[PATCH v15 07/11] arm64: syscall: Introduce syscall_exit_to_user_mode_work()

[Jinjie Ruan]

Refactor the system call exit path to align with the generic entry
framework. This consolidates thread flag checking, rseq handling, and
syscall tracing into a structure that mirrors the generic
syscall_exit_to_user_mode_work() implementation.

[Rationale]
The generic entry code employs a hierarchical approach for
syscall exit work:

1. syscall_exit_to_user_mode_work(): The entry point that handles
   rseq and checks if further exit work (tracing/audit) is required.

2. syscall_exit_work(): Performs the actual tracing, auditing, and
   ptrace reporting.

[Changes]
- Rename and Encapsulate: Rename syscall_trace_exit() to
  syscall_exit_work() and make it static, as it is now an internal
  helper for the exit path.

- New Entry Point: Implement syscall_exit_to_user_mode_work() to
  replace the manual flag-reading logic in el0_svc_common(). This
  function now encapsulates the rseq_syscall() call and the
  conditional execution of syscall_exit_work().

- Simplify el0_svc_common(): Remove the complex conditional checks
  for tracing and CONFIG_DEBUG_RSEQ at the end of the syscall path,
  delegating this responsibility to the new helper.

- Helper Migration: Move has_syscall_work() to asm/syscall.h
  to allow its reuse across ptrace.c and syscall.c.

- Clean up RSEQ: Remove the explicit IS_ENABLED(CONFIG_DEBUG_RSEQ)
  check in the caller, as rseq_syscall() is already a no-op when the
  config is disabled.

Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>
Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
v15
- Make syscall_exit_to_user_mode_work() __always_inline to keep
  the fast-path performance as Sashiko pointed out.
---
 arch/arm64/include/asm/syscall.h | 18 +++++++++++++++++-
 arch/arm64/kernel/ptrace.c       |  5 +----
 arch/arm64/kernel/syscall.c      | 20 +-------------------
 3 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 30b203ef156b..b331e09b937f 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -8,6 +8,7 @@
 #include <uapi/linux/audit.h>
 #include <linux/compat.h>
 #include <linux/err.h>
+#include <linux/rseq.h>
 
 typedef long (*syscall_fn_t)(const struct pt_regs *regs);
 
@@ -121,6 +122,21 @@ static inline int syscall_get_arch(struct task_struct *task)
 }
 
 int syscall_trace_enter(struct pt_regs *regs, unsigned long flags);
-void syscall_trace_exit(struct pt_regs *regs, unsigned long flags);
+void syscall_exit_work(struct pt_regs *regs, unsigned long flags);
+
+static inline bool has_syscall_work(unsigned long flags)
+{
+	return unlikely(flags & _TIF_SYSCALL_WORK);
+}
+
+static __always_inline void syscall_exit_to_user_mode_work(struct pt_regs *regs)
+{
+	unsigned long flags = read_thread_flags();
+
+	rseq_syscall(regs);
+
+	if (has_syscall_work(flags) || flags & _TIF_SINGLESTEP)
+		syscall_exit_work(regs, flags);
+}
 
 #endif	/* __ASM_SYSCALL_H */
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 15a45eeb56da..256aa20377e1 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -28,7 +28,6 @@
 #include <linux/hw_breakpoint.h>
 #include <linux/regset.h>
 #include <linux/elf.h>
-#include <linux/rseq.h>
 
 #include <asm/compat.h>
 #include <asm/cpufeature.h>
@@ -2454,10 +2453,8 @@ int syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 	return syscall;
 }
 
-void syscall_trace_exit(struct pt_regs *regs, unsigned long flags)
+void syscall_exit_work(struct pt_regs *regs, unsigned long flags)
 {
-	rseq_syscall(regs);
-
 	audit_syscall_exit(regs);
 
 	if (flags & _TIF_SYSCALL_TRACEPOINT)
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index f6f87b042995..dac7bcc4bbdf 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -54,11 +54,6 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 	syscall_set_return_value(current, regs, 0, ret);
 }
 
-static inline bool has_syscall_work(unsigned long flags)
-{
-	return unlikely(flags & _TIF_SYSCALL_WORK);
-}
-
 static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 			   const syscall_fn_t syscall_table[])
 {
@@ -120,21 +115,8 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	}
 
 	invoke_syscall(regs, scno, sc_nr, syscall_table);
-
-	/*
-	 * The tracing status may have changed under our feet, so we have to
-	 * check again. However, if we were tracing entry, then we always trace
-	 * exit regardless, as the old entry assembly did.
-	 */
-	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
-		flags = read_thread_flags();
-		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
-			return;
-	}
-
 trace_exit:
-	flags = read_thread_flags();
-	syscall_trace_exit(regs, flags);
+	syscall_exit_to_user_mode_work(regs);
 }
 
 void do_el0_svc(struct pt_regs *regs)
-- 
2.34.1