linux-kernel - Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

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Message-ID: <20200924084555.GJ6442@alley>
Date:   Thu, 24 Sep 2020 10:45:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

On Thu 2020-09-24 14:40:58, Sergey Senozhatsky wrote:
> On (20/09/23 17:11), Petr Mladek wrote:
> >
> > AFAIK, there is one catch. We need to use va_copy() around
> > the 1st call because va_format can be proceed only once.
> >
> 
> Current printk() should be good enough for reporting, say, "Kernel
> stack overflow" errors. Is extra pressure that va_copy() adds something
> that we need to consider?

The thing is that vsprintf() traverses the arguments using va_arg().
It modifies internal values so that the next va_arg() will read
the next value.

If we want to call vsprintf() twice then we need to reset the internal
va_list states. My understanding is that va_copy() is the only legal
way when we are already nested inside va_start()/va_end().

See also the commit 5756b76e4db643d8f7 ("vsprintf: make %pV handling
compatible with kasprintf()").

Best Regards,
Petr