[PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[Jinjie Ruan]

Currently, only x86 genuinely implements and supports Syscall User
Dispatch (SUD). Multiple architectures provide a stub
arch_syscall_is_vdso_sigreturn() returning 'false' simply to satisfy
GENERIC_ENTRY compilation, which creates a false impression of feature
support.

Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH to decouple this mechanism
from GENERIC_ENTRY. Select it exclusively on x86 and remove the redundant
stub functions from other architectures.

Link: https://lore.kernel.org/linux-arm-kernel/akZgV0Y4YAmB43_g@J2N7QTR9R3.cambridge.arm.com/
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
 arch/Kconfig                         | 1 +
 arch/loongarch/include/asm/syscall.h | 6 ------
 arch/powerpc/include/asm/syscall.h   | 5 -----
 arch/riscv/include/asm/syscall.h     | 5 -----
 arch/s390/include/asm/syscall.h      | 5 -----
 arch/x86/Kconfig                     | 1 +
 6 files changed, 2 insertions(+), 21 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 0c01521c2f3f..393d0fb75eac 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -116,6 +116,7 @@ config GENERIC_ENTRY
 
 config SYSCALL_USER_DISPATCH
 	bool "Syscall User Dispatch"
+	depends on ARCH_SUPPORTS_SYSCALL_USER_DISPATCH
 	depends on GENERIC_ENTRY
 	default y
 	help
diff --git a/arch/loongarch/include/asm/syscall.h b/arch/loongarch/include/asm/syscall.h
index df8ea223c77b..d9275010f548 100644
--- a/arch/loongarch/include/asm/syscall.h
+++ b/arch/loongarch/include/asm/syscall.h
@@ -84,10 +84,4 @@ static inline int syscall_get_arch(struct task_struct *task)
 	return AUDIT_ARCH_LOONGARCH64;
 #endif
 }
-
-static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
-{
-	return false;
-}
-
 #endif	/* __ASM_LOONGARCH_SYSCALL_H */
diff --git a/arch/powerpc/include/asm/syscall.h b/arch/powerpc/include/asm/syscall.h
index 834fcc4f7b54..4b3c52ed6e9d 100644
--- a/arch/powerpc/include/asm/syscall.h
+++ b/arch/powerpc/include/asm/syscall.h
@@ -139,9 +139,4 @@ static inline int syscall_get_arch(struct task_struct *task)
 	else
 		return AUDIT_ARCH_PPC64;
 }
-
-static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
-{
-	return false;
-}
 #endif	/* _ASM_SYSCALL_H */
diff --git a/arch/riscv/include/asm/syscall.h b/arch/riscv/include/asm/syscall.h
index 8067e666a4ca..987c9a78806f 100644
--- a/arch/riscv/include/asm/syscall.h
+++ b/arch/riscv/include/asm/syscall.h
@@ -112,11 +112,6 @@ static inline void syscall_handler(struct pt_regs *regs, ulong syscall)
 	regs->a0 = fn(regs);
 }
 
-static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
-{
-	return false;
-}
-
 asmlinkage long sys_riscv_flush_icache(uintptr_t, uintptr_t, uintptr_t);
 
 asmlinkage long sys_riscv_hwprobe(struct riscv_hwprobe *, size_t, size_t,
diff --git a/arch/s390/include/asm/syscall.h b/arch/s390/include/asm/syscall.h
index 4271e4169f45..5f310caad1fc 100644
--- a/arch/s390/include/asm/syscall.h
+++ b/arch/s390/include/asm/syscall.h
@@ -89,11 +89,6 @@ static inline int syscall_get_arch(struct task_struct *task)
 	return AUDIT_ARCH_S390X;
 }
 
-static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
-{
-	return false;
-}
-
 #define SYSCALL_FMT_0
 #define SYSCALL_FMT_1 , "0" (r2)
 #define SYSCALL_FMT_2 , "d" (r3) SYSCALL_FMT_1
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bdad90f210e4..f3f8ad107eeb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -335,6 +335,7 @@ config X86
 	select SCHED_SMT			if SMP
 	select ARCH_SUPPORTS_SCHED_CLUSTER	if SMP
 	select ARCH_SUPPORTS_SCHED_MC		if SMP
+	select ARCH_SUPPORTS_SYSCALL_USER_DISPATCH
 	select HAVE_SINGLE_FTRACE_DIRECT_OPS	if X86_64 && DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 
 config INSTRUCTION_DECODER
-- 
2.34.1

