[PATCH v17 12/15] arm64: ptrace: Make return type of arm64_syscall_trace_enter() bool

[Jinjie Ruan]

In preparation for migrating arm64 to the generic entry infrastructure,
decouple the decision to execute the syscall from the syscall number
itself. The ptrace and seccomp now returns a boolean flag rather than
overloading NO_SYSCALL (-1).

Changes:
- arm64_syscall_trace_enter() returns bool:
  - false: abort syscall execution
  - true: proceed, with syscall number reread from regs
- If we decide to execute the syscall, reread the syscall number
  and check if it is NO_SYSCALL again. Because when a tracer sets
  the syscall number to -1 and provides a custom return value in x0,
  this avoids calling invoke_syscall() which would overwrite the custom
  value with -ENOSYS by sys_ni_syscall().

Behavioral impact (unchanged):
- User-issued syscall(-1): unchanged, returns -ENOSYS.
- Tracer skip with custom x0: unchanged, returns custom x0
- Tracer skip without setting x0: unchanged, returns original
  argument (garbage)
- Normal syscalls and seccomp rejects: unaffected

No functional changes.

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 | 2 +-
 arch/arm64/kernel/ptrace.c       | 8 ++++----
 arch/arm64/kernel/syscall.c      | 6 +++++-
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
index 7cc872fde019..f19d4a8824ba 100644
--- a/arch/arm64/include/asm/syscall.h
+++ b/arch/arm64/include/asm/syscall.h
@@ -121,7 +121,7 @@ static inline int syscall_get_arch(struct task_struct *task)
 	return AUDIT_ARCH_AARCH64;
 }
 
-int arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags);
+bool arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags);
 void arm64_syscall_exit_work(struct pt_regs *regs, unsigned long flags);
 
 static __always_inline void arm64_syscall_exit_to_user_mode_work(struct pt_regs *regs)
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 10ead67b4eaf..b2b28dcb2826 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -2468,14 +2468,14 @@ static inline void syscall_enter_audit(struct pt_regs *regs)
 }
 #endif
 
-int arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
+bool arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 {
 	int ret;
 
 	if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
 		ret = report_syscall_entry(regs);
 		if (ret || (flags & _TIF_SYSCALL_EMU))
-			return NO_SYSCALL;
+			return false;
 
 		/* ptrace might have changed thread flags */
 		flags = read_thread_flags();
@@ -2484,7 +2484,7 @@ int arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 	/* Do the secure computing after ptrace; failures should be fast. */
 	if (unlikely(flags & _TIF_SECCOMP)) {
 		if (!__seccomp_permit_syscall())
-			return NO_SYSCALL;
+			return false;
 	}
 
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
@@ -2493,7 +2493,7 @@ int arm64_syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 	if (unlikely(audit_context()))
 		syscall_enter_audit(regs);
 
-	return syscall_get_nr(current, regs);
+	return true;
 }
 
 static inline bool report_single_step(unsigned long flags)
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 3488afd45d20..72c6e8b7ab21 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -108,7 +108,11 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 		 */
 		if (scno == NO_SYSCALL)
 			syscall_set_return_value(current, regs, -ENOSYS, 0);
-		scno = arm64_syscall_trace_enter(regs, read_thread_flags());
+		if (!arm64_syscall_trace_enter(regs, read_thread_flags()))
+			goto trace_exit;
+
+		/* Reread the syscall number as it might have been modified */
+		scno = syscall_get_nr(current, regs);
 		if (scno == NO_SYSCALL)
 			goto trace_exit;
 	}
-- 
2.34.1