kboot: Create function for error checking.

Linux has the convention of returning -ERRNO to flag errors from its
system calls. Sometimes other negative values are returned that are
success...  However, only values -1 to -4096 (inclusive) are really
errors. The rest are either truncated values that only look negative (so
use long instead of int), or are things like addresses or legal unsigned
file offsets or similar that are successful returns. Filter out the
latter.

Sponsored by:		Netflix

stand/kboot/include/host_syscall.h

@@ -190,20 +190,36 @@ ssize_t host_write(int fd, const void *buf, size_t nbyte);
 	host_mmap(0, size, HOST_PROT_READ | HOST_PROT_WRITE, \
 	    HOST_MAP_PRIVATE | HOST_MAP_ANONYMOUS, -1, 0);
 
+/*
+ * Since we have to interface with the 'raw' system call, we have to cope with
+ * Linux's conventions. To run on the most architectures possible, they don't
+ * return errors through some CPU flag, but instead, return a negative value for
+ * an error, and a positive one for success. However, there's some issues since
+ * addresses have to be returned, some of which are also negative, so Linus
+ * declared that no successful result could be -4096 to 0. This implements
+ * that quirk so we can check return values easily.
+ */
+static __inline bool
+is_linux_error(long e)
+{
+	return (e < 0 && e >= -4096);
+}
+
 /*
  * Translate Linux errno to FreeBSD errno. The two system have idenitcal errors
  * for 1-34. After that, they differ. Linux also has errno that don't map
  * exactly to FreeBSD's errno, plus the Linux errno are arch dependent >
  * 34. Since we just need to do this for simple cases, use the simple mapping
  * function where -1 to -34 are translated to 1 to 34 and all others are EINVAL.
- * Pass the linux return value, which will be the -errno.
+ * Pass the linux return value, which will be the -errno. Linux returns these
+ * values as a 'long' which has to align to CPU register size, so accept that
+ * size as the error so the assert can catch more values.
  */
 static __inline int
-host_to_stand_errno(int e)
+host_to_stand_errno(long e)
 {
-	assert(e < 0);
+	assert(is_linux_error(e));
 
 	return((-e) > 34 ? EINVAL : (-e));
 }
-
 #endif