selftests/seccomp: powerpc: Fix seccomp return value testing

On powerpc, the errno is not inverted, and depends on ccr.so being
set. Add this to a powerpc definition of SYSCALL_RET_SET().

Co-developed-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Link: https://lore.kernel.org/linux-kselftest/20200911181012.171027-1-cascardo@canonical.com/
Fixes: 5d83c2b ("selftests/seccomp: Add powerpc support")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-13-keescook@chromium.org
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>

Author: kees

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -1750,6 +1750,21 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 # define ARCH_REGS		struct pt_regs
 # define SYSCALL_NUM(_regs)	(_regs).gpr[0]
 # define SYSCALL_RET(_regs)	(_regs).gpr[3]
+# define SYSCALL_RET_SET(_regs, _val)				\
+	do {							\
+		typeof(_val) _result = (_val);			\
+		/*						\
+		 * A syscall error is signaled by CR0 SO bit	\
+		 * and the code is stored as a positive value.	\
+		 */						\
+		if (_result < 0) {				\
+			SYSCALL_RET(_regs) = -result;		\
+			(_regs).ccr |= 0x10000000;		\
+		} else {					\
+			SYSCALL_RET(_regs) = result;		\
+			(_regs).ccr &= ~0x10000000;		\
+		}						\
+	} while (0)
 #elif defined(__s390__)
 # define ARCH_REGS		s390_regs
 # define SYSCALL_NUM(_regs)	(_regs).gprs[2]