Re: [patch 03/18] entry: Provide [syscall_]enter_from_user_mode_randomize_stack()

On 7/8/2026 3:06 AM, Thomas Gleixner wrote:
> Randomizing the syscall stack can only happen after state is established
> via enter_from_user_mode() or syscall_enter_from_user_mode(). The earlier
> it happens the better.
> 
> Provide two new macros to consolidate that:
> 
>   - enter_from_user_mode_randomize_stack()
> 	enter_from_user_mode();
> 	add_random_kstack_offset_irqsoff();
> 
>   - syscall_enter_from_user_mode_randomize_stack()
> 	enter_from_user_mode_randomize_stack();
> 	syscall_enter_from_user_mode_work();
>     
> to reduce boiler plate code.
> 
> Those are macros and not inline functions as the latter would limit the
> stack randomization scope to the inline function itself.
> 
> Signed-off-by: Thomas Gleixner <[email protected]>
> ---
>  include/linux/entry-common.h |   56 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -6,6 +6,7 @@
>  #include <linux/irq-entry-common.h>
>  #include <linux/livepatch.h>
>  #include <linux/ptrace.h>
> +#include <linux/randomize_kstack.h>
>  #include <linux/resume_user_mode.h>
>  #include <linux/seccomp.h>
>  #include <linux/sched.h>
> @@ -150,6 +151,61 @@ static __always_inline long syscall_ente
>  }
>  
>  /**
> + * enter_from_user_mode_randomize_stack - Establish state and add stack randomization
> + *					  before invoking syscall_enter_from_user_mode_work()
> + * @regs:	Pointer to currents pt_regs
> + *
> + * Invoked from architecture specific syscall entry code with interrupts
> + * disabled. The calling code has to be non-instrumentable. When the function
> + * returns all state is correct, interrupts are still disabled and the
> + * subsequent functions can be instrumented.
> + *
> + * Implemented as a macro so that the stack randomization is effective
> + * throughout the function in which it is invoked. An inline would only make it
> + * effective in the scope of the inline function.
> + */
> +#define enter_from_user_mode_randomize_stack(regs)			\
> +do {									\
> +	enter_from_user_mode(regs);					\
> +	instrumentation_begin();					\
> +	add_random_kstack_offset_irqsoff();				\
> +	instrumentation_end();						\
> +} while (0)


Perhaps this new function can also be reused when the ARM64 is switched
to the generic entry as the irq also disabled now.

--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -64,7 +64,7 @@ static void noinstr arm64_exit_to_kernel_mode(struct
pt_regs *regs,

 static __always_inline void arm64_syscall_enter_from_user_mode(struct
pt_regs *regs)
 {
-       enter_from_user_mode(regs);
+       enter_from_user_mode_randomize_stack(regs);
        mte_disable_tco_entry(current);
        sme_enter_from_user_mode();
 }
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index e0a98fac3b85..42ac02573b66 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -6,7 +6,6 @@
 #include <linux/errno.h>
 #include <linux/nospec.h>
 #include <linux/ptrace.h>
-#include <linux/randomize_kstack.h>
 #include <linux/syscalls.h>

 #include <asm/debug-monitors.h>
@@ -42,8 +41,6 @@ static void invoke_syscall(struct pt_regs *regs,
unsigned int scno,
 {
        long ret;

-       add_random_kstack_offset();
-
        if (likely(scno < sc_nr)) {
                syscall_fn_t syscall_fn;
                syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];


> +
> +/**
> + * syscall_enter_from_user_mode_randomize_stack - Establish state and check and handle work
> + *						  before invoking a syscall
> + * @regs:	Pointer to currents pt_regs
> + * @syscall:	The syscall number
> + *
> + * Invoked from architecture specific syscall entry code with interrupts
> + * disabled. The calling code has to be non-instrumentable. When the
> + * function returns all state is correct, interrupts are enabled and the
> + * subsequent functions can be instrumented.
> + *
> + * This is the combination of enter_from_user_mode_randomize_stack() and
> + * syscall_enter_from_user_mode_work() to be used when there is no
> + * architecture specific work to be done between the two.
> + *
> + * Returns: The original or a modified syscall number. See
> + * syscall_enter_from_user_mode_work() for further explanation.
> + *
> + * Implemented as a macro to make stack randomization effective in the calling
> + * scope.
> + */
> +#define syscall_enter_from_user_mode_randomize_stack(regs, syscall)	\
> +({									\
> +	enter_from_user_mode_randomize_stack(regs);			\
> +									\
> +	instrumentation_begin();					\
> +	local_irq_enable();						\
> +	long _ret = syscall_enter_from_user_mode_work(regs, syscall);	\
> +	instrumentation_end();						\
> +									\
> +	_ret;								\
> +})
> +
> +/**
>   * syscall_enter_from_user_mode - Establish state and check and handle work
>   *				  before invoking a syscall
>   * @regs:	Pointer to currents pt_regs
>