Re: [PATCH RFC 4/5] tools/nolibc: add support for stack protector

[Willy Tarreau]

Hi Thomas,

thanks for this patchset. I must confess it's not very clear to me which
class of programs using nolibc could benefit from stack protection, but
if you think it can improve the overall value (even if just by allowing
to test more combinations), I'm fine with this given that it doesn't
remove anything.

I'm having a few comments below:

On Tue, Mar 07, 2023 at 10:22:33PM +0000, Thomas Weißschuh wrote:
> diff --git a/tools/include/nolibc/stackprotector.h b/tools/include/nolibc/stackprotector.h
> new file mode 100644
> index 000000000000..ca1360b7afd8
> --- /dev/null
> +++ b/tools/include/nolibc/stackprotector.h
> @@ -0,0 +1,48 @@
> +/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
> +/*
> + * Stack protector support for NOLIBC
> + * Copyright (C) 2023 Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> + */
> +
> +#ifndef _NOLIBC_STACKPROTECTOR_H
> +#define _NOLIBC_STACKPROTECTOR_H
> +
> +#include "arch.h"
> +
> +#if defined(NOLIBC_STACKPROTECTOR)
> +
> +#if !defined(__ARCH_SUPPORTS_STACK_PROTECTOR)
> +#error "nolibc does not support stack protectors on this arch"
> +#endif
> +
> +#include "sys.h"
> +#include "stdlib.h"
> +
> +__attribute__((weak,noreturn,section(".text.nolibc_stack_chk")))
> +void __stack_chk_fail(void)
> +{
> +	write(STDERR_FILENO, "!!Stack smashing detected!!\n", 28);
> +	abort();
> +}

Don't you think you should call the syscall directly here like you
did for __stack_chk_init() and/or declare the function with the
no_stackprotector attribute ? I'm wondering if there could be a
risk that it fails again if called from a bad condition. If you're
certain it cannot, maybe just explain it in a 2-line comment above
the function so that others don't ask the same in the future.

> +__attribute__((weak,no_stack_protector,section(".text.nolibc_stack_chk")))
> +void __stack_chk_init(void)
> +{
> +	// raw syscall assembly as calling a function would trigger the
> +	// stackprotector itself
> +	my_syscall3(__NR_getrandom, &__stack_chk_guard, sizeof(__stack_chk_guard), 0);

For full-line comments, the regular C-style "/* */" is preferred (and
please also use the multi-line format when needed). "//" tends to be
reserved for short ones at the end of a line.

> +	// a bit more randomness in case getrandom() fails
> +	__stack_chk_guard |= (uintptr_t) &__stack_chk_guard;

Using |= will in fact remove randomness rather than add, because it
will turn some zero bits to ones but not the opposite. Maybe you'd
want to use "^=" or "+=" instead ?

Thanks,
Willy