[PATCH] parisc: Fix ptrace syscall number modification

["Dmitry V. Levin" <ldv@altlinux.org>]

Commit 910cd32e552e ("parisc: Fix and enable seccomp filter support")
introduced a regression in ptrace-based syscall tampering: when tracer
changes syscall number to -1, the kernel fails to initialize %r28 with
-ENOSYS and subsequently fails to return the error code of the failed
syscall to userspace.

This erroneous behaviour could be observed with a simple strace syscall
fault injection command which is expected to print something like this:

$ strace -a0 -ewrite -einject=write:error=enospc echo hello
write(1, "hello\n", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "echo: ", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "write error", 11) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "\n", 1) = -1 ENOSPC (No space left on device) (INJECTED)
+++ exited with 1 +++

After commit 910cd32e552ea09caa89cdbe328e468979b030dd it loops printing
something like this instead:

write(1, "hello\n", 6../strace: Failed to tamper with process 12345: unexpectedly got no error (return value 0, error 0)
) = 0 (INJECTED)

This bug was found by strace test suite.

Fixes: 910cd32e552e ("parisc: Fix and enable seccomp filter support")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
---

 N.B. I have no parisc box to test the patch.

 arch/parisc/kernel/ptrace.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c
index 2582df1c529b..9177f3e68b93 100644
--- a/arch/parisc/kernel/ptrace.c
+++ b/arch/parisc/kernel/ptrace.c
@@ -308,15 +308,17 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 
 long do_syscall_trace_enter(struct pt_regs *regs)
 {
-	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
-	    tracehook_report_syscall_entry(regs)) {
-		/*
-		 * Tracing decided this syscall should not happen or the
-		 * debugger stored an invalid system call number. Skip
-		 * the system call and the system call restart handling.
-		 */
-		regs->gr[20] = -1UL;
-		goto out;
+	if (test_thread_flag(TIF_SYSCALL_TRACE)) {
+		regs->gr[28] = -ENOSYS;
+		if (tracehook_report_syscall_entry(regs)) {
+			/*
+			 * Tracing decided this syscall should not happen or the
+			 * debugger stored an invalid system call number. Skip
+			 * the system call and the system call restart handling.
+			 */
+			regs->gr[20] = -1UL;
+			return -1;
+		}
 	}
 
 	/* Do the secure computing check after ptrace. */
-- 
ldv