netdev - Re: [PATCH] bpf: make bpf_stackmap

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Date:	Wed, 9 Mar 2016 19:38:47 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Alexei Starovoitov <ast@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: make bpf_stackmap_copy conditionally called

On Thu, Mar 10, 2016 at 02:43:42AM +0100, Arnd Bergmann wrote:
> Changing the bpf syscall to use the new bpf_stackmap_copy() helper for
> BPF_MAP_TYPE_STACK_TRACE causes a link error when CONFIG_PERF_EVENTS
> is disabled:
> 
> kernel/built-in.o: In function `map_lookup_elem':
> :(.text+0x7fca4): undefined reference to `bpf_stackmap_copy'
> 
> This patch simply avoids handling that case, which may or may not
> be the correct answer here.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 557c0c6e7df8 ("bpf: convert stackmap to pre-allocation")
...
> -	} else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE) {
> +	} else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE &&
> +		   IS_ENABLED(CONFIG_PERF_EVENTS)) {
>  		err = bpf_stackmap_copy(map, key, value);
>  	} else {

yes. this is also ok-ish fix.
I've sent different version already:
http://patchwork.ozlabs.org/patch/595617/

I considered the option like yours but it's relying on gcc doing
dead code elimination of 'if (false) {}' branch and though kernel
is never compiled with -O0. I didn't want to take the risk.
I'm fine with either approach though.