linux-kernel - Re: [PATCH] flow_dissector: work around stack frame size warning

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Message-ID: <20200530164314.GA31920@pc-3.home>
Date:   Sat, 30 May 2020 18:43:14 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Vlad Buslov <vladbu@...lanox.com>,
        Xin Long <lucien.xin@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] flow_dissector: work around stack frame size warning

On Fri, May 29, 2020 at 10:13:58PM +0200, Arnd Bergmann wrote:
> The fl_flow_key structure is around 500 bytes, so having two of them
> on the stack in one function now exceeds the warning limit after an
> otherwise correct change:
> 
> net/sched/cls_flower.c:298:12: error: stack frame size of 1056 bytes in function 'fl_classify' [-Werror,-Wframe-larger-than=]
> 
> I suspect the fl_classify function could be reworked to only have one
> of them on the stack and modify it in place, but I could not work out
> how to do that.
> 
> As a somewhat hacky workaround, move one of them into an out-of-line
> function to reduce its scope. This does not necessarily reduce the stack
> usage of the outer function, but at least the second copy is removed
> from the stack during most of it and does not add up to whatever is
> called from there.
> 
> I now see 552 bytes of stack usage for fl_classify(), plus 528 bytes
> for fl_mask_lookup().
> 
> Fixes: 58cff782cc55 ("flow_dissector: Parse multiple MPLS Label Stack Entries")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
Sorry, I didn't see that, as my .config has CONFIG_FRAME_WARN=2048,
which seems to be the default for x86_64.

Acked-by: Guillaume Nault <gnault@...hat.com>