[PATCH v2 2/5] x86/boot/64: Clear BSS as early as possible

[Brian Gerst]

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>
---
 arch/x86/include/asm/setup.h |  2 --
 arch/x86/kernel/head64.c     | 12 ------------
 arch/x86/kernel/head_64.S    | 20 ++++++++++++++++++++
 arch/x86/xen/enlighten_pv.c  |  2 --
 arch/x86/xen/xen-head.S      |  5 ++++-
 5 files changed, 24 insertions(+), 17 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 914eb32581c7..5534f4aaf73b 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -126,8 +126,6 @@ void *extend_brk(size_t size, size_t align);
 
 extern void probe_roms(void);
 
-void clear_bss(void);
-
 #ifdef __i386__
 
 asmlinkage void __init __noreturn i386_start_kernel(void);
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index fd28b53dbac5..9f19635c6390 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -172,16 +172,6 @@ void __init do_early_exception(struct pt_regs *regs, int trapnr)
 	early_fixup_exception(regs, trapnr);
 }
 
-/* Don't add a printk in there. printk relies on the PDA which is not initialized 
-   yet. */
-void __init clear_bss(void)
-{
-	memset(__bss_start, 0,
-	       (unsigned long) __bss_stop - (unsigned long) __bss_start);
-	memset(__brk_base, 0,
-	       (unsigned long) __brk_limit - (unsigned long) __brk_base);
-}
-
 static unsigned long get_cmd_line_ptr(void)
 {
 	unsigned long cmd_line_ptr = boot_params.hdr.cmd_line_ptr;
@@ -246,8 +236,6 @@ asmlinkage __visible void __init __noreturn x86_64_start_kernel(char * real_mode
 		vmemmap_base		= __VMEMMAP_BASE_L5;
 	}
 
-	clear_bss();
-
 	/*
 	 * This needs to happen *before* kasan_early_init() because latter maps stuff
 	 * into that page.
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
+
+	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
+	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
-- 
2.55.0