[PATCH v16 11/18] arm64: ptrace: Align syscall exit work semantics with generic entry

[Jinjie Ruan]

Streamline syscall_exit_to_user_mode_work() to align arm64's syscall
exit behavior with the generic entry framework.

[Rationale]
1. Unconditional RSEQ Execution: Relocate rseq_syscall() from the slow-path
   helper to the very beginning of syscall_exit_to_user_mode_work(). This
   ensures that RSEQ validation executes unconditionally across all exit
   scenarios, preventing it from being incorrectly bypassed on fast paths
   when CONFIG_DEBUG_RSEQ is active.

2. Centralized Exit Work Gating: Introduce the `_TIF_SYSCALL_EXIT_WORK`
   mask to aggregate exit thread flags and gate the execution of
   syscall_exit_work().

   Gating audit_syscall_exit() behind this exit-time check introduces
   no functional changes. The `SYSCALL_AUDIT` flag and its context are
   statically allocated via audit_alloc() at fork time and only freed via
   audit_free() at do_exit(). Since the flag remains persistent and static
   throughout syscall execution, checking `_TIF_SYSCALL_AUDIT` in the mask
   is fully equivalent to evaluating audit_context() inside
   audit_syscall_exit().

[Changes]
- Introduce the `_TIF_SYSCALL_EXIT_WORK` mask to bundle exit-specific
  flags.

- Relocate rseq_syscall() to run unconditionally on the outermost layer.

- Gate syscall_exit_work() via the new aggregated flag check to mirror
  the generic entry loop behavior.

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>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm64/include/asm/syscall.h     | 6 +++++-
 arch/arm64/include/asm/thread_info.h | 3 +++
 arch/arm64/kernel/ptrace.c           | 3 ---
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 72461c22bb5e..b982398f8765 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);
 
@@ -127,7 +128,10 @@ static __always_inline void syscall_exit_to_user_mode_work(struct pt_regs *regs)
 {
 	unsigned long flags = read_thread_flags();
 
-	syscall_exit_work(regs, flags);
+	rseq_syscall(regs);
+
+	if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
+		syscall_exit_work(regs, flags);
 }
 
 #endif	/* __ASM_SYSCALL_H */
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/ptrace.c b/arch/arm64/kernel/ptrace.c
index 9ebe3389451c..05ceb9f2d038 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>
@@ -2455,8 +2454,6 @@ int syscall_trace_enter(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)
-- 
2.34.1