Re: [PATCH v7 1/3] xen: common: add ability to enable stack protector

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.03.2025 03:34, Volodymyr Babchuk wrote:
> Both GCC and Clang support -fstack-protector feature, which add stack
> canaries to functions where stack corruption is possible. This patch
> makes general preparations to enable this feature on different
> supported architectures:
> 
>  - Added CONFIG_HAS_STACK_PROTECTOR option so each architecture
>    can enable this feature individually
>  - Added user-selectable CONFIG_STACK_PROTECTOR option
>  - Implemented code that sets up random stack canary and a basic
>    handler for stack protector failures
> 
> Stack guard value is initialized in two phases:
> 
> 1. Pre-defined randomly-selected value.
> 
> 2. Own implementation linear congruent random number generator. It
> relies on get_cycles() being available very early. If get_cycles()
> returns zero, it would leave pre-defined value from the previous
> step.
> 
> boot_stack_chk_guard_setup() is declared as inline, so it can be

It's an always-inline function, and that is so important that it should
be got right in the description as well.

> --- /dev/null
> +++ b/xen/common/stack-protector.c
> @@ -0,0 +1,21 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#include <xen/init.h>
> +#include <xen/lib.h>
> +#include <xen/random.h>
> +#include <xen/time.h>
> +
> +/*
> + * Initial value is chosen by a fair dice roll.
> + * It will be updated during boot process.
> + */
> +#if BITS_PER_LONG == 32
> +unsigned long __ro_after_init __stack_chk_guard = 0xdd2cc927UL;
> +#else
> +unsigned long __ro_after_init __stack_chk_guard = 0x2d853605a4d9a09cUL;
> +#endif
> +
> +void asmlinkage __stack_chk_fail(void)

The use of asmlinkage here comes close to an abuse: The Misra deviation is
about C code called from assembly code only. This isn't the case here; instead
it's a function that the compiler generates calls to without source code
explicitly saying so.

This imo wants approving from the Misra side as well, and even if approved
likely requires a justifying code comment.

> --- /dev/null
> +++ b/xen/include/xen/stack-protector.h
> @@ -0,0 +1,39 @@
> +#ifndef __XEN_STACK_PROTECTOR_H__
> +#define __XEN_STACK_PROTECTOR_H__
> +
> +extern unsigned long __stack_chk_guard;
> +
> +/*
> + * This function should be called from a C function that escapes stack
> + * canary tracking (by calling reset_stack_and_jump() for example).
> + */
> +static always_inline void boot_stack_chk_guard_setup(void)
> +{
> +#ifdef CONFIG_STACK_PROTECTOR
> +
> +     /*

Nit: Hard tab slipped in.

Jan