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Date: Thu, 23 Nov 2017 13:46:48 +0100
From: Petr Mladek <pmladek@suse.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Prarit Bhargava <prarit@redhat.com>,
        Mark Salyzyn <salyzyn@android.com>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>, Joe Perches <joe@perches.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC patch 5/7] crash: Add VMCOREINFO_FIELD_AND_OFFSET()
Message-ID: <20171123124648.s4oigunxjfzvhtqh@pathway.suse.cz>
References: <20171115181531.322572387@linutronix.de>
 <20171115182657.542096222@linutronix.de>
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On Wed 2017-11-15 19:15:36, Thomas Gleixner wrote:
> Changing the time stamp storage in the printk ringbuffer entries would
> break existing tools which rely on the availability of the 'ts_nsec' field
> in vmcore info.
> 
> Provide a new macro which allows to store the offset of a member of struct
> printk_log with a different name. This allows to change the underlying
> storage and without breaking tools.

It seems that this cheating is not enough. I tried to use it in printk
(6th patch in this patch set) and the "crash" tool complained:

===== cut =====
WARNING: log buf data structure(s) have changed

log: invalid structure member offset: log_ts_nsec
     FILE: kernel.c  LINE: 5055  FUNCTION: dump_log_entry()
===== cut =====


The crash tool get this value using the following code:

  MEMBER_OFFSET_INIT(log_ts_nsec, "printk_log", "ts_nsec");
  + MEMBER_OFFSET()
    + datatype_info((X), (Y), NULL)

, where the last function does something like:

long
datatype_info(char *name, char *member, struct datatype_member *dm)
{
	struct gnu_request *req;

	req->command = GNU_GET_DATATYPE;
	req->name = name;
	req->member = member;

	gdb_interface(req);

I am not completely sure what it does. I guess that gdb
is able to get information about the real types either
from vmlinux binary or vmcore. It is not able to
find "ts_nsec" in the struct "printk_log" and fails.


I am afraid that we would need to fix crash for
the modified struct printk_log. Then this hack
will not be needed.

Best Regards,
Petr