[PATCH v16 12/18] arm64: syscall: Use exit-specific flags check in el0_svc_common()

[Jinjie Ruan]

Use exit-specific _TIF_SYSCALL_EXIT_WORK mask to filter out entry-only
flags during the system call exit path checks. This aligns arm64 with
the generic entry framework's SYSCALL_WORK_EXIT semantics.

[Rationale]
The current syscall exit path re-evaluates the thread flags using
the global _TIF_SYSCALL_WORK mask. However, _TIF_SYSCALL_WORK includes
flags that are strictly relevant to system call entry processing:

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 fully
  executed, reporting a separate syscall exit stop is unnecessary.

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

- Optimize re-evaluation check: Use _TIF_SYSCALL_EXIT_WORK inside the
  el0_svc_common() fast-path block to prevent redundant exit work execution
  when entry-only flags fluctuate or are synchronously modified under
  the hood. The outermost gating maintains _TIF_SYSCALL_WORK to preserve
  architectural entry-exit symmetry for tracers.

- Cleanup: Remove the has_syscall_work() helper as it is no longer
  needed, supporting direct flag comparison to clearly distinguish between
  entry and exit mandates.

[Impact]
Unnecessary exit tracing and auditing processing are safely bypassed when
entry-specific flags fluctuate during the fast-path re-check block. This
safely streamlines the syscall exit sequence to mirror the generic entry
loop behaviors without breaking debugger expectations.

No functional changes intended

Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
Reviewed-by: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>
Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm64/kernel/syscall.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 6de1fe281d61..5dd94bece929 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[])
 {
@@ -95,7 +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
@@ -125,9 +120,9 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	 * 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)) {
+	if (!(unlikely(flags & _TIF_SYSCALL_WORK)) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
 		flags = read_thread_flags();
-		if (has_syscall_work(flags) || flags & _TIF_SINGLESTEP)
+		if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
 			syscall_exit_to_user_mode_work(regs);
 		return;
 	}
-- 
2.34.1