syscall_user_dispatch: Make it configurable in Kconfig

Syscall User Dispatch is presently built under CONFIG_GENERIC_SYSCALL and
cannot be disabled independently.

Add CONFIG_SYSCALL_USER_DISPATCH to make it an optional feature.

Signed-off-by: Gregory Price <gourry@gourry.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260706140020.873735-2-gourry@gourry.net

arch/Kconfig

@@ -114,6 +114,16 @@ config GENERIC_ENTRY
 	select GENERIC_IRQ_ENTRY
 	select GENERIC_SYSCALL
 
+config SYSCALL_USER_DISPATCH
+	bool "Syscall User Dispatch"
+	depends on GENERIC_ENTRY
+	default y
+	help
+	  Syscall User Dispatch lets a thread have its own system calls
+	  intercepted and redirected to a userspace signal handler based
+	  on a prctl() configured instruction pointer range.
+	  If unsure, say Y.
+
 config KPROBES
 	bool "Kprobes"
 	depends on HAVE_KPROBES

include/linux/entry-common.h

@@ -9,6 +9,7 @@
 #include <linux/resume_user_mode.h>
 #include <linux/seccomp.h>
 #include <linux/sched.h>
+#include <linux/syscall_user_dispatch.h>
 
 #include <asm/entry-common.h>
 #include <asm/syscall.h>
@@ -55,7 +56,6 @@ static __always_inline int arch_ptrace_report_syscall_entry(struct pt_regs *regs
 }
 #endif
 
-bool syscall_user_dispatch(struct pt_regs *regs);
 long trace_syscall_enter(struct pt_regs *regs, long syscall);
 void trace_syscall_exit(struct pt_regs *regs, long ret);
 
@@ -232,10 +232,8 @@ static __always_inline void syscall_exit_work(struct pt_regs *regs, unsigned lon
 	 * of these syscalls is unknown.
 	 */
 	if (work & SYSCALL_WORK_SYSCALL_USER_DISPATCH) {
-		if (unlikely(current->syscall_dispatch.on_dispatch)) {
-			current->syscall_dispatch.on_dispatch = false;
+		if (syscall_user_dispatch_clear_on_dispatch())
 			return;
-		}
 	}
 
 	audit_syscall_exit(regs);

include/linux/syscall_user_dispatch.h

@@ -6,9 +6,23 @@
 #define _SYSCALL_USER_DISPATCH_H
 
 #include <linux/thread_info.h>
+#include <linux/sched.h>
 #include <linux/syscall_user_dispatch_types.h>
 
-#ifdef CONFIG_GENERIC_ENTRY
+struct pt_regs;
+
+#ifdef CONFIG_SYSCALL_USER_DISPATCH
+
+bool syscall_user_dispatch(struct pt_regs *regs);
+
+static __always_inline bool syscall_user_dispatch_clear_on_dispatch(void)
+{
+	if (likely(!current->syscall_dispatch.on_dispatch))
+		return false;
+
+	current->syscall_dispatch.on_dispatch = false;
+	return true;
+}
 
 int set_syscall_user_dispatch(unsigned long mode, unsigned long offset,
 			      unsigned long len, char __user *selector);
@@ -24,6 +38,16 @@ int syscall_user_dispatch_set_config(struct task_struct *task, unsigned long siz
 
 #else
 
+static __always_inline bool syscall_user_dispatch(struct pt_regs *regs)
+{
+	return false;
+}
+
+static __always_inline bool syscall_user_dispatch_clear_on_dispatch(void)
+{
+	return false;
+}
+
 static inline int set_syscall_user_dispatch(unsigned long mode, unsigned long offset,
 					    unsigned long len, char __user *selector)
 {
@@ -46,6 +70,6 @@ static inline int syscall_user_dispatch_set_config(struct task_struct *task,
 	return -EINVAL;
 }
 
-#endif /* CONFIG_GENERIC_ENTRY */
+#endif /* CONFIG_SYSCALL_USER_DISPATCH */
 
 #endif /* _SYSCALL_USER_DISPATCH_H */

include/linux/syscall_user_dispatch_types.h

@@ -4,7 +4,7 @@
 
 #include <linux/types.h>
 
-#ifdef CONFIG_GENERIC_ENTRY
+#ifdef CONFIG_SYSCALL_USER_DISPATCH
 
 struct syscall_user_dispatch {
 	char __user	*selector;

kernel/entry/Makefile

@@ -13,5 +13,6 @@ CFLAGS_REMOVE_common.o	 = -fstack-protector -fstack-protector-strong
 CFLAGS_common.o		+= -fno-stack-protector
 
 obj-$(CONFIG_GENERIC_IRQ_ENTRY) 	+= common.o
-obj-$(CONFIG_GENERIC_SYSCALL) 		+= syscall-common.o syscall_user_dispatch.o
+obj-$(CONFIG_GENERIC_SYSCALL) 		+= syscall-common.o
+obj-$(CONFIG_SYSCALL_USER_DISPATCH)	+= syscall_user_dispatch.o
 obj-$(CONFIG_VIRT_XFER_TO_GUEST_WORK)	+= virt.o

tools/testing/selftests/syscall_user_dispatch/config

@@ -1 +1 @@
-CONFIG_GENERIC_ENTRY=y
+CONFIG_SYSCALL_USER_DISPATCH=y