[PATCH v2] treewide: Flush pcap and log files, if used, before exiting

15 Aug 2025

I didn't imagine that occasionally truncated pcap and log files, as a
result of commit d0006fa784a7 ("treewide: use _exit() over exit()"),
would be such a big deal, until I tried to debug TCP issues with this
beauty:

  while true; do ./pasta --trace -l /tmp/pasta.log -p /tmp/pasta.pcap --config-net -t 5555 -- socat TCP-LISTEN:5555 OPEN:/tmp/large.rcv,trunc & (sleep 0.3; socat -T2 OPEN:large.bin TCP:88.198.0.164:5555; ); wait; diff large.bin /tmp/large.rcv || break; done

...flush files and pcap if we're using them. Ignore fsync() errors for
the log file as we obviously can't reliably log them.

Signed-off-by: Stefano Brivio 
---
v2: Fix commit reference in commit message

 README.md |  2 +-
 log.c     |  2 +-
 log.h     |  1 +
 passt.c   |  1 +
 pasta.c   |  2 ++
 pcap.c    |  2 +-
 pcap.h    |  2 ++
 tap.c     |  4 +++-
 util.c    | 15 +++++++++++++++
 util.h    |  1 +
 10 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 54fed07..8f188f4 100644
--- a/README.md
+++ b/README.md
@@ -291,7 +291,7 @@ speeding up local connections, and usually requiring NAT. _pasta_:
 * ✅ all capabilities dropped, other than `CAP_NET_BIND_SERVICE` (if granted)
 * ✅ with default options, user, mount, IPC, UTS, PID namespaces are detached
 * ✅ no external dependencies (other than a standard C library)
-* ✅ restrictive seccomp profiles (30 syscalls allowed for _passt_, 41 for
+* ✅ restrictive seccomp profiles (33 syscalls allowed for _passt_, 43 for
   _pasta_ on x86_64)
 * ✅ examples of [AppArmor](/passt/tree/contrib/apparmor) and
   [SELinux](/passt/tree/contrib/selinux) profiles available
diff --git a/log.c b/log.c
index 33b89fc..21e3673 100644
--- a/log.c
+++ b/log.c
@@ -35,7 +35,7 @@ static int	log_sock = -1;		/* Optional socket to system logger */
 static char	log_ident[BUFSIZ];	/* Identifier string for openlog() */
 static int	log_mask;		/* Current log priority mask */
 
-static int	log_file = -1;		/* Optional log file descriptor */
+int		log_file = -1;		/* Optional log file descriptor */
 static size_t	log_size;		/* Maximum log file size in bytes */
 static size_t	log_written;		/* Currently used bytes in log file */
 static size_t	log_cut_size;		/* Bytes to cut at start on rotation */
diff --git a/log.h b/log.h
index 08aa88c..c8473c0 100644
--- a/log.h
+++ b/log.h
@@ -41,6 +41,7 @@ void logmsg_perror(int pri, const char *format, ...)
 		_exit(EXIT_FAILURE);					\
 	} while (0)
 
+extern int log_file;
 extern int log_trace;
 extern bool log_conf_parsed;
 extern bool log_stderr;
diff --git a/passt.c b/passt.c
index 388d10f..a4ec115 100644
--- a/passt.c
+++ b/passt.c
@@ -170,6 +170,7 @@ static void exit_handler(int signal)
 {
 	(void)signal;
 
+	fsync_pcap_and_log();
 	_exit(EXIT_SUCCESS);
 }
 
diff --git a/pasta.c b/pasta.c
index c207692..687406b 100644
--- a/pasta.c
+++ b/pasta.c
@@ -70,6 +70,8 @@ void pasta_child_handler(int signal)
 	if (pasta_child_pid &&
 	    !waitid(P_PID, pasta_child_pid, &infop, WEXITED | WNOHANG)) {
 		if (infop.si_pid == pasta_child_pid) {
+			fsync_pcap_and_log();
+
 			if (infop.si_code == CLD_EXITED)
 				_exit(infop.si_status);
 
diff --git a/pcap.c b/pcap.c
index 46d11a2..e9d8d80 100644
--- a/pcap.c
+++ b/pcap.c
@@ -37,7 +37,7 @@
 
 #define PCAP_VERSION_MINOR 4
 
-static int pcap_fd = -1;
+int pcap_fd = -1;
 
 struct pcap_pkthdr {
 	uint32