selftests/seccomp: powerpc: Set syscall return during ptrace syscall exit

Some archs (like powerpc) only support changing the return code during
syscall exit when ptrace is used. Test entry vs exit phases for which
portions of the syscall number and return values need to be set at which
different phases. For non-powerpc, all changes are made during ptrace
syscall entry, as before. For powerpc, the syscall number is changed at
ptrace syscall entry and the syscall return value is changed on ptrace
syscall exit.
Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Suggested-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Link: https://lore.kernel.org/linux-kselftest/20200911181012.171027-1-cascardo@canonical.com/
Fixes: 58d0a862 ("seccomp: add tests for ptrace hole")
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/lkml/20200921075300.7iylzof2w5vrutah@wittgenstein/Signed-off-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -1765,6 +1765,7 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 			(_regs).ccr &= ~0x10000000;		\
 		}						\
 	} while (0)
+# define SYSCALL_RET_SET_ON_PTRACE_EXIT
 #elif defined(__s390__)
 # define ARCH_REGS		s390_regs
 # define SYSCALL_NUM(_regs)	(_regs).gprs[2]
@@ -1853,6 +1854,18 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 	} while (0)
 #endif
 
+/*
+ * Some architectures (e.g. powerpc) can only set syscall
+ * return values on syscall exit during ptrace.
+ */
+const bool ptrace_entry_set_syscall_nr = true;
+const bool ptrace_entry_set_syscall_ret =
+#ifndef SYSCALL_RET_SET_ON_PTRACE_EXIT
+	true;
+#else
+	false;
+#endif
+
 /*
  * Use PTRACE_GETREGS and PTRACE_SETREGS when available. This is useful for
  * architectures without HAVE_ARCH_TRACEHOOK (e.g. User-mode Linux).
@@ -2006,11 +2019,15 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
 	 */
 	if (entry)
 		self->syscall_nr = get_syscall(_metadata, tracee);
-	else
-		return;
 
-	syscall_nr = &syscall_nr_val;
-	syscall_ret = &syscall_ret_val;
+	/*
+	 * Depending on the architecture's syscall setting abilities, we
+	 * pick which things to set during this phase (entry or exit).
+	 */
+	if (entry == ptrace_entry_set_syscall_nr)
+		syscall_nr = &syscall_nr_val;
+	if (entry == ptrace_entry_set_syscall_ret)
+		syscall_ret = &syscall_ret_val;
 
 	/* Now handle the actual rewriting cases. */
 	switch (self->syscall_nr) {