Re: [PATCH bpf-next v2] bpf: Upgrade bpf_{g,s}etsockopt return values

[Ji Rongfeng]

On 2022/12/7 19:19, Ji Rongfeng wrote:
> On 2022/12/7 2:36, Martin KaFai Lau wrote:
> > On 12/2/22 9:39 AM, Ji Rongfeng wrote:
> > > Returning -EINVAL almost all the time when error occurs is not very
> > > helpful for the bpf prog to figure out what is wrong. This patch
> > > upgrades some return values so that they will be much more helpful.
> > >
> > > * return -ENOPROTOOPT when optname is unsupported
> > >
> > >    The same as {g,s}etsockopt() syscall does. Before this patch,
> > >    bpf_setsockopt(TCP_SAVED_SYN) already returns -ENOPROTOOPT, which
> > >    may confuse the user, as -EINVAL is returned on other unsupported
> > >    optnames. This patch also rejects TCP_SAVED_SYN right in
> > >    sol_tcp_sockopt() when getopt is false, since do_tcp_setsockopt()
> > >    is just the executor and it's not its duty to discover such error
> > >    in bpf. We should maintain a precise allowlist to control whether
> > >    an optname is supported and allowed to enter the executor or not.
> > >    Functions like do_tcp_setsockopt(), their behaviour are not fully
> > >    controllable by bpf. Imagine we let an optname pass, expecting
> > >    -ENOPROTOOPT will be returned, but someday that optname is
> > >    actually processed and unfortunately causes deadlock when calling
> > >    from bpf. Thus, precise access control is essential.
> >
> > Please leave the current -EINVAL to distinguish between optnames 
> > rejected by bpf and optnames rejected by the do_*_{get,set}sockopt().
>
> To reach that goal, it would be better for us to pick a value other than 
> -ENOPROTOOPT or -EINVAL. This patch actually makes sk-related errors, 
> level-reletad errors, optname-related errors and opt{val,len}-related 
> errors distinguishable, as they should be, by leaving -EINVAL to 
> opt{val,len}-related errors only. man setsockopt:
>
>  > EINVAL optlen invalid in setsockopt().  In some cases this error
>  >        can also occur for an invalid value in optval (e.g., for
>  >        the IP_ADD_MEMBERSHIP option described in ip(7)).
>
> With an unique return value, the bpf prog developer will be able to know 
> that the error is "unsupported or unknown optname" for sure, saving time 
> on figuring the actual cause of the error. In production environment, 
> the bpf prog will be able to test whether an optname is available in 
> current bpf env and decide what to do next also, which is very useful.
>
> > > * return -EOPNOTSUPP on level-related errors
> > >
> > >    In do_ip_getsockopt(), -EOPNOTSUPP will be returned if level !=
> > >    SOL_IP. In ipv6_getsockopt(), -ENOPROTOOPT will be returned if
> > >    level != SOL_IPV6. To be distinguishable, the former is chosen.
> >
> > I would leave this one as is also.  Are you sure the do_ip_*sockopt 
> > cannot handle sk_family == AF_INET6?  afaict, bpf is rejecting those 
> > optnames instead.
>
> -EOPNOTSUPP is just picked here as an unique return value representing 
> "unknown level or unsupported sk_family or mismatched protocol in 
> bpf_{g,s}etsockopt()". I'm ok if you want to pick another unique value 
> for them or pick three unique values for each type of error : )

Sorry, I meant "three unique values for three types of error", which is 
growing more and more sensible in my mind as I'm thinking about it.

> > > * return -EBADFD when sk is not a full socket
> > >
> > >    -EPERM or -EBUSY was an option, but in many cases one of them
> > >    will be returned, especially under level SOL_TCP. -EBADFD is the
> > >    better choice, since it is hardly returned in all cases. The bpf
> > >    prog will be able to recognize it and decide what to do next.
> >
> > This one makes sense and is useful.