[PATCH v2] KVM/x86: make function emulator_do_task_switch as noinline_for_stack

[PATCH v2] KVM/x86: make function emulator_do_task_switch as noinline_for_stack

[flyingpenghao]

From: Peng Hao <flyingpeng@tencent.com>

When KASAN is enabled and built with clang:
clang report
arch/x86/kvm/emulate.c:3022:5: error: stack frame size (2488) exceeds limit (2048) in 'emulator_task_switch' [-Werror,-Wframe-larger-than]
int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
    ^

since emulator_do_task_switch() consumes a lot of stack space, mark it as
noinline_for_stack to prevent it from blowing up emulator_task_switch()'s
stack size.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
---
 arch/x86/kvm/emulate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5d4c86133453..bbc185b9725d 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2918,7 +2918,7 @@ static int task_switch_32(struct x86_emulate_ctxt *ctxt, u16 old_tss_sel,
 	return load_state_from_tss32(ctxt, &tss_seg);
 }
 
-static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
+static noinline_for_stack int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
 				   u16 tss_selector, int idt_index, int reason,
 				   bool has_error_code, u32 error_code)
 {
-- 
2.27.0

Re: [PATCH v2] KVM/x86: make function emulator_do_task_switch as noinline_for_stack

[Sean Christopherson]

On Fri, Jul 12, 2024, flyingpenghao@gmail.com wrote:
> From: Peng Hao <flyingpeng@tencent.com>
> 
> When KASAN is enabled and built with clang:
> clang report
> arch/x86/kvm/emulate.c:3022:5: error: stack frame size (2488) exceeds limit (2048) in 'emulator_task_switch' [-Werror,-Wframe-larger-than]
> int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
>     ^
> 
> since emulator_do_task_switch() consumes a lot of stack space, mark it as
> noinline_for_stack to prevent it from blowing up emulator_task_switch()'s
> stack size.

No, sprinkling noinline to combat KASAN stack frame sizes is not a maintainable
approach.  Practically speaking, KASAN + -Werror + FRAME_WARN=2048 is not a sane
config.

> Signed-off-by: Peng Hao <flyingpeng@tencent.com>
> ---
>  arch/x86/kvm/emulate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 5d4c86133453..bbc185b9725d 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2918,7 +2918,7 @@ static int task_switch_32(struct x86_emulate_ctxt *ctxt, u16 old_tss_sel,
>  	return load_state_from_tss32(ctxt, &tss_seg);
>  }
>  
> -static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
> +static noinline_for_stack int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
>  				   u16 tss_selector, int idt_index, int reason,
>  				   bool has_error_code, u32 error_code)
>  {
> -- 
> 2.27.0
>