powerpc: Use trap metadata to prevent double restart rather than zeroing trap

It's not very nice to zero trap for this, because then system calls no
longer have trap_is_syscall(regs) invariant, and we can't distinguish
between sc and scv system calls (in a later patch).

Take one last unused bit from the low bits of the pt_regs.trap word
for this instead. There is not a really good reason why it should be
in trap as opposed to another field, but trap has some concept of
flags and it exists. Ideally I think we would move trap to 2-byte
field and have 2 more bytes available independently.

Add a selftests case for this, which can be seen to fail if
trap_norestart() is changed to return false.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make them static inlines]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200507121332.2233629-4-mpe@ellerman.id.au

arch/powerpc/include/asm/ptrace.h

@@ -182,13 +182,13 @@ extern int ptrace_put_reg(struct task_struct *task, int regno,
 
 #ifdef __powerpc64__
 #ifdef CONFIG_PPC_BOOK3S
-#define TRAP_FLAGS_MASK		0
-#define TRAP(regs)		((regs)->trap)
+#define TRAP_FLAGS_MASK		0x10
+#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
 #define FULL_REGS(regs)		true
 #define SET_FULL_REGS(regs)	do { } while (0)
 #else
-#define TRAP_FLAGS_MASK		0x1
-#define TRAP(regs)		((regs)->trap & ~0x1)
+#define TRAP_FLAGS_MASK		0x11
+#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
 #define FULL_REGS(regs)		(((regs)->trap & 1) == 0)
 #define SET_FULL_REGS(regs)	((regs)->trap |= 1)
 #endif
@@ -202,8 +202,8 @@ extern int ptrace_put_reg(struct task_struct *task, int regno,
  * On 4xx we use the next bit to indicate whether the exception
  * is a critical exception (1 means it is).
  */
-#define TRAP_FLAGS_MASK		0xF
-#define TRAP(regs)		((regs)->trap & ~0xF)
+#define TRAP_FLAGS_MASK		0x1F
+#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
 #define FULL_REGS(regs)		(((regs)->trap & 1) == 0)
 #define SET_FULL_REGS(regs)	((regs)->trap |= 1)
 #define IS_CRITICAL_EXC(regs)	(((regs)->trap & 2) != 0)
@@ -227,6 +227,16 @@ static inline bool trap_is_syscall(struct pt_regs *regs)
 	return TRAP(regs) == 0xc00;
 }
 
+static inline bool trap_norestart(struct pt_regs *regs)
+{
+	return regs->trap & 0x10;
+}
+
+static inline void set_trap_norestart(struct pt_regs *regs)
+{
+	regs->trap |= 0x10;
+}
+
 #define arch_has_single_step()	(1)
 #ifndef CONFIG_BOOK3S_601
 #define arch_has_block_step()	(true)