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

From: Cong Wang
Date: Sat May 30 2020 - 00:49:58 EST

On Fri, May 29, 2020 at 1:14 PM Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxx>

I think this is probably the quickest way to amend this warning,

Acked-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>