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

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Date:   Sun, 14 Oct 2018 20:53:46 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Al Viro <viro@...IV.linux.org.uk>
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 06:18:55PM +0100, Al Viro wrote:
> On Sun, Oct 14, 2018 at 03:25:09PM +0200, Christian Brauner wrote:
> 
> > +static unsigned long sysctl_strtoul_lenient(const char *cp, char **endp,
> > +					    unsigned int base, bool *overflow)
> > +{
> > +	unsigned long long result;
> > +	unsigned int rv;
> > +
> > +	cp = _parse_integer_fixup_radix(cp, &base);
> > +	rv = _parse_integer(cp, base, &result);
> > +	if ((rv & KSTRTOX_OVERFLOW) ||
> > +	    (result != (unsigned long long)(unsigned long)result))
> > +		*overflow = true;
> > +	else
> > +		*overflow = false;
> 
> 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.

> 	if (expr)
> 		foo = true;
> 	else
> 		foo = flase;
> is a fairly unidiomatic way to spell foo = expr;
> 
> And... is there anything that would really care if this "overflow" thing had
> been replaced by simply returning ~0UL on such?  That would appear to be
> a lot more natural API...

Yes, I thought about this but I really didn't want to risk breaking
anything that relies on the weird old behavior. We can change it to that
and assume that anything that doesn't explicitly set a maximum value
wants to be capped at ULONG_MAX. Fine with me.