linux-kernel - Re: [PATCH v4 12/20] vsprintf: add new `%pA` format specifier

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Date:   Thu, 24 Feb 2022 10:55:33 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux <rust-for-linux@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Gary Guo <gary@...yguo.net>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v4 12/20] vsprintf: add new `%pA` format specifier

On Tue 2022-02-22 11:35:39, Rasmus Villemoes wrote:
> On 22/02/2022 10.29, Petr Mladek wrote:
> > On Mon 2022-02-14 13:12:24, Miguel Ojeda wrote:
> >> On Mon, Feb 14, 2022 at 11:52 AM Rasmus Villemoes
> >> <linux@...musvillemoes.dk> wrote:
> >>>
> >>> I think the point is for vsnprintf() to call (back) into Rust code.
> >>
> >> Indeed, this is the case.
> >>
> >>> That said, I don't like the !CONFIG_RUST version to return NULL, that
> >>> will surely crash moments later.
> >>>
> >>> So I prefer something like
> >>>
> >>> [rust.h]
> >>> // no CONFIG_RUST conditional
> >>> +char *rust_fmt_argument(char* buf, char* end, void *ptr);
> >>>
> >>> [vsprintf.c]
> >>> +       case 'A':
> >>> +               if (IS_ENABLED(CONFIG_RUST))
> >>> +                   return rust_fmt_argument(buf, end, ptr);
> >>> +               else
> >>> +                   return string_nocheck(buf, end, "[%pA in non-Rust
> >>> code?!]", default_str_spec);
> > 
> > Any long message might cause buffer overflow when the caller expects
> > fixed short string.
> 
> If the caller (1) uses a %p extension from C code which should only be
> used from Rust and (2) uses sprintf() or another variant where he
> doesn't provide the real buffer bounds, well, then he certainly gets to
> keep the pieces.
> 
> It is a much worse problem that if CONFIG_RUST is enabled, we can't know
> that we were actually called from Rust (but when !CONFIG_RUST, we
> certainly know that we weren't), so we could call into rust_fmt_argument
> with a pointer which certainly doesn't point to the/a data structure
> which that Rust code expects. But we can't do anything about it, we will
> just have to rely on static analysis to flag any use of %pA in C code.

Yeah. !CONFIG_RUST would trigger the warning and help to find the
sinners but it is not reliable. Static analysic might be better...

> > The most safe solution would be to use WARN_ONCE(). 
> 
> Preferably no, we shouldn't call into the printk machinery from within
> vsnprintf(). I know I've added a few myself (AFAIR for use of %n or
> other unsupported specifiers, and for overflow of precision/field
> width), and I've often thought about a way to get rid of them while
> still making sure some message eventually gets logged (once).

WARN_ONCE() in vsprintf() code is much more acceptable these days
with the lockless ringbuffer.

Best Regards,
Petr