linux-kernel - Re: [RFC[ Alloc in vsprintf

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Date:   Sun, 26 Jun 2022 13:39:01 -0700
From:   Joe Perches <joe@...ches.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC[ Alloc in vsprintf

On Sun, 2022-06-26 at 13:19 -0700, Linus Torvalds wrote:
> On Sun, Jun 26, 2022 at 12:53 PM Joe Perches <joe@...ches.com> wrote:
> > 
> > In a reply to the printbufs thread, I wrote a proposal to use an
> > alloc to reduce stack in vsprintf when CONFIG_KALLSYMS is enabled.
> > 
> > No one has replied to this but I think it's somewhat sensible.
> 
> I think that's a bad idea.

Somewhat sensible not sensible...

> Those things are *literally* called from panic situations, which may
> be while holding core memory allocation locks, or similar.

True, and special_hex_number was used on alloc failure.

> Now, you are correct that the stack buffer is annoying. But I think
> the proper way to fix that is to say "we already *have* the target
> buffer, let's use it".

OK, and that's true for all the temp stack buffers in every %p<foo>.

> That does require teaching the sprint_symbol() functions that they
> need to take a "length of buffer" and return how much they used, but
> that would seem to be a sensible thing anyway, and what the code
> should always have done?

Unnecessary stack and/or unnecessary buffers for printbufs are
just unnecessary. 

> It's bad policy to just pass in a buffer without length, and I think
> it was always broken. Nasty. That KSYM_SYMBOL_LEN is magically taking
> care of it all, but it's ugly as heck, wouldn't you say?

Yup.