Re: [PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[Thomas Weißschuh]

On Thu, Jul 09, 2026 at 03:28:03PM +0800, Jinjie Ruan wrote:
> Currently, only x86 genuinely implements and supports Syscall User
> Dispatch (SUD). Multiple architectures provide a stub
> arch_syscall_is_vdso_sigreturn() returning 'false' simply to satisfy
> GENERIC_ENTRY compilation, which creates a false impression of feature
> support.
> 
> Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH to decouple this mechanism
> from GENERIC_ENTRY. Select it exclusively on x86 and remove the redundant
> stub functions from other architectures.

Nice, this confused me before.

> Link: https://lore.kernel.org/linux-arm-kernel/akZgV0Y4YAmB43_g@J2N7QTR9R3.cambridge.arm.com/
> Cc: Thomas Gleixner <tglx@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>  arch/Kconfig                         | 1 +
>  arch/loongarch/include/asm/syscall.h | 6 ------
>  arch/powerpc/include/asm/syscall.h   | 5 -----
>  arch/riscv/include/asm/syscall.h     | 5 -----
>  arch/s390/include/asm/syscall.h      | 5 -----
>  arch/x86/Kconfig                     | 1 +
>  6 files changed, 2 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 0c01521c2f3f..393d0fb75eac 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -116,6 +116,7 @@ config GENERIC_ENTRY
>  
>  config SYSCALL_USER_DISPATCH
>  	bool "Syscall User Dispatch"
> +	depends on ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

ARCH_SUPPORTS_SYSCALL_USER_DISPATCH is not declared as a config option,
so the 'select' will not do anything. Needs this:

config ARCH_SUPPORTS_SYSCALL_USER_DISPATCH
	bool

>  	depends on GENERIC_ENTRY
>  	default y
>  	help

(...)

Re: [PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

New issues:
- [Critical] The Kconfig symbol `ARCH_SUPPORTS_SYSCALL_USER_DISPATCH` is referenced in `depends on` and `select` but is never defined anywhere, silently disabling Syscall User Dispatch globally.

Pre-existing issues:
- [High] This problem wasn't introduced by this patch, but pre-exists in the state restoration logic: tasks that configure Syscall User Dispatch with `PR_SYS_DISPATCH_INCLUSIVE_ON` cannot have their configuration restored via ptrace, breaking Checkpoint/Restore (CRIU).
--

--- Patch [1]: [PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH ---
commit 46bd6e141d4dc4cf13cdaf9787b9a2959d21a56a
Author: Jinjie Ruan <ruanjinjie@huawei.com>

syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

This commit introduces ARCH_SUPPORTS_SYSCALL_USER_DISPATCH to decouple
Syscall User Dispatch from GENERIC_ENTRY. It selects the new config
exclusively on x86 and removes redundant stub functions from other
architectures.

Link: https://lore.kernel.org/linux-arm-kernel/akZgV0Y4YAmB43_g@J2N7QTR9R3.cambridge.arm.com/

> diff --git a/arch/Kconfig b/arch/Kconfig
> index 0c01521c2f3f2..393d0fb75eac4 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -116,6 +116,7 @@ config GENERIC_ENTRY
>  
>  config SYSCALL_USER_DISPATCH
>  	bool "Syscall User Dispatch"
> +	depends on ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[Severity: Critical]
Does this regression disable Syscall User Dispatch globally? It appears
ARCH_SUPPORTS_SYSCALL_USER_DISPATCH is referenced here but never defined in
any Kconfig file. Since the symbol is undefined, Kconfig evaluates it to
false, which prevents CONFIG_SYSCALL_USER_DISPATCH from being enabled and
could break the userspace ABI where prctl(PR_SET_SYSCALL_USER_DISPATCH)
returns -EINVAL.

>  	depends on GENERIC_ENTRY
>  	default y
>  	help

[ ... ]

> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b5571667a89b2..2bff333d29e84 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -336,6 +336,7 @@ config X86
>  	select SCHED_SMT			if SMP
>  	select ARCH_SUPPORTS_SCHED_CLUSTER	if SMP
>  	select ARCH_SUPPORTS_SCHED_MC		if SMP
> +	select ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[Severity: Critical]
Could this select statement be ineffective? Since
ARCH_SUPPORTS_SYSCALL_USER_DISPATCH is not defined anywhere, selecting it
here has no effect.

