linux-kernel - Re: [PATCH 1/2] sysctl: add overflow detection to proc_get

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Date:   Mon, 15 Oct 2018 01:03:10 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christian Brauner <christian@...uner.io>
Cc:     keescook@...omium.org, linux-kernel@...r.kernel.org,
        ebiederm@...ssion.com, mcgrof@...nel.org,
        akpm@...ux-foundation.org, joe.lawrence@...hat.com,
        longman@...hat.com
Subject: Re: [PATCH 1/2] sysctl: add overflow detection to proc_get_long()

On Sun, Oct 14, 2018 at 08:53:46PM +0200, Christian Brauner wrote:

> > Yecchh...  First of all, the cast back to unsigned long long is completely
> > pointless.  What's more,
> 
> Sorry, seriously asking: why? This was meant to handle the case where
> sizeof(unsigned long long) != sizeof(unsigned long) and I just looked at
> _kstrtoul() which does the same:
> 
> int _kstrtoul(const char *s, unsigned int base, unsigned long *res)
> {
> 	unsigned long long tmp;
> 	int rv;
> 
> 	rv = kstrtoull(s, base, &tmp);
> 	if (rv < 0)
> 		return rv;
> 	if (tmp != (unsigned long long)(unsigned long)tmp)
> 		return -ERANGE;
> 	*res = tmp;
> 	return 0;
> }
> 
> Sorry, if I'm being dense here.

C quiz:
	given that type of e1 is unsigned long long and type of e2 -
unsigned long, what conversions are going to happen in e1 == e2?

[relevant part of C standard: 6.5.9 (Equality operators),
6.3.1.8 (Usual arithmetic conversions)]