linux-kernel - Re: [PATCH v3 2/4] vsscanf(): Integer overflow is a conversion failure

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Date:   Mon, 12 Jun 2023 12:53:06 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Lee Jones <lee@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 2/4] vsscanf(): Integer overflow is a conversion
 failure

On 10/06/2023 22.40, Demi Marie Obenour wrote:
> sscanf() and friends currently ignore integer overflow, but this is a
> bad idea.  It is much better to detect integer overflow errors and
> consider this a conversion failure.

Perhaps. And maybe I even agree. But not like this:

>  	while (*fmt) {
>  		/* skip any white space in format */
> @@ -3464,6 +3474,9 @@ int vsscanf(const char *buf, const char *fmt, va_list args)
>  			break;
>  		++fmt;
>  
> +		allow_overflow = *fmt == '!';
> +		fmt += (int)allow_overflow;
> +

You can't do that. Or, at least, you won't be able to actually use %!d
anywhere, because the compiler will yell at you:

lib/vsprintf.c: In function ‘foobar’:
lib/vsprintf.c:3727:26: error: unknown conversion type character ‘!’ in
format [-Werror=format=]
 3727 |  ret = sscanf("12345", "%!d", &val);
      |                          ^

So NAK.

Also, when you make significant changes to the sscanf implementation,
I'd expect the diffstat for the patch series to contain lib/test_scanf.c.

Rasmus