[PATCH v15 08/11] arm64/ptrace: Define and use _TIF_SYSCALL_EXIT_WORK

[Jinjie Ruan]

Introduce _TIF_SYSCALL_EXIT_WORK to filter out entry-only flags
during the syscall exit path. This aligns arm64 with the generic
entry framework's SYSCALL_WORK_EXIT semantics.

[Rationale]
The current syscall exit path uses _TIF_SYSCALL_WORK to decide whether
to invoke syscall_exit_work(). However, _TIF_SYSCALL_WORK includes
flags that are only relevant during syscall entry:

1. _TIF_SECCOMP: Seccomp filtering (__secure_computing) only runs
  on entry. There is no seccomp callback for syscall exit.

2. _TIF_SYSCALL_EMU: In PTRACE_SYSEMU mode, the syscall is
  intercepted and skipped on entry. Since the syscall is never
  executed, reporting a syscall exit stop is unnecessary.

[Changes]
- Define _TIF_SYSCALL_EXIT_WORK: A new mask containing only flags
  requiring exit processing: _TIF_SYSCALL_TRACE, _TIF_SYSCALL_AUDIT,
  and _TIF_SYSCALL_TRACEPOINT.

- Update exit path: Use _TIF_SYSCALL_EXIT_WORK in
  syscall_exit_to_user_mode_work() to avoid redundant calls to
  audit and ptrace reporting when only entry-flags are set.

- Cleanup: Remove the has_syscall_work() helper as it is no longer
  needed. Direct flag comparison is now used to distinguish between
  entry and exit work requirements.

[Impact]
audit_syscall_exit() and report_syscall_exit() will no longer be
triggered for seccomp-only or emu-only syscalls. This matches the
generic entry behavior and improves efficiency by skipping unnecessary
exit processing.

Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Will Deacon <will at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Reviewed-by: Linus Walleij <linusw at kernel.org>
Reviewed-by: Yeoreum Yun <yeoreum.yun at arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
---
 arch/arm64/include/asm/syscall.h     | 7 +------
 arch/arm64/include/asm/thread_info.h | 3 +++
 arch/arm64/kernel/syscall.c          | 2 +-
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index b331e09b937f..b982398f8765 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -124,18 +124,13 @@ static inline int syscall_get_arch(struct task_struct *task)
 int syscall_trace_enter(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)
+	if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
 		syscall_exit_work(regs, flags);
 }
 
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 5d7fe3e153c8..56a2c9426a32 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -112,6 +112,9 @@ void arch_setup_new_exec(void);
 				 _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
 				 _TIF_SYSCALL_EMU)
 
+#define _TIF_SYSCALL_EXIT_WORK	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
+				 _TIF_SYSCALL_TRACEPOINT)
+
 #ifdef CONFIG_SHADOW_CALL_STACK
 #define INIT_SCS							\
 	.scs_base	= init_shadow_call_stack,			\
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index dac7bcc4bbdf..6ac71a0282d5 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -90,7 +90,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 		return;
 	}
 
-	if (has_syscall_work(flags)) {
+	if (unlikely(flags & _TIF_SYSCALL_WORK)) {
 		/*
 		 * The de-facto standard way to skip a system call using ptrace
 		 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
-- 
2.34.1