netdev - Re: [PATCH bpf-next 1/2] bpf: add get_netns

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Message-ID: <a66af5a8-1aa4-481a-b57e-b3076cc520b0@linux.dev>
Date: Fri, 7 Mar 2025 15:06:29 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Mahe Tardy <mahe.tardy@...il.com>
Cc: daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
 andrii@...nel.org, jolsa@...nel.org, bpf@...r.kernel.org,
 Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: add get_netns_cookie helper to tracing
 programs

On 3/6/25 9:03 AM, Mahe Tardy wrote:
>>>> The immediate question is whether sock_net(sk) must be non-NULL for tracing.
>>> We discussed this offline with Daniel Borkmann and we think that it
>>> might not be the question. The get_netns_cookie(NULL) call allows us to
>>> compare against get_netns_cookie(sock) to see whether the sock's netns
>>> is equal to the init netns and thus dispatch different logic.
>> bpf_get_netns_cookie(NULL) should be fine.
>>
>> I meant to ask if sock_net(sk) may return NULL for a non NULL sk. Please check.
> Oh sorry for the confusion, I investigated with my humble kernel
> knowledge: essentially sock_net(sk) is doing sk->sk_net->net, retrieving
> the net struct representing the network namespace, to later extract the
> cookie, and thus dereference the returned pointer (here is the concern).
> The sk_net intermediary (in reality __sk_common.skc_net) is here because
> of the possibility of switching on/off network namespaces via
> CONFIG_NET_NS. It's a possible_net_t type containing (or not) the struct
> net pointer, explaining why we use write/read_pnet to no-op or return
> the global net ns.
> 
> Now by adding this helper to tracing progs, it allows to call this
> function in any function entry or function exit, but unlike kprobes,
> it's not possible to just hook at an obvious arbitrary point in the code
> where the net ns would be NULL in the sock struct. With that in mind, I
> failed to crash the kernel tracing a function (some candidates were
> inlined). I mostly grepped for sock_net_set, but I lack the knowledge to

Thanks for checking.

I took a quick look at the callers of sock_net_set. I suspect 
"fentry/sk_prot_alloc" and "lsm/sk_alloc" could have a NULL?

> guarantee that this could not happen right now or in the future. Maybe
> that would be just safer to add a check and return 0 in that case if
> that's ok? Not sure since the helper returns an 8-byte long opaque
> number which thus includes 0 as a valid value.

I assume net_cookie 0 is invalid, but then it leaks the implementation details 
of what is a valid cookie in a uapi helper

  * u64 bpf_get_netns_cookie(void *ctx)
  * ...
  *      Return
  *              A 8-byte long opaque number

Note that, the tracing program can already read most fields of the sk, including 
sk->sk_net.net->net_cookie. Therefore, what this patch aims to achieve has 
already been supported in tracing. It can also save a helper call.

The only thing that may be missing in your use case is determining the init_net. 
I don't think reading a global kernel variable has been supported yet. Not sure 
if init_net must have net_cookie 1. Otherwise, we could consider to add a kfunc 
to return &init_net, which could be used to compare with sk->sk_net.net. Having 
a pointer to &init_net might be more useful for other tracing use cases in general.