parse: add a helper to obtain an uptime

will be used in the next patch

https://jira.sw.ru/browse/PSBM-67502

note: man for /proc/uptime says that uptime is in seconds and for now
the format is "seconds.centiseconds", where ecentiseconds is 2 digits

note: now uptime is in csec but I prefer saving it in usec, that allows
us to be reuse these image field when/if we have more accurate value.

v8: add length specifier to parse only centiseconds
v9: put uptime to u_int64_t directly, define CSEC_PER_SEC
v10: switch to uint64_t from u_int64_t, comment about usec in image
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/proc_parse.h

@@ -107,5 +107,6 @@ int parse_children(pid_t pid, pid_t **_c, int *_n);
 
 extern bool is_vma_range_fmt(char *line);
 extern void parse_vmflags(char *buf, u32 *flags, u64 *madv, int *io_pf);
+extern int parse_uptime(uint64_t *upt);
 
 #endif /* __CR_PROC_PARSE_H__ */

criu/proc_parse.c

@@ -2629,3 +2629,28 @@ err:
 	xfree(ch);
 	return -1;
 }
+
+#define CSEC_PER_SEC 100
+
+__maybe_unused int parse_uptime(uint64_t *upt)
+{
+	unsigned long sec, csec;
+	FILE *f;
+
+	f = fopen("/proc/uptime", "r");
+	if (!f) {
+		pr_perror("Failed to fopen /proc/uptime");
+		return -1;
+	}
+
+	if (fscanf(f, "%lu.%2lu", &sec, &csec) != 2) {
+		pr_perror("Failed to parse /proc/uptime");
+		fclose(f);
+		return -1;
+	}
+
+	*upt = sec * USEC_PER_SEC + csec * (USEC_PER_SEC / CSEC_PER_SEC);
+
+	fclose(f);
+	return 0;
+}