Re: Patch "net: flow_dissector: use DEBUG_NET_WARN_ON_ONCE" should probably be ported to 5.4, 5.10 and 5.15 LTS.

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 21, 2025 at 11:07:48PM -0800, Xingyu Li wrote:
> Hi,
> 
> We noticed that the patch 120f1c857a73 should be ported to 5.4, 5.10
> and 5.15 LTS according to bug introducing commits. Also, they can be
> applied to the latest version of these three branches without
> conflicting.
> Its bug introducing commit is 9b52e3f267a6. According to our manual
> analysis,  the  commit (9b52e3f267a6) introduced a
> `WARN_ON_ONCE(!net);` statement in the `__skb_flow_dissect` function
> within `net/core/flow_dissector.c`. This change began triggering
> warnings (splat messages) when `net` is `NULL`, which can happen in
> legitimate use cases, such as when `__skb_get_hash()` is called by the
> nftables tracing infrastructure to identify packets in traces. The
> patch provided replaces this `WARN_ON_ONCE(!net);` with
> `DEBUG_NET_WARN_ON_ONCE(!net);`, which is more appropriate for
> situations where `net` can be `NULL` without it indicating a critical
> issue. This change prevents unnecessary warning messages from
> appearing, which can clutter logs and potentially mask real issues.
> Therefore, the prior commit introduced the issue (the unnecessary
> warnings when `net` is `NULL`), and the patch fixes this by adjusting
> the warning mechanism.

Have you tested the commit to ensure that it actually works?

Hint, I just tried, it breaks the build on all of the above branches,
which is probably why it was not backported.

Please test stuff before asking others to test it for you :(

thanks,

greg k-h