linux-kernel - RE: [PATCH v2 03/28] vsprintf: %pf(%p)

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Message-ID: <141b17a257274fc38291cdb07e0e23b6@AcuMS.aculab.com>
Date:   Thu, 19 May 2022 21:06:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kent Overstreet' <kent.overstreet@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...r.kernel.org" <linux-mm@...r.kernel.org>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>
CC:     "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "willy@...radead.org" <willy@...radead.org>
Subject: RE: [PATCH v2 03/28] vsprintf: %pf(%p)

From: Kent Overstreet
> Sent: 19 May 2022 18:24
> 
> This implements two new format strings: both do the same thing, one more
> compatible with current gcc format string checking, the other that we'd
> like to standardize:
> 
>  %p(%p) - more compatible
>  %(%p)  - more prettier
> 
> Both can take variable numbers of arguments, i.e. %(%p,%p,%p).
> 
> They're used to indicate that snprintf or pr_buf should interpret the
> next argument as a pretty-printer function to call, and subsequent
> arguments within the parentheses should be passed to the pretty-printer.

I suspect this a very good way to blow the kernel stack.
The highest stack use is already very likely to be inside
the printf code in an error path somewhere.

...
> The goal is to replace most of our %p format extensions with this
> interface, and to move pretty-printers out of the core vsprintf.c code -

One advantage of the current scheme is that is reasonably safe
and easy to use.
Perhaps too many extra formats have been added recently.
This all seems like a recipe for disaster with functions being
called with the wrong number of parameters
(I can't think how you can compile-time check it).

Double copying using a temporary buffer isn't the end of the world.
It is only a problem because pr_cont() is basically impossible.
But since kernel printf ought to be formatted to reasonable
line length that isn't really an issue.
printf() is expensive an extra memory copy is probably noise.

...
> Currently, we can only call pretty printers with pointer arguments. This
> could be changed to also allow at least integer arguments in the future
> by using libffi.

I'm sure I remember something else trying to use that.
IIRC it is basically broken by design.

	David

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