netdev - RE: KASAN+netlink, was: [PATCH] [net-next?] hns: avoid stack overflow with CONFIG

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Date:   Wed, 8 Feb 2017 16:00:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Johannes Berg' <johannes@...solutions.net>,
        Arnd Bergmann <arnd@...db.de>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Andrey Ryabinin" <a.ryabinin@...sung.com>,
        "nikolay@...ulusnetworks.com" <nikolay@...ulusnetworks.com>,
        "nicolas.dichtel@...nd.com" <nicolas.dichtel@...nd.com>,
        "adobriyan@...il.com" <adobriyan@...il.com>,
        linux-wireless <linux-wireless@...r.kernel.org>
Subject: RE: KASAN+netlink, was: [PATCH] [net-next?] hns: avoid stack
 overflow with CONFIG_KASAN

> From: Johannes Berg
> Sent: 08 February 2017 12:24
...
> Btw, what's causing this to start with? Can't the compiler reuse the
> stack places?

Only if it realises they've gone out of scope - which probably
doesn't happen when the functions are inlined.
The address of the parameter can be saved by the calling function
and used in a later call.

Something like this is valid:

int foo(int *p, int v)
{
	static int *sv;
	int old = -1;
	if (sv) {old = *sv; *sv = v;}
	sv = v;
	return old;
}

void bar(...) {
	int a, b;
	...
	foo(&a, 0);
	...
	foo(&b, 1);
	...
	foo(NULL, 2);
	...

If the compiler starts sharing stack it all goes wrong.

	David