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

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index f2be9e960c2e..a46c3fdb6853 100644
--- a/arch/powerpc/kernel/signal.c
+++ b/arch/powerpc/kernel/signal.c
@@ -201,6 +201,9 @@ static void check_syscall_restart(struct pt_regs *regs, struct k_sigaction *ka,
 	if (!trap_is_syscall(regs))
 		return;
 
+	if (trap_norestart(regs))
+		return;
+
 	/* error signalled ? */
 	if (!(regs->ccr & 0x10000000))
 		return;
@@ -258,7 +261,7 @@ static void do_signal(struct task_struct *tsk)
 	if (ksig.sig <= 0) {
 		/* No signal to deliver -- put the saved sigmask back */
 		restore_saved_sigmask();
-		tsk->thread.regs->trap = 0;
+		set_trap_norestart(tsk->thread.regs);
 		return;               /* no signals delivered */
 	}
 
@@ -285,7 +288,7 @@ static void do_signal(struct task_struct *tsk)
 		ret = handle_rt_signal64(&ksig, oldset, tsk);
 	}
 
-	tsk->thread.regs->trap = 0;
+	set_trap_norestart(tsk->thread.regs);
 	signal_setup_done(ret, &ksig, test_thread_flag(TIF_SINGLESTEP));
 }