linux-kernel - Re: [PATCH 6/8] tools/nolibc: tests: add test for -fstack-protector

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Date:   Sun, 26 Mar 2023 22:38:39 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Willy Tarreau <w@....eu>
Cc:     Thomas Weißschuh <thomas@...ch.de>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/8] tools/nolibc: tests: add test for -fstack-protector

On Sun, Mar 26, 2023 at 08:45:04PM +0200, Willy Tarreau wrote:
> On Sun, Mar 26, 2023 at 01:42:35PM -0500, Thomas Weißschuh  wrote:
> > 
> > Mar 26, 2023 13:30:21 Alexey Dobriyan <adobriyan@...il.com>:
> > 
> > > Willy Tarreau wrote:
> > >> #if defined(__clang__)
> > >> __attribute__((optnone))
> > >> #elif defined(__GNUC__)
> > >> __attribute__((optimize("O0")))
> > >> #endif
> > >> static int smash_stack(void)
> > >> {
> > >>     char buf[100];
> > >>
> > >>     for (size_t i = 0; i < 200; i++)
> > >>         buf[i] = 'P';
> > >>
> > >>     return 1;
> > >> }
> > >
> > > If you want to corrupt the stack, corrupt the stack!
> > 
> > I do!
> > 
> > > asm(
> > > ".globl f\n"
> > > "f:\n"
> > > "movq $0, (%rsp)\n"
> > > "ret\n"
> > > ".type f,@function\n"
> > > ".size f,.-f"
> > > );
> > > > No problems with optimisation levels.
> > 
> > Wouldn't this be architecture-specific?
> 
> Yes it would.

Which is OK. Corrupting return address is very arch-specific.

> I'm not seeing any issue with your approach instead, let's
> keep it as-is for now (also it does what the stack protector is supposed
> to catch anyway).

There are no guarantess about stack layout and dead writes.
The test doesn't corrupt stack reliably, just 99.99% reliably.