arm64: head.S: use ordinary stack frame for __primary_switched()

Instead of stashing the value of the link register in x28 before setting
up the stack and calling into C code, create an ordinary PCS compatible
stack frame so that we can push the return address onto the stack.

Since exception handlers require a stack as well, assign the stack pointer
register before installing the vector table.

Note that this accounts for the difference between THREAD_START_SP and
THREAD_SIZE, given that the stack pointer is always decremented before
calling into any C code.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 4dee51045e790283d42641c30e06e31af1ec8e54..29a734ee0770261a8bba710d95553ed89db5471d 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -415,13 +415,18 @@ ENDPROC(__create_page_tables)
  *
  *   x0 = __PHYS_OFFSET
  */
-       .set    initial_sp, init_thread_union + THREAD_START_SP
 __primary_switched:
-       mov     x28, lr                         // preserve LR
+       adrp    x4, init_thread_union
+       add     sp, x4, #THREAD_SIZE
+       msr     sp_el0, x4                      // Save thread_info
+
        adr_l   x8, vectors                     // load VBAR_EL1 with virtual
        msr     vbar_el1, x8                    // vector table address
        isb
 
+       stp     xzr, x30, [sp, #-16]!
+       mov     x29, sp
+
        str_l   x21, __fdt_pointer, x5          // Save FDT pointer
 
        ldr_l   x4, kimage_vaddr                // Save the offset between
@@ -436,11 +441,6 @@ __primary_switched:
        bl      __pi_memset
        dsb     ishst                           // Make zero page visible to PTW
 
-       adr_l   sp, initial_sp, x4
-       mov     x4, sp
-       and     x4, x4, #~(THREAD_SIZE - 1)
-       msr     sp_el0, x4                      // Save thread_info
-       mov     x29, #0
 #ifdef CONFIG_KASAN
        bl      kasan_early_init
 #endif
@@ -452,8 +452,8 @@ __primary_switched:
        bl      kaslr_early_init                // parse FDT for KASLR options
        cbz     x0, 0f                          // KASLR disabled? just proceed
        orr     x23, x23, x0                    // record KASLR offset
-       ret     x28                             // we must enable KASLR, return
-                                               // to __primary_switch()
+       ldp     x29, x30, [sp], #16             // we must enable KASLR, return
+       ret                                     // to __primary_switch()
 0:
 #endif
        b       start_kernel