@@ -54,8 +54,9 @@ struct pt_regs
 			};
 			unsigned long result;
 			unsigned long exit_flags;
+			unsigned long entry_flags;
 			/* Maintain 16 byte interrupt stack alignment */
-			unsigned long __pt_regs_pad[3];
+			unsigned long __pt_regs_pad[2];
 		};
 	};
 #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_KUAP)

@@ -233,6 +234,25 @@ static inline unsigned long frame_pointer(struct pt_regs *regs)
 #define current_pt_regs() \
 	((struct pt_regs *)((unsigned long)task_stack_page(current) + THREAD_SIZE) - 1)
 
+/*
+ * SYSCALL_ENTRY_RET_SET: seccomp or ptrace called syscall_set_return_value()
+ * and wants the syscall skipped; regs->gpr[3] already holds the return value.
+ */
+#define SYSCALL_ENTRY_RET_SET	BIT(0)
+
+static inline void set_syscall_entry_ret(struct pt_regs *regs)
+{
+	regs->entry_flags |= SYSCALL_ENTRY_RET_SET;
+}
+
+static inline bool test_and_clear_syscall_entry_ret(struct pt_regs *regs)
+{
+	bool set = !!(regs->entry_flags & SYSCALL_ENTRY_RET_SET);
+
+	regs->entry_flags &= ~SYSCALL_ENTRY_RET_SET;
+	return set;
+}
+
 /*
  * The 4 low bits (0xf) are available as flags to overload the trap word,
  * because interrupt vectors have minimum alignment of 0x10. TRAP_FLAGS_MASK

@@ -98,6 +98,12 @@ static inline void syscall_set_return_value(struct task_struct *task,
 			regs->gpr[3] = val;
 		}
 	}
+	/*
+	 * Mark that a return value has been explicitly set by seccomp or
+	 * ptrace so that system_call_exception() can skip the syscall
+	 * unconditionally, even when the user requested syscall(-1).
+	 */
+	set_syscall_entry_ret(regs);
 }
 
 static inline void syscall_get_arguments(struct task_struct *task,

@@ -56,7 +56,8 @@ struct pt_regs
 	unsigned long dsisr;		/* on 4xx/Book-E used for ESR */
 	unsigned long result;		/* Result of a system call */
 	unsigned long exit_flags;	/* System call exit flags */
-	unsigned long __pt_regs_pad[3];	/* Maintain 16 byte interrupt stack alignment */
+	unsigned long entry_flags;	/* System call entry flags */
+	unsigned long __pt_regs_pad[2];	/* Maintain 16 byte interrupt stack alignment */
 };
 
 #endif /* __ASSEMBLER__ */

@@ -117,7 +118,8 @@ struct pt_regs
 #define PT_DSISR 42
 #define PT_RESULT 43
 #define PT_EXIT_FLAGS 44
-#define PT_PAD 47 /* 3 times */
+#define PT_ENTRY_FLAGS 45
+#define PT_PAD 46 /* 2 times */
 #define PT_DSCR 48
 #define PT_REGS_COUNT 48
 

@@ -235,6 +235,8 @@ void __init pt_regs_check(void)
 		     offsetof(struct user_pt_regs, dsisr));
 	BUILD_BUG_ON(offsetof(struct pt_regs, result) !=
 		     offsetof(struct user_pt_regs, result));
+	BUILD_BUG_ON(offsetof(struct pt_regs, entry_flags) !=
+		     offsetof(struct user_pt_regs, entry_flags));
 
 	BUILD_BUG_ON(sizeof(struct user_pt_regs) > sizeof(struct pt_regs));
 

@@ -19,9 +19,27 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 	long ret;
 	syscall_fn f;
 
+	/*
+	 * Zero entry_flags before syscall_enter_from_user_mode() so that
+	 * syscall_set_return_value() can set SYSCALL_ENTRY_RET_SET as an
+	 * unambiguous out-of-band signal.  The field is not initialised by
+	 * the entry assembly.
+	 */
+	regs->entry_flags = 0;
 	add_random_kstack_offset();
 	r0 = syscall_enter_from_user_mode(regs, r0);
 
+	/*
+	 * Seccomp or ptrace may have set a return value and requested that
+	 * the syscall be skipped. syscall_set_return_value() sets
+	 * SYSCALL_ENTRY_RET_SET in regs->entry_flags as an
+	 * unambiguous out-of-band signal. This avoids the ambiguity of
+	 * using r0 == -1 as the skip sentinel when the user themselves
+	 * called syscall(-1).
+	 */
+	if (unlikely(test_and_clear_syscall_entry_ret(regs)))
+		return regs->gpr[3];
+
 	if (unlikely(r0 >= NR_syscalls)) {
 		if (unlikely(trap_is_unsupported_scv(regs))) {
 			/* Unsupported scv vector */