linux-kernel - Re: [PATCH] pstore: Remove duplicate invoking of persistent_ram

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Message-ID: <1540630374669.30217@xiaomi.com>
Date:   Sat, 27 Oct 2018 08:52:58 +0000
From:   Peng15 Wang 王鹏 <wangpeng15@...omi.com>
To:     Kees Cook <keescook@...omium.org>
CC:     "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: Remove duplicate invoking of persistent_ram_zap()



________________________________________
>From: Kees Cook <keescook@...omium.org>
>Sent: Friday, October 26, 2018 17:44
>To: Peng15 Wang 王鹏
>Cc: anton@...msg.org; ccross@...roid.com; tony.luck@...el.com; linux-kernel@...r.kernel.org
>Subject: Re: [PATCH] pstore: Remove duplicate invoking of persistent_ram_zap()

>On Fri, Oct 26, 2018 at 4:41 AM, Peng15 Wang 王鹏 <wangpeng15@...omi.com> wrote:
>> From: Peng Wang <wangpeng15@...omi.com>
>>
>> When initialing przs with invalid data in buffer(no PERSISTENT_RAM_SIG),
>> function call path is like this:
>>
>> ramoops_init_prz ->
>> |
>> |-> persistent_ram_new -> persistent_ram_post_init -> persistent_ram_zap
>> |
>> |--> persistent_ram_zap
>
>There does appear to be a duplicate call to persistent_ram_zap() in this case.
>
>> As we can see, persistent_ram_zap() is called twice.
>> We can avoid this by removing it in ramoops_init_prz(),
>>and only call it in persistent_ram_post_init().
>
>However, I think the proposed fix doesn't work the way it should.
>
>There are two prz init paths: ramoops_init_prz() (a single prz) and
>ramoops_init_przs (multiple przs). The "dump" and "ftrace" cases use
>the latter. In those, there is no call to persistent_ram_zap() if the
>buffer is valid.
>
>In other words:
>
>ramoops_init_prz() unconditionally calls persistent_ram_zap(). (And
>may call it twice if there is a mismatch of the magic header.)
>
>ramoops_init_przs() only calls persistent_ram_zap() when the magic
>header is wrong.
>
>The proposed patch unconditionally zaps all regions, which means we'd
>lose "dump" and "ftrace" across the next reboot.
>
>Perhaps we could make it an option to persistent_ram_new()?
>
>--
>Kees Cook

Thanks for your reply. 

You are right, this patch does zap regions unconditionally when it comes to "dump" and
"ftrace". Sorry for the inconvenience owing to my previous mistake.

I have tried adding an option to persistent_ram_new() according to your advice and will send
a V2 version patch later. Could you please kindly pay any attention to it? Thank you!