>  	select HAVE_SINGLE_FTRACE_DIRECT_OPS	if X86_64 && DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>  
>  config INSTRUCTION_DECODER

[ ... ]

[Severity: High]
This is a pre-existing issue, but does the state restoration logic break for
tasks configuring Syscall User Dispatch with PR_SYS_DISPATCH_INCLUSIVE_ON?

When a task configures PR_SYS_DISPATCH_INCLUSIVE_ON,
task_set_syscall_user_dispatch() inverts offset and len:

kernel/entry/syscall_user_dispatch.c:task_set_syscall_user_dispatch() {
    ...
	case PR_SYS_DISPATCH_INCLUSIVE_ON:
		/* ... */
		offset += len;
		len = -len;
    ...
}

However, syscall_user_dispatch_get_config() reads these inverted values but
sets the mode to PR_SYS_DISPATCH_ON (which is EXCLUSIVE_ON), losing the
INCLUSIVE_ON distinction:

kernel/entry/syscall_user_dispatch.c:syscall_user_dispatch_get_config() {
    ...
	if (test_task_syscall_work(task, SYSCALL_USER_DISPATCH))
		cfg.mode = PR_SYS_DISPATCH_ON;
	else
		cfg.mode = PR_SYS_DISPATCH_OFF;

	cfg.offset = sd->offset;
	cfg.len = sd->len;
    ...
}

During restoration, task_set_syscall_user_dispatch() evaluates this state
under the EXCLUSIVE_ON branch, where the overflow check rejects the inverted
wrap-around parameters:

kernel/entry/syscall_user_dispatch.c:task_set_syscall_user_dispatch() {
    ...
	case PR_SYS_DISPATCH_EXCLUSIVE_ON:
		/* ... */
		if (offset && offset + len <= offset)
			return -EINVAL;
    ...
}

Could this cause ptrace(PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG) to
unconditionally fail with -EINVAL for inclusive mode configurations,
breaking Checkpoint/Restore?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260709072803.3843948-1-ruanjinjie@huawei.com?part=1

Re: [PATCH] syscall_user_dispatch: Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH

[Jinjie Ruan]

On 7/9/2026 3:36 PM, Thomas Weißschuh wrote:
> On Thu, Jul 09, 2026 at 03:28:03PM +0800, Jinjie Ruan wrote:
>> Currently, only x86 genuinely implements and supports Syscall User
>> Dispatch (SUD). Multiple architectures provide a stub
>> arch_syscall_is_vdso_sigreturn() returning 'false' simply to satisfy
>> GENERIC_ENTRY compilation, which creates a false impression of feature
>> support.
>>
>> Introduce ARCH_SUPPORTS_SYSCALL_USER_DISPATCH to decouple this mechanism
>> from GENERIC_ENTRY. Select it exclusively on x86 and remove the redundant
>> stub functions from other architectures.
> 
> Nice, this confused me before.
> 
>> Link: https://lore.kernel.org/linux-arm-kernel/akZgV0Y4YAmB43_g@J2N7QTR9R3.cambridge.arm.com/
>> Cc: Thomas Gleixner <tglx@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
>> Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
>> Suggested-by: Mark Rutland <mark.rutland@arm.com>
>> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
>> ---
>>  arch/Kconfig                         | 1 +
>>  arch/loongarch/include/asm/syscall.h | 6 ------
>>  arch/powerpc/include/asm/syscall.h   | 5 -----
>>  arch/riscv/include/asm/syscall.h     | 5 -----
>>  arch/s390/include/asm/syscall.h      | 5 -----
>>  arch/x86/Kconfig                     | 1 +
>>  6 files changed, 2 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 0c01521c2f3f..393d0fb75eac 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -116,6 +116,7 @@ config GENERIC_ENTRY
>>  
>>  config SYSCALL_USER_DISPATCH
>>  	bool "Syscall User Dispatch"
>> +	depends on ARCH_SUPPORTS_SYSCALL_USER_DISPATCH
> 
> ARCH_SUPPORTS_SYSCALL_USER_DISPATCH is not declared as a config option,
> so the 'select' will not do anything. Needs this:
> 
> config ARCH_SUPPORTS_SYSCALL_USER_DISPATCH
> 	bool

Yes, this is missing here.
> 
>>  	depends on GENERIC_ENTRY
>>  	default y
>>  	help
> 
> (...)