linux-kernel - Re: [PATCH] printk: inject caller information into the body of message

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Date:   Tue, 9 Oct 2018 16:52:29 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        kbuild test robot <fengguang.wu@...el.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of
 message

On Tue 2018-10-09 05:48:33, Tetsuo Handa wrote:
> On 2018/10/09 1:03, Petr Mladek wrote:
> > On Mon 2018-10-08 19:31:58, Tetsuo Handa wrote:
> >>   A structure named "struct printk_buffer" is introduced for buffering
> >>   up to LOG_LINE_MAX bytes of printk() output which did not end with '\n'.
> >>
> >>   A caller is allowed to allocate/free "struct printk_buffer" using
> >>   kzalloc()/kfree() if that caller is in a location where it is possible
> >>   to do so.
> >>
> >>   A macro named "DEFINE_PRINTK_BUFFER()" is defined for allocating
> >>   "struct printk_buffer" from the stack memory or in the .bss section.
> >>
> >>   But since sizeof("struct printk_buffer") is nearly 1KB, it might not be
> >>   preferable to allocate "struct printk_buffer" from the stack memory.
> >>   In that case, a caller can use best-effort buffering mode. Two functions
> >>   get_printk_buffer() and put_printk_buffer() are provided for that mode.
> >>
> >>   get_printk_buffer() tries to assign a "struct printk_buffer" from
> >>   statically preallocated array. It returns NULL if all static
> >>   "struct printk_buffer" are in use.
> >>
> >>   put_printk_buffer() flushes and releases the "struct printk_buffer".
> >>   put_printk_buffer() must match corresponding get_printk_buffer() as with
> >>   rcu_read_unlock() must match corresponding rcu_read_lock().
> > 
> > One problem with this API is when it is used in more complicated code
> > and put_printk_buffer() is not called in some path. I mean leaking.
> > We might get out of buffers easily.
> 
> Then, as an debugging config option for statically preallocated buffers,
> we could record how get_printk_buffer() was called, like lockdep records
> where a lock was taken.

Another solution might be to store some timestamp (jiffies?) into
struct printk_buffer when a new message is added. Then we could flush
stalled buffers in get_printk_buffer() with some warning.

Unfortunately, it might be unsafe to put the stalled buffers.
Well, it might be safe if there is a lock less access. I wonder
if we could reuse the printk_safe code here.

Anyway, I would like to have a solution before we add the new
API into the kernel. We would need it sooner or later anyway.
And I would like to be sure that the API is sane.


> > A solution might be to store some information about the owner and
> > put the buffer also when a non-buffered printk is called from
> > the same context.
> > 
> > It might even make it easier to use. If we are able to guess the
> > buffer by the context, we do not need to pass it as an argument.
> 
> It would be nice if we can omit passing "struct printk_buffer" argument.
> But that results in "implicit contexts" which Linus has rejected
> ( https://lkml.kernel.org/CA+55aFx+5R-vFQfr7+Ok9Yrs2adQ2Ma4fz+S6nCyWHY_-2mrmw@mail.gmail.com ).

Yeah and the arguments for explicit context make sense when
I reread them again.

Best Regards,
Petr