UPSTREAM: seccomp: Use -1 marker for end of mode 1 syscall list

The terminator for the mode 1 syscalls list was a 0, but that could be
a valid syscall number (e.g. x86_64 __NR_read). By luck, __NR_read was
listed first and the loop construct would not test it, so there was no
bug. However, this is fragile. Replace the terminator with -1 instead,
and make the variable name for mode 1 syscall lists more descriptive.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
(cherry picked from commit fe4bfff86ec54773df3db79e8112e3b0f820c799)
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Bug: 176068146
Change-Id: I3d91bf57236f2c20d71f22fa9c0ef7b0b8869bcf

include/asm-generic/seccomp.h

@@ -33,7 +33,7 @@ static inline const int *get_compat_mode1_syscalls(void)
 	static const int mode1_syscalls_32[] = {
 		__NR_seccomp_read_32, __NR_seccomp_write_32,
 		__NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
-		0, /* null terminated */
+		-1, /* negative terminated */
 	};
 	return mode1_syscalls_32;
 }