Re: [patch 08/18] x86/syscall: Use [syscall_]enter_from_user_mode_randomize_stack()

[Radu Rendec]

On Tue, 2026-07-07 at 21:06 +0200, Thomas Gleixner wrote:
> These functions integrate the stack randomization.
> 
> syscall_enter_from_user_mode_randomize_stack() has the advantage that the
> randomization happens early right after enter_from_user_mode().
> 
> In both cases also the overhead of get/put_cpu_var() in
> add_random_kstack_offset() is avoided.
> 
> No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
>  arch/x86/entry/syscall_32.c         |   19 +++++--------------
>  arch/x86/entry/syscall_64.c         |    3 +--
>  arch/x86/include/asm/entry-common.h |    1 -
>  3 files changed, 6 insertions(+), 17 deletions(-)
> 
> --- a/arch/x86/entry/syscall_32.c
> +++ b/arch/x86/entry/syscall_32.c
> @@ -142,10 +142,9 @@ static __always_inline bool int80_is_ext
>  	 * int80_is_external() below which calls into the APIC driver.
>  	 * Identical for soft and external interrupts.
>  	 */
> -	enter_from_user_mode(regs);
> +	enter_from_user_mode_randomize_stack(regs);
>  
>  	instrumentation_begin();
> -	add_random_kstack_offset();
>  
>  	/* Validate that this is a soft interrupt to the extent possible */
>  	if (unlikely(int80_is_external()))
> @@ -210,11 +209,9 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
>  {
>  	int nr;
>  
> -	enter_from_user_mode(regs);
> +	enter_from_user_mode_randomize_stack(regs);
>  
>  	instrumentation_begin();
> -	add_random_kstack_offset();
> -
>  	/*
>  	 * FRED pushed 0 into regs::orig_ax and regs::ax contains the
>  	 * syscall number.
> @@ -252,10 +249,10 @@ DEFINE_FREDENTRY_RAW(int80_emulation)
>  	 * orig_ax, the int return value truncates it. This matches
>  	 * the semantics of syscall_get_nr().
>  	 */
> -	nr = syscall_enter_from_user_mode(regs, nr);
> +	nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
> +
>  	instrumentation_begin();
>  
> -	add_random_kstack_offset();
>  	do_syscall_32_irqs_on(regs, nr);
>  
>  	instrumentation_end();
> @@ -268,15 +265,9 @@ static noinstr bool __do_fast_syscall_32
>  	int nr = syscall_32_enter(regs);
>  	int res;
>  
> -	/*
> -	 * This cannot use syscall_enter_from_user_mode() as it has to
> -	 * fetch EBP before invoking any of the syscall entry work
> -	 * functions.
> -	 */
> -	enter_from_user_mode(regs);
> +	enter_from_user_mode_randomize_stack(regs);
>  
>  	instrumentation_begin();
> -	add_random_kstack_offset();
>  	local_irq_enable();
>  	/* Fetch EBP from where the vDSO stashed it. */
>  	if (IS_ENABLED(CONFIG_X86_64)) {
> --- a/arch/x86/entry/syscall_64.c
> +++ b/arch/x86/entry/syscall_64.c
> @@ -86,10 +86,9 @@ static __always_inline bool do_syscall_x
>  /* Returns true to return using SYSRET, or false to use IRET */
>  __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
>  {
> -	nr = syscall_enter_from_user_mode(regs, nr);
> +	nr = syscall_enter_from_user_mode_randomize_stack(regs, nr);
>  
>  	instrumentation_begin();
> -	add_random_kstack_offset();
>  
>  	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
>  		/* Invalid system call, but still a system call. */
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -2,7 +2,6 @@
>  #ifndef _ASM_X86_ENTRY_COMMON_H
>  #define _ASM_X86_ENTRY_COMMON_H
>  
> -#include <linux/randomize_kstack.h>
>  #include <linux/user-return-notifier.h>
>  
>  #include <asm/nospec-branch.h>

Reviewed-by: Radu Rendec <radu@xxxxxxxxxx>