logging: Respect log-size-max immediately after open

When a container keeps restarting, conmon restarts and opens the log
file with O_APPEND every single time, resetting bytes_written to 0
with a non-empty file.
If the container fails before writting log-size-max bytes then the
log keeps growing indefinitely, potentially using up all the available
space.

This should be kept in check: initialize bytes_written to the file's
current size, so that it can be reset if required (it's possible that
the file gets re-open before the first write if it was already above the
limit, but that should almost never happen in practice)

In order to make this simpler make bytes written global variables

src/ctr_logging.c

@@ -3,6 +3,7 @@
 #include "cli.h"
 #include "config.h"
 #include <string.h>
+#include <sys/stat.h>
 
 // if the systemd development files were found, we can log to systemd
 #ifdef USE_JOURNALD
@@ -40,6 +41,8 @@ static int64_t log_global_size_max = -1;
 /* k8s log file parameters */
 static int k8s_log_fd = -1;
 static char *k8s_log_path = NULL;
+int64_t k8s_bytes_written;
+int64_t k8s_total_bytes_written;
 
 /* journald log file parameters */
 // short ID length
@@ -114,6 +117,15 @@ void configure_log_drivers(gchar **log_drivers, int64_t log_size_max_, int64_t l
 		if (k8s_log_fd < 0)
 			pexit("Failed to open log file");
 
+		struct stat statbuf;
+		if (fstat(k8s_log_fd, &statbuf) == 0) {
+			k8s_bytes_written = statbuf.st_size;
+		} else {
+			nwarnf("Could not stat log file %s, assuming 0 size", k8s_log_path);
+			k8s_bytes_written = 0;
+		}
+		k8s_total_bytes_written = k8s_bytes_written;
+
 		if (!use_journald_logging && tag)
 			nexit("k8s-file doesn't support --log-tag");
 	}
@@ -336,9 +348,7 @@ static int write_journald(int pipe, char *buf, ssize_t buflen)
 static int write_k8s_log(stdpipe_t pipe, const char *buf, ssize_t buflen)
 {
 	writev_buffer_t bufv = {0};
-	static int64_t bytes_written = 0;
 	int64_t bytes_to_be_written = 0;
-	static int64_t total_bytes_written = 0;
 
 	/*
 	 * Use the same timestamp for every line of the log in this buffer.
@@ -363,17 +373,15 @@ static int write_k8s_log(stdpipe_t pipe, const char *buf, ssize_t buflen)
 		}
 
 		/* If the caller specified a global max, enforce it before writing */
-		if (log_global_size_max > 0 && total_bytes_written >= log_global_size_max)
+		if (log_global_size_max > 0 && k8s_total_bytes_written >= log_global_size_max)
 			break;
 
 		/*
 		 * We re-open the log file if writing out the bytes will exceed the max
 		 * log size. We also reset the state so that the new file is started with
 		 * a timestamp.
 		 */
-		if ((log_size_max > 0) && (bytes_written + bytes_to_be_written) > log_size_max) {
-			bytes_written = 0;
-
+		if ((log_size_max > 0) && (k8s_bytes_written + bytes_to_be_written) > log_size_max) {
 			if (writev_buffer_flush(k8s_log_fd, &bufv) < 0) {
 				nwarn("failed to flush buffer to log");
 				/*
@@ -418,8 +426,8 @@ static int write_k8s_log(stdpipe_t pipe, const char *buf, ssize_t buflen)
 			}
 		}
 
-		bytes_written += bytes_to_be_written;
-		total_bytes_written += bytes_to_be_written;
+		k8s_bytes_written += bytes_to_be_written;
+		k8s_total_bytes_written += bytes_to_be_written;
 	next:
 		/* Update the head of the buffer remaining to output. */
 		buf += line_len;
@@ -606,6 +614,9 @@ static void reopen_k8s_file(void)
 	/* Close the current k8s_log_fd */
 	close(k8s_log_fd);
 
+	/* Open with O_TRUNC: reset bytes written */
+	k8s_bytes_written = 0;
+
 	/* Open the log path file again */
 	k8s_log_fd = open(k8s_log_path_tmp, O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, 0600);
 	if (k8s_log_fd < 0)