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

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 3 Nov 2022 11:23:08 +0000

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?

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