linux-kernel - Re: [PATCH] trace: events: fix error directive in argument list

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Message-ID: <20190329231231.suwcypwnyunvysyd@ltop.local>
Date:   Sat, 30 Mar 2019 00:12:32 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Hariprasad Kelam <hariprasad.kelam@...il.com>, mingo@...hat.com,
        roopa@...ulusnetworks.com, davem@...emloft.net,
        linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH] trace: events: fix  error directive in argument list

On Wed, Mar 27, 2019 at 11:53:30AM -0400, Steven Rostedt wrote:
> On Tue, 26 Mar 2019 01:23:03 +0530
> Hariprasad Kelam <hariprasad.kelam@...il.com> wrote:
> 
> > This patch fixes below spare errors.
> > 
> > Sparse error:
> > make C=2 CF=-D__CHECK_ENDIAN__ M=net/core
> > ./include/trace/events/neigh.h:73:1: error: directive in argument list
> > ./include/trace/events/neigh.h:78:1: error: directive in argument list
> > ./include/trace/events/neigh.h:150:1: error: directive in argument list
> > ./include/trace/events/neigh.h:155:1: error: directive in argument list
> > 
> 
> I have nothing really against these patches, but why is the current
> code considered wrong?
> 
> Note, TRACE_EVENTS() are "special macros". They hold structure
> definitions and full code inside the argument list. There should be no
> reason that this is causing a warning.

The problem is that #ifdefs at line 73 & 150 are inside TRACE_EVENT()'s
invocation and this is undefined behaviour. From the standard:
	... If there are sequences of preprocessing tokens within the
	list of arguments that would otherwise act as preprocessing
	directives, the behavior is undefined. 

	[C90 6.8.3, C99 & C11 6.10.3p11]

GCC can handle it (and sparse too) but yes sparse complains about it.

-- Luc Van Oostenryck