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

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Message-Id: <20200601.115222.628560714795578081.davem@davemloft.net>
Date:   Mon, 01 Jun 2020 11:52:22 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     arnd@...db.de
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        kuba@...nel.org, gnault@...hat.com, vladbu@...lanox.com,
        lucien.xin@...il.com, pablo@...filter.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] flow_dissector: work around stack frame size warning

From: Arnd Bergmann <arnd@...db.de>
Date: Fri, 29 May 2020 22:13:58 +0200

> 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>

Applied, thanks.