Re: [PATCH v4 4/5] pstore: add pmsg

[Lukasz Stelmach <stlman@poczta.fm>]

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On 28.01.2015 18:28, Mark Salyzyn wrote:
> On 01/13/2015 04:16 PM, Łukasz Stelmach wrote:
>>> A secured user-space accessible pstore object. Writes
>>> to /dev/pmsg0 are appended to the buffer, on reboot
>>> the persistent contents are available in
>>> /sys/fs/pstore/pmsg-ramoops-[ID].
>>>
>>> One possible use is syslogd, or other daemon, can
>>> write messages, then on reboot provides a means to
>>> triage user-space activities leading up to a panic
>>> as a companion to the pstore dmesg or console logs.
>>>
>>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>>> ---
>> I am not an expert but this smells like duplicating /dev/kmsg. If
>> I remember correctly since about Linux 3.5 /dev/kmsg is writable for the
>> user-space and every single process (modulo MAC/DAC) can log there. The
>> messages from user-space are preserved accross reboots as a part of the
>> kmsg/printk buffer anyway.
>>
>> What is the advantege of pmsg0 over /dev/kmsg?
> 
> - Precious little user-space content goes to kmsg (otherwise you
> can ask why is there a syslogd?), there is a reason for this, user
> space is notorious for containing Personal Identifiable Information
> whereas kernel information does not.

Sure it does too: MAC addresses, UUIDs, serial numbers. With mobile
devices these are actually PII.

> - pmsg0 can take a lot of content (with a ramoops backend) and
> will not disrupt/DOS the kernel logs.

Documentation/ramoops.txt says it is for logging kernel oopses
and panics not user logs.

> - State, Binary or packetized content can go to /dev/pmsg0 and
> not interfere with the text content in kmsg

Indeed kmsg can't store arbitrary binary data. However it can,
store key/value pairs next to text and there is base64 too.
Yes, this one seems a little bit better than kmsg.

> - /dev/pmsg0 write is atomic

devkmsg_write + vprintk_emit are atomic too.

> - /dev/pmsg0 is write only, there is no access to the live content
> _unless_ there is a reboot.

Why do you consider this an advantage?

> - Personal identification which abounds in user space could be placed
> into /dev/pmsg0, and there is no way except a reboot in order to
> extract the content, and then /sys/fs/pstore/pmsg-ramoops-0 can be
> deleted, or heavily MAC and DAC controlled to enforce protection
> (doing so to kmsg would be unlivable)

Read access to /dev/kmsg can be limited too.

I think that the goals you present can be met with less code.
You could try adding support for multiple /dev/kmsg instances
for example. How about that?

-- 
Było mi bardzo miło.                   Twoje oczy lubią mnie
>Łukasz<                                     i to mnie zgubi  (c)SNL


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