On Wed, 2021-06-23 at 18:53 +0200, Pavel Machek wrote:

Hi!

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From: Paolo Abeni <pabeni@redhat.com>

[ Upstream commit 61e710227e97172355d5f150d5c78c64175d9fb2 ]

warn_bad_map() produces a kernel WARN on bad input coming
from the network. Use pr_debug() to avoid spamming the system
log.

So... we switched from WARN _ONCE_ to pr_debug, as many times as we
detect the problem.

Should this be pr_debug_once?

Thank you for double checking this!

In the MPTCP code, we use pr_debug() statements as a debug tool, e.g.
when enabled, it could print per-packet info with no restriction. 

There are (a few) similar use in the plain TCP code.

pr_debug() is not supposed to be enabled on any production system,
while the WARN_ONCE could trigger automated tools for irrelevant
network noise.

Correct me if I'm wrong, but I believe pr_debug will result in
messages being stored in the dmesg buffer, even on production
systems.

If DYNAMIC_DEBUG is enabled, and the system administator explicitly
enables the relevant debug message writing suitable data in
/sys/kernel/debug/dynamic_debug/control, yes, the message will be
printed on dmsg.

With DYNAMIC_DEBUG disabled, patching the kernel and defining the DEBUG
macro in the net/mptcp/protocol.c file you will get again the debug
message.

Neither of the above fit productions systems IMHO, and I'm not aware of
other options to trigger the relevant debug message.

I personally have CONFIG_DYNAMIC_DEBUG=y in my kconfig and I enable the
relevant debug message as needed in my devel activity.

Cheers,

Paolo