From: "Samo Pogačnik" <samo_pogacnik@t-2.net>
To: Petr Mladek <pmladek@suse.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
Jiri Slaby <jirislaby@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
John Ogness <john.ogness@linutronix.de>,
linux-kernel@vger.kernel.org,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [PATCH] ttyprintk: Add TTY hangup callback.
Date: Mon, 26 Apr 2021 18:42:05 +0200 [thread overview]
Message-ID: <d4d0603716e5cb99a7a9a93d4f767278ac318557.camel@t-2.net> (raw)
In-Reply-To: <YIaPQzktArmoWbLr@alley>
Dne 26.04.2021 (pon) ob 12:00 +0200 je Petr Mladek napisal(a):
> On Sat 2021-04-24 11:57:47, Samo Pogačnik wrote:
> > Dne 24.04.2021 (sob) ob 10:16 +0900 je Tetsuo Handa napisal(a):
> > > On 2021/04/24 4:47, Samo Pogačnik wrote:
> > > > At any point the tpk_buffer is potentially multiplexed/interleaved by
> > > > parts
> > > > of
> > > > required output of any concurrent user, as buffs are being delivered by
> > > > the
> > > > scheduled writes.
> > >
> > > As long as one line is printed by one printk() call,
> > > CONFIG_PRINTK_CALLER=y is
> > > helpful enough to distinguish multilplexed/interleaved messages. I
> > > consider
> > > that
> > > ttyprintk offers additional advantage over printk() for allow buffering
> > > one
> > > line
> > > of message from userspace.
>
> It does not matter how much buffering games you play. As long as you
> use printk() to store single lines into the kernel logbuffer they
> alway could be interleaved with lines from other processes/CPUs.
Exactly. The only purpose of ttyprintk buffering is to mark any begining of
lines occurring within the userspace-string written into ttyprintk TTY. The
marked lines do not originate in the kernel source code, which is not obvious
otherwise (imho this is importannt). Even the CONFIG_PRINTK_CALLER=y does not
give this information, if the task ID printed does not live anymore.
>
> > > >
> > > > As per user buffers look promising with output formatting, the FDs
> > > > passing
> > > > between tasks lead to the same single buffer (Greg already mentioned
> > > > that).
> > >
> > > Those programs which use FD passing know what they are doing. If they
> > > still
> > > want
> > > one line of message printed via ttyprintk interface, they must do their
> > > buffering
> > > before trying to write() to ttyprintk's file descriptor.
>
> Lines might get interleaved when using printk().
> What is special about messages passed via ttyprintk()?
They do not originate in the kernel code.
> How many processes are using it?
In case of redirection any proces, that is writing to console.
> Do they print many lines?
?
> Is it really worth any added complexity?
No.
>
> > On the other hand, my main concern is how to provide a reliable system wide
> > collection of all console output via ttyprintk console redirection, while
> > normal
> > operation of system console is preserved (except its output being detoured
> > via
> > printk and as such logged together with kernel output). Such logging is
> > particularly useful for after-the-fact inspection of system operation.
>
> I am not sure if I understand the problem. But why does ttyprintk need
> any buffer at all. AFAIK, the use-case is to pass any written data into the
> kernel logbuffer via printk()?
(see above - it is not something the kernel is telling you)
>
> Why tpk_write() does not call printk() directly?
(see above)
>
> If you call printk() directly, the caller_id would be from the process
> that really wrote the data/message.
It can be a kernel-code originating message printk-ed on behalf of a user task
or a kernel-code originating message on behalf of a kernel task. Or it may be a
user-code originating message on behalf of its task, when printk-ed via
ttyprintk.
>
> > That being said i am thinking about how to permanently enable this
> > redirection
> > as early as possible (i.e. via kernel command line option). I had a working
> > prototype for that some time ago (never posted). Would anybody like to see
> > such
> > functionality?
>
> Please, do not add any complex code if it does not cause real life
> problems.
>
Noted, thanks.
Best regards, Samo
next prev parent reply other threads:[~2021-04-26 16:42 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-03 4:14 [PATCH] tty: use printk_safe context at tty_msg() Tetsuo Handa
2021-04-03 6:52 ` kernel test robot
2021-04-03 10:11 ` [PATCH] printk: Make multiple inclusion of kernel/printk/internal.h safe Tetsuo Handa
2021-04-06 4:51 ` [PATCH] tty: use printk_safe context at tty_msg() Jiri Slaby
2021-04-06 5:31 ` Tetsuo Handa
2021-04-06 7:10 ` Greg Kroah-Hartman
2021-04-06 11:16 ` Tetsuo Handa
2021-04-06 13:42 ` Greg Kroah-Hartman
2021-04-06 15:10 ` Petr Mladek
2021-04-06 16:22 ` Tetsuo Handa
2021-04-06 19:10 ` Greg Kroah-Hartman
2021-04-07 9:20 ` Petr Mladek
2021-04-07 13:26 ` [PATCH v2] tty: use printk_deferred() " Tetsuo Handa
2021-04-07 13:48 ` Greg Kroah-Hartman
2021-04-07 14:24 ` Tetsuo Handa
2021-04-12 10:39 ` How to handle concurrent access to /dev/ttyprintk ? Tetsuo Handa
2021-04-12 10:44 ` Greg Kroah-Hartman
2021-04-12 11:25 ` Tetsuo Handa
2021-04-12 12:04 ` Greg Kroah-Hartman
2021-04-14 0:45 ` Tetsuo Handa
2021-04-14 11:11 ` Tetsuo Handa
2021-04-14 16:15 ` Samo Pogačnik
2021-04-15 0:22 ` [PATCH] ttyprintk: Add TTY hangup callback Tetsuo Handa
2021-04-18 11:16 ` Samo Pogačnik
2021-04-22 10:02 ` Greg Kroah-Hartman
2021-04-23 4:22 ` Jiri Slaby
2021-04-23 9:55 ` Samo Pogačnik
2021-04-23 10:12 ` Tetsuo Handa
2021-04-23 19:47 ` Samo Pogačnik
2021-04-24 1:16 ` Tetsuo Handa
2021-04-24 9:57 ` Samo Pogačnik
2021-04-26 10:00 ` Petr Mladek
2021-04-26 16:42 ` Samo Pogačnik [this message]
2021-04-27 10:08 ` Petr Mladek
2021-04-27 11:31 ` Samo Pogačnik
2021-04-23 10:28 ` Jiri Slaby
2021-04-23 12:23 ` [PATCH] ttyprintk: Add TTY port shutdown callback Samo Pogačnik
2021-04-12 12:41 ` How to handle concurrent access to /dev/ttyprintk ? Samo Pogačnik
2021-04-13 9:41 ` Petr Mladek
2021-04-13 11:10 ` Samo Pogačnik
2021-04-13 14:32 ` Petr Mladek
2021-04-13 15:22 ` Samo Pogačnik
2021-04-14 17:36 ` Petr Mladek
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