PPC32: Implement force_successful_syscall_return.

We use a bit in the thread_info flags for this.  The bit is cleared on syscall
entry and set by force_successful_syscall_return.  If the bit is set on syscall
exit, the normal check whether the value from the syscall function is an
error code is disabled and the value is just returned to userspace.

arch/ppc/kernel/entry.S

@@ -173,8 +173,10 @@ _GLOBAL(DoSyscall)
 	cmpli	0,r0,NR_syscalls
 	bge-	66f
 	rlwinm	r10,r1,0,0,18	/* current_thread_info() */
-	lwz	r10,TI_FLAGS(r10)
-	andi.	r10,r10,_TIF_SYSCALL_TRACE
+	lwz	r11,TI_FLAGS(r10)
+	rlwinm	r11,r11,0,~_TIF_FORCE_NOERROR
+	stw	r11,TI_FLAGS(r10)
+	andi.	r11,r11,_TIF_SYSCALL_TRACE
 	bne-	syscall_dotrace
 syscall_dotrace_cont:
 	lis	r10,sys_call_table@h
@@ -192,7 +194,11 @@ ret_from_syscall:
 	mr	r6,r3
 	li	r11,-_LAST_ERRNO
 	cmpl	0,r3,r11
+	rlwinm	r12,r1,0,0,18	/* current_thread_info() */
 	blt+	30f
+	lwz	r11,TI_FLAGS(r12)
+	andi.	r11,r11,_TIF_FORCE_NOERROR
+	bne	30f
 	neg	r3,r3
 	cmpi	0,r3,ERESTARTNOHAND
 	bne	22f
@@ -205,7 +211,6 @@ ret_from_syscall:
 30:	LOAD_MSR_KERNEL(r10,MSR_KERNEL)	/* doesn't include MSR_EE */
 	SYNC
 	MTMSRD(r10)
-	rlwinm	r12,r1,0,0,18	/* current_thread_info() */
 	lwz	r9,TI_FLAGS(r12)
 	andi.	r0,r9,(_TIF_SYSCALL_TRACE|_TIF_SIGPENDING|_TIF_NEED_RESCHED)
 	bne-	syscall_exit_work

include/asm-ppc/ptrace.h

@@ -49,6 +49,8 @@ struct pt_regs {
 #define instruction_pointer(regs) ((regs)->nip)
 #define user_mode(regs) (((regs)->msr & MSR_PR) != 0)
 
+#define force_successful_syscall_return()   set_thread_flag(TIF_FORCE_NOERROR)
+
 /*
  * We use the least-significant bit of the trap field to indicate
  * whether we have saved the full set of registers, or only a

include/asm-ppc/thread_info.h

@@ -86,6 +86,8 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
 #define TIF_POLLING_NRFLAG	4	/* true if poll_idle() is polling
 					   TIF_NEED_RESCHED */
+#define TIF_FORCE_NOERROR	5	/* don't return error from current
+					   syscall even if result < 0 */
 
 /* as above, but as bit values */
 #define _TIF_SYSCALL_TRACE	(1<<TIF_SYSCALL_TRACE)
@@ -93,6 +95,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_SIGPENDING		(1<<TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1<<TIF_NEED_RESCHED)
 #define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)
+#define _TIF_FORCE_NOERROR	(1<<TIF_FORCE_NOERROR)
 
 #endif /* __KERNEL__ */