linux-kernel - Re: [RFC PATCH] checkpatch: check for function calls with struct or union on stack

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Date:   Sat, 28 Jul 2018 10:20:44 -0700
From:   Joe Perches <joe@...ches.com>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        Andy Whitcroft <apw@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] checkpatch: check for function calls with struct or
 union on stack

(unintentionally sent partial reply, better now)

On Sat, 2018-07-28 at 08:25 +0200, Julia Lawall wrote:
> On Fri, 27 Jul 2018, Joe Perches wrote:
[]
> > It might make sense for this sort of check to be
> > added to coccinelle or maybe as a compiler warning
> > when the struct is larger than some size.
> > 
> > Original thread for Julia:
> > https://lore.kernel.org/patchwork/patch/967890/
> 
> Coccinelle doesn't directly know the size of the structure, but it can
> count the number of fields.  Maybe a case with an update in the function
> body

Perhaps this might be the most useful to check.

>  or at least 3 fields is worth reporting on?

Maybe, maybe not.  For instance:

lib/vsprintf.c uses struct printf_spec which is
5 fields totaling 8 bytes and that fits nicely in
a single register on x86-64 so there are good
reasons why structs could be passed by value.

Maybe structs with arrays or other structs would
make more sense.