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Subject[RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks
Thanks to all the recent x86 entry code refactoring, most tasks' kernel
stacks start at the same offset right above their saved pt_regs,
regardless of which syscall was used to enter the kernel. That creates
a nice convention which makes it straightforward to identify the
"bottom" of the stack, which can be useful for stack walking code which
needs to verify the stack is sane.

However there are still a few types of tasks which don't yet follow that

1) CPU idle tasks, aka the "swapper" tasks

2) freshly forked TIF_FORK tasks which don't have a stack at all

Make the idle tasks conform to the new stack bottom convention by
starting their stack at a sizeof(pt_regs) offset from the end of the
stack page.

Signed-off-by: Josh Poimboeuf <>
arch/x86/kernel/head_64.S | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 6dbd2c0..0b12311 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -296,8 +296,9 @@ ENTRY(start_cpu)
* REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
* address given in m16:64.
- movq initial_code(%rip),%rax
- pushq $0 # fake return address to stop unwinder
+ call 1f # put return address on stack for unwinder
+1: xorq %rbp, %rbp # clear frame pointer
+ movq initial_code(%rip), %rax
pushq $__KERNEL_CS # set correct cs
pushq %rax # target address in negative space
@@ -325,7 +326,7 @@ ENDPROC(start_cpu0)
.quad INIT_PER_CPU_VAR(irq_stack_union)
- .quad init_thread_union+THREAD_SIZE-8
+ .quad init_thread_union + THREAD_SIZE - SIZEOF_PTREGS

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