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 <[e-mail]>
Cc: Will Drewry <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

arch/mips/include/asm/seccomp.h

···
 	static const int syscalls_O32[] = {
 		__NR_O32_Linux + 3, __NR_O32_Linux + 4,
 		__NR_O32_Linux + 1, __NR_O32_Linux + 193,
-		0, /* null terminated */
+		-1, /* negative terminated */
 	};
 	static const int syscalls_N32[] = {
 		__NR_N32_Linux + 0, __NR_N32_Linux + 1,
 		__NR_N32_Linux + 58, __NR_N32_Linux + 211,
-		0, /* null terminated */
+		-1, /* negative terminated */
 	};
 
 	if (IS_ENABLED(CONFIG_MIPS32_O32) && test_thread_flag(TIF_32BIT_REGS))

include/asm-generic/seccomp.h

···
 	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;
 }

kernel/seccomp.c

···
  */
 static const int mode1_syscalls[] = {
 	__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
-	0, /* null terminated */
+	-1, /* negative terminated */
 };
 
 static void __secure_computing_strict(int this_syscall)
 {
-	const int *syscall_whitelist = mode1_syscalls;
+	const int *allowed_syscalls = mode1_syscalls;
 #ifdef CONFIG_COMPAT
 	if (in_compat_syscall())
-		syscall_whitelist = get_compat_mode1_syscalls();
+		allowed_syscalls = get_compat_mode1_syscalls();
 #endif
 	do {
-		if (*syscall_whitelist == this_syscall)
+		if (*allowed_syscalls == this_syscall)
 			return;
-	} while (*++syscall_whitelist);
+	} while (*++allowed_syscalls != -1);
 
 #ifdef SECCOMP_DEBUG
 	dump_stack();