Re: [PATCH 2/5] x86/boot/64: Clear BSS as early as possible

[Nikolay Borisov]

On 7/24/26 06:02, Brian Gerst wrote:
> Currently, the BSS section is not cleared until x86_64_start_kernel().
> Using unitialized BSS data before that point leads to difficult to debug
> problems.  Fix this by moving clear_bss() to as early as possible.
>
> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>

<snip>


> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 7ed5520dd52e..15ef5ea52b9a 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -37,6 +37,8 @@
>   	.code64
>   SYM_CODE_START_NOALIGN(startup_64)
>   	UNWIND_HINT_END_OF_STACK
> +	cld
> +
>   	/*
>   	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0,
>   	 * and someone has loaded an identity mapped page table
> @@ -61,6 +63,8 @@ SYM_CODE_START_NOALIGN(startup_64)
>   	/* Set up the stack for verify_cpu() */
>   	leaq	__top_init_kernel_stack(%rip), %rsp
>   
> +	call	clear_bss
> +
>   	/*
>   	 * Set up GSBASE.
>   	 * Note that on SMP the boot CPU uses the init data section until
> @@ -140,6 +144,22 @@ SYM_CODE_START_NOALIGN(startup_64)
>   	jmp	*.Lcommon_startup_64(%rip)
>   SYM_CODE_END(startup_64)
>   
> +SYM_FUNC_START(clear_bss)
> +	xorl	%eax, %eax
> +
> +	leaq	__bss_start(%rip), %rdi
> +	leaq	__bss_stop(%rip), %rcx
> +	subq	%rdi, %rcx
> +	rep stosb

Why not stosq (with appropriate adjustment to xor %rax, %rax as well)? 
The length is guaranteed to be multiple of page_size ergo 8 byte as well ?

> +
> +	leaq	__brk_base(%rip), %rdi
> +	leaq	__brk_limit(%rip), %rcx
> +	subq	%rdi, %rcx
> +	rep stosb
> +
> +	RET
> +SYM_FUNC_END(clear_bss)
> +
>   	__INITRODATA
>   SYM_DATA_LOCAL(.Lcommon_startup_64, .quad common_startup_64)
>   
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 2c64b388f616..6aa13d19c0a4 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1334,8 +1334,6 @@ asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
>   	if (!si)
>   		return;
>   
> -	clear_bss();
> -
>   	xen_start_info = si;
>   
>   	__text_gen_insn(&early_xen_iret_patch,
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 5dad6c51cdc3..9b56659246fe 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -31,6 +31,9 @@ SYM_CODE_START(startup_xen)
>   
>   	leaq	__top_init_kernel_stack(%rip), %rsp
>   
> +	movq	%rsi, %r15

Why is this change necessary, rsi is ont used in clear_bss ?
> +	call	clear_bss
> +
>   	/*
>   	 * Set up GSBASE.
>   	 * Note that, on SMP, the boot cpu uses init data section until
> @@ -41,7 +44,7 @@ SYM_CODE_START(startup_xen)
>   	xorl	%edx, %edx
>   	wrmsr
>   
> -	mov	%rsi, %rdi
> +	mov	%r15, %rdi
>   	call xen_start_kernel
>   SYM_CODE_END(startup_xen)
>   	__FINIT