 AM Russell King (Oracle) >>> <linux@armlinux.org.uk> 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. > .
From the above discussion, there are two different solutions: 1. Modify bpf_skb_is_valid_access to ensure that 32-bit can only load the 32-bit pointer or the full 64-bit value, and 64-bit can only load the 64-bit pointer 2. Modify libbpf, CO_RE skips adjust load's mem size and retains the 8-byte load. The two fixes will be added in the next version. Please review the solution to be selected.
Thanks, Yang
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