From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <systemtap-return-26538-listarch-systemtap=sources.redhat.com@sourceware.org>
Received: (qmail 103981 invoked by alias); 25 Apr 2019 08:13:59 -0000
Mailing-List: contact systemtap-help@sourceware.org; run by ezmlm
Precedence: bulk
List-Id: <systemtap.sourceware.org>
List-Subscribe: <mailto:systemtap-subscribe@sourceware.org>
List-Post: <mailto:systemtap@sourceware.org>
List-Help: <mailto:systemtap-help@sourceware.org>, <http://sourceware.org/lists.html#faqs>
Sender: systemtap-owner@sourceware.org
Received: (qmail 103973 invoked by uid 89); 25 Apr 2019 08:13:58 -0000
Authentication-Results: sourceware.org; auth=none
X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,SPF_PASS autolearn=ham version=3.3.1 spammy=H*r:sk:systemt
X-HELO: mx1.suse.de
Received: from mx2.suse.de (HELO mx1.suse.de) (195.135.220.15) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 25 Apr 2019 08:13:57 +0000
Received: from relay2.suse.de (unknown [195.135.220.254])	by mx1.suse.de (Postfix) with ESMTP id 54BF7ABD0	for <systemtap@sourceware.org>; Thu, 25 Apr 2019 08:13:55 +0000 (UTC)
Message-ID: <1556180324.3080.5.camel@suse.cz>
Subject: Re: Error when using @container_of
From: Giovanni Gherdovich <ggherdovich@suse.cz>
To: systemtap@sourceware.org
Date: Thu, 25 Apr 2019 08:13:00 -0000
In-Reply-To: <1555927560.2268.2.camel@suse.cz>
References: <1555927560.2268.2.camel@suse.cz>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
X-IsSubscribed: yes
X-SW-Source: 2019-q2/txt/msg00034.txt.bz2

On Mon, 2019-04-22 at 12:06 +0200, Giovanni Gherdovich wrote:
> Hello,
> 
> when I use the macro @container_of in systemtap I'm getting an error I don't
> understand.
> 
> This is my tapset:
> 
>     #!/root/systemtap-latest/bin/stap
> 
>     probe kernel.function("__update_load_avg_se") {
>             printf("%x\n", @container_of($se, "struct task_struct", se));
>             exit();
>     }
> 
> and this is the error:
> 
>     # ./repro.stp
>     semantic error: 'struct task_struct' (./include/linux/sched.h:602) is being
>     accessed instead of a member such as '->acct_rss_mem1': operator '@cast' at
>     /root/systemtap-latest/share/systemtap/tapset/container_of.stpm:2:5
> 
>             source:     @cast(@ptr - @offsetof(@type, @member), @type)
>                         ^
>             in expansion of macro: operator '@container_of' at ./repro.stp:4:17
>             source:         printf("%x\n", @container_of($se, "struct task_struct", se));
>                                            ^
> 
>     Pass 2: analysis failed.  [man error::pass2]

Hi again,

I'm posting here for future reference: I've been explained that my mistake was
to not use the address-of operator '&' after @container_of. Since my original
goal was to get the PID of a sched entity, the snipped above can be fixed
with:

    probe kernel.function("__update_load_avg_se") {
	    p = &@container_of($se, "struct task_struct", se);
	    printf("pid: %d\n", p->pid);
	    exit();
    }

or even by chaining a field access with -> without using '&' like:

    probe kernel.function("__update_load_avg_se") {
	    printf("pid: %d\n", @container_of($se, "struct task_struct", se)->pid);
	    exit();
    }

In hindsight I should say that the error message makes perfect sense: "you're
accessing the struct itself, you should access a field instead!". What I missed
is that I had to take the address of the struct.


Regards,
Giovanni Gherdovich