[PATCH 3/8] arm64/kernel: use a proper stack frame in __mmap_switched()

[Ard Biesheuvel]

Considering that we can expect stack accesses from the moment we assign
VBAR_EL1, let's initialize the stack pointer first in __mmap_switched(),
and set up a proper stack frame while we're at it.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 arch/arm64/kernel/head.S | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 9d8f928c355c..f441fc73a7a2 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -421,9 +421,14 @@ kernel_img_size:
 /*
  * The following fragment of code is executed with the MMU enabled.
  */
-	.set	initial_sp, init_thread_union + THREAD_START_SP
 __mmap_switched:
-	mov	x28, lr				// preserve LR
+	adrp	x4, init_thread_union
+	add	sp, x4, #THREAD_SIZE
+	msr	sp_el0, x4			// Save thread_info
+
+	stp	xzr, x30, [sp, #-16]!
+	mov	x29, sp
+
 	adr_l	x8, vectors			// load VBAR_EL1 with virtual
 	msr	vbar_el1, x8			// vector table address
 	isb
@@ -475,16 +480,9 @@ __mmap_switched:
 	dsb	sy				// with MMU off
 #endif
 
-	adr_l	sp, initial_sp, x4
-	mov	x4, sp
-	and	x4, x4, #~(THREAD_SIZE - 1)
-	msr	sp_el0, x4			// Save thread_info
-
 	ldr_l	x4, kimage_vaddr		// Save the offset between
 	sub	x4, x4, x24			// the kernel virtual and
 	str_l	x4, kimage_voffset, x5		// physical mappings
-
-	mov	x29, #0
 #ifdef CONFIG_KASAN
 	bl	kasan_early_init
 #endif
@@ -494,8 +492,8 @@ __mmap_switched:
 	bl	kaslr_early_init		// parse FDT for KASLR options
 	cbz	x0, 0f				// KASLR disabled? just proceed
 	mov	x23, x0				// record KASLR offset
-	ret	x28				// we must enable KASLR, return
-						// to __enable_mmu()
+	ldp	x29, x30, [sp], #16		// we must enable KASLR, return
+	ret					// to __enable_mmu()
 0:
 #endif
 	b	start_kernel
-- 
2.5.0