[PATCH v13 RESEND 08/14] arm64/ptrace: Skip syscall exit reporting for PTRACE_SYSEMU_SINGLESTEP

[Jinjie Ruan]

Align the syscall exit reporting logic with the generic entry
framework by skipping the exit stop when PTRACE_SYSEMU_SINGLESTEP is
in effect.

[Rationale]
When a tracer uses PTRACE_SYSEMU_SINGLESTEP, both _TIF_SYSCALL_EMU
and _TIF_SINGLESTEP flags are set. Currently, arm64 reports a syscall
exit stop whenever _TIF_SINGLESTEP is set, regardless of the
emulation state.

However, as per the generic entry implementation (see
include/linux/entry-common.h):
"If SYSCALL_EMU is set, then the only reason to report is when SINGLESTEP
is set (i.e. PTRACE_SYSEMU_SINGLESTEP). This syscall instruction has been
already reported in syscall_trace_enter()."

Since PTRACE_SYSEMU intercepts and skips the actual syscall
execution, reporting a subsequent exit stop is redundant and
inconsistent with the expected behavior of emulated system calls.

[Changes]
- Introduce report_single_step(): Add a helper to encapsulate the
  logic for deciding whether to report a single-step stop at syscall
  exit. It returns false if _TIF_SYSCALL_EMU is set, ensuring the
  emulated syscall does not trigger a duplicate report.

- Update syscall_exit_work(): Use the new helper to determine
  the stepping state instead of directly checking _TIF_SINGLESTEP.

[Impact]
- PTRACE_SINGLESTEP: Continues to report exit stops for actual
  instructions.

- PTRACE_SYSEMU: Continues to skip exit stops.

- PTRACE_SYSEMU_SINGLESTEP: Now correctly skips the redundant exit
  stop, aligning arm64 with the generic entry infrastructure.

Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm64/kernel/ptrace.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 3cac9668aaa8..766de3584cff 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -2454,14 +2454,25 @@ int syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 	return ret ? : syscall;
 }
 
+static inline bool report_single_step(unsigned long flags)
+{
+	if (flags & _TIF_SYSCALL_EMU)
+		return false;
+
+	return flags & _TIF_SINGLESTEP;
+}
+
 static void syscall_exit_work(struct pt_regs *regs, unsigned long flags)
 {
+	bool step;
+
 	audit_syscall_exit(regs);
 
 	if (flags & _TIF_SYSCALL_TRACEPOINT)
 		trace_sys_exit(regs, syscall_get_return_value(current, regs));
 
-	if (flags & (_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP))
+	step = report_single_step(flags);
+	if (step || flags & _TIF_SYSCALL_TRACE)
 		report_syscall_exit(regs);
 }
 
-- 
2.34.1