linux-kernel - Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

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Message-ID: <202008271517.ECC1F1F8F@keescook>
Date:   Thu, 27 Aug 2020 15:20:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Denis Efremov <efremov@...ux.com>
Cc:     Julia Lawall <julia.lawall@...ia.fr>,
        Joe Perches <joe@...ches.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cocci <cocci@...teme.lip6.fr>,
        accessrunner-general@...ts.sourceforge.net,
        Alex Dewar <alex.dewar90@...il.com>
Subject: Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

On Fri, Aug 28, 2020 at 12:01:34AM +0300, Denis Efremov wrote:
> Just FYI, I've send an addition to the device_attr_show.cocci script[1] to turn
> simple cases of snprintf (e.g. "%i") to sprintf. Looks like many developers would
> like it more than changing snprintf to scnprintf. As for me, I don't like the idea
> of automated altering of the original logic from bounded snprint to unbouded one
> with sprintf.

Agreed. This just makes me cringe. If the API design declares that when
a show() callback starts, buf has been allocated with PAGE_SIZE bytes,
then that's how the logic should proceed, and it should be using
scnprintf...

show(...) {
	size_t remaining = PAGE_SIZE;

	...
	remaining -= scnprintf(buf, remaining, "fmt", var args ...);
	remaining -= scnprintf(buf, remaining, "fmt", var args ...);
	remaining -= scnprintf(buf, remaining, "fmt", var args ...);

	return PAGE_SIZE - remaining;
}

-- 
Kees Cook