ARC: [Review] Preparing to fix incorrect syscall restarts due to signals

To avoid multiple syscall restarts (multiple signals) or no restart at
all (sigreturn), we need just an extra bit of state "literally 1 bit" in
struct pt_regs. orig_r8 is the best place to do this, however given the
way it is encoded currently, we can't add anything simplistically.

Current orig_r8:
* syscalls   -> 1 to NR_SYSCALLS
* Exceptions -> NR_SYSCALLS + 1
* Break-point-> NR_SYSCALLS + 2

In new scheme it is a bit-field
* lower short word contains the  exact event type (and a new bit to represent
   restart semantics : if syscall was already / can't be restarted)
* upper short word optionally containing the syscall num - needed by
  likes of tracehooks etc

This patch only changes how orig_r8 is organised and nothing should
change behaviourily.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>

arch/arc/include/asm/entry.h

@@ -343,18 +343,12 @@
  *-------------------------------------------------------------*/
 .macro SAVE_ALL_EXCEPTION   marker
 
+	st      \marker, [sp, 8]
+	st      r0, [sp, 4]    /* orig_r0, needed only for sys calls */
+
 	/* Restore r9 used to code the early prologue */
 	EXCPN_PROLOG_RESTORE_REG  r9
 
-	/* Save the complete regfile now */
-
-	/* orig_r8 marker:
-	 * syscalls   -> 1 to NR_SYSCALLS
-	 * Exceptions -> NR_SYSCALLS + 1
-	 * Break-point-> NR_SYSCALLS + 2
-	 */
-	st      \marker, [sp, 8]
-	st      r0, [sp, 4]    /* orig_r0, needed only for sys calls */
 	SAVE_CALLER_SAVED
 	st.a    r26, [sp, -4]   /* gp */
 	st.a    fp, [sp, -4]
@@ -384,14 +378,25 @@
  * Save scratch regs for exceptions
  *-------------------------------------------------------------*/
 .macro SAVE_ALL_SYS
-	SAVE_ALL_EXCEPTION  (NR_syscalls + 1)
+	SAVE_ALL_EXCEPTION  orig_r8_IS_EXCPN
 .endm
 
 /*--------------------------------------------------------------
  * Save scratch regs for sys calls
  *-------------------------------------------------------------*/
 .macro SAVE_ALL_TRAP
-	SAVE_ALL_EXCEPTION  r8
+	/*
+	 * Setup pt_regs->orig_r8.
+	 * Encode syscall number (r8) in upper short word of event type (r9)
+	 * N.B. #1: This is already endian safe (see ptrace.h)
+	 *      #2: Only r9 can be used as scratch as it is already clobbered
+	 *          and it's contents are no longer needed by the latter part
+	 *          of exception prologue
+	 */
+	lsl  r9, r8, 16
+	or   r9, r9, orig_r8_IS_SCALL
+
+	SAVE_ALL_EXCEPTION  r9
 .endm
 
 /*--------------------------------------------------------------
@@ -442,7 +447,7 @@
 	ld  r9, [@int1_saved_reg]
 
 	/* now we are ready to save the remaining context :) */
-	st     -1, [sp, 8]    /* orig_r8, -1 for interuppt level one */
+	st      orig_r8_IS_IRQ1, [sp, 8]    /* Event Type */
 	st      0, [sp, 4]    /* orig_r0 , N/A for IRQ */
 	SAVE_CALLER_SAVED
 	st.a    r26, [sp, -4]   /* gp */

arch/arc/include/asm/ptrace.h

@@ -50,7 +50,17 @@ struct pt_regs {
 	long r0;
 	long sp;	/* user/kernel sp depending on where we came from  */
 	long orig_r0;
-	long orig_r8;	/*to distinguish bet excp, sys call, int1 or int2 */
+
+	/*to distinguish bet excp, syscall, irq */
+	union {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+		/* so that assembly code is same for LE/BE */
+		unsigned long orig_r8:16, event:16;
+#else
+		unsigned long event:16, orig_r8:16;
+#endif
+		long orig_r8_word;
+	};
 };
 
 /* Callee saved registers - need to be saved only when you are scheduled out */
@@ -87,9 +97,8 @@ struct callee_regs {
 	sp;			\
 })
 
-/* return 1 if in syscall, 0 if Intr or Exception */
-#define in_syscall(regs) (((regs->orig_r8) >= 0 && \
-			   (regs->orig_r8 <= NR_syscalls)) ? 1 : 0)
+#define in_syscall(regs)    (regs->event & orig_r8_IS_SCALL)
+#define in_brkpt_trap(regs) (regs->event & orig_r8_IS_BRKPT)
 
 #define current_pt_regs()					\
 ({								\
@@ -101,6 +110,13 @@ struct callee_regs {
 
 #endif /* !__ASSEMBLY__ */
 
+#define orig_r8_IS_SCALL		0x0001
+#define orig_r8_IS_SCALL_RESTARTED	0x0002
+#define orig_r8_IS_BRKPT		0x0004
+#define orig_r8_IS_EXCPN		0x0004
+#define orig_r8_IS_IRQ1			0x0010
+#define orig_r8_IS_IRQ2			0x0020
+
 #endif /* __KERNEL__ */
 
 #ifndef __ASSEMBLY__

arch/arc/kernel/asm-offsets.c

@@ -46,7 +46,7 @@ int main(void)
 	BLANK();
 
 	DEFINE(PT_status32, offsetof(struct pt_regs, status32));
-	DEFINE(PT_orig_r8, offsetof(struct pt_regs, orig_r8));
+	DEFINE(PT_orig_r8, offsetof(struct pt_regs, orig_r8_word));
 	DEFINE(PT_sp, offsetof(struct pt_regs, sp));
 	DEFINE(PT_r0, offsetof(struct pt_regs, r0));
 	DEFINE(PT_r1, offsetof(struct pt_regs, r1));

arch/arc/kernel/entry.S

@@ -350,8 +350,8 @@ ARC_EXIT EV_Extension
 
 trap_with_param:
 
-	;make sure orig_r8 is a positive value
-	st  NR_syscalls + 2, [sp, PT_orig_r8]
+	; stop_pc info by gdb needs this info
+	st  orig_r8_IS_BRKPT, [sp, PT_orig_r8]
 
 	mov r0, r12
 	lr  r1, [efa]