kboot: Create routines to read Linux tiny files

Most of the files in /sys/ and /proc/ are small with one value. Create
two routines to help us read the file and decode that value.

Sponsored by:		Netflix

stand/kboot/Makefile

@@ -30,7 +30,8 @@ SRCS=	\
 		kbootfdt.c \
 		main.c \
 		termios.c \
-		vers.c \
+		util.c \
+		vers.c
 
 CFLAGS.gfx_fb_stub.c += -I${SRCTOP}/contrib/pnglite -I${SRCTOP}/sys/teken
 

stand/kboot/kboot.h

@@ -19,4 +19,8 @@ void fdt_arch_fixups(void *fdtp);
 uint64_t kboot_get_phys_load_segment(void);
 uint8_t kboot_get_kernel_machine_bits(void);
 
+/* util.c */
+bool file2str(const char *fn, char *buffer, size_t buflen);
+bool file2u64(const char *fn, uint64_t *val);
+
 #endif /* KBOOT_H */

stand/kboot/util.c

@@ -0,0 +1,46 @@
+/*-
+ * Copyright 2022 Netflix, Inc
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "stand.h"
+#include "host_syscall.h"
+#include "kboot.h"
+
+bool
+file2str(const char *fn, char *buffer, size_t buflen)
+{
+	int fd;
+	ssize_t len;
+
+	fd = host_open(fn, HOST_O_RDONLY, 0);
+	if (fd == -1)
+		return false;
+	len = host_read(fd, buffer, buflen - 1);
+	if (len < 0) {
+		host_close(fd);
+		return false;
+	}
+	buffer[len] = '\0';
+	/*
+	 * Trim trailing white space
+	 */
+	while (isspace(buffer[len - 1]))
+		buffer[--len] = '\0';
+	host_close(fd);
+	return true;
+}
+
+bool
+file2u64(const char *fn, uint64_t *val)
+{
+	unsigned long v;
+	char buffer[80];
+
+	if (!file2str(fn, buffer, sizeof(buffer)))
+		return false;
+	v = strtoull(buffer, NULL, 0);	/* XXX check return values? */
+	*val = v;
+	return true;
+}