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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 26 Jun 2022 13:19:21 -0700
Message-ID: <CAHk-=whwyxSpzgr+roEr7_V5wVenw9fV3EOAZhAYCAuRdEyChQ@mail.gmail.com>
Subject: Re: [RFC[ Alloc in vsprintf
To:     Joe Perches <joe@perches.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        David Laight <David.Laight@aculab.com>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
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On Sun, Jun 26, 2022 at 12:53 PM Joe Perches <joe@perches.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.

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

The last thing we want to do is make a hard-to-debug panic be even
*harder* to debug because you get a deadlock when oopsing.

(And yes, I realize that the symbol name lookup can have problems too,
but thats' kind of fundamental to %pS, while a kzmalloc isn't.

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".

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?

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?

NOTE! The attached patch is completely broken.  I did not do that
interface change to the kallsyms code. The patch is literally meant to
be just an explanation of what I mean, not a working patch.

                  Linus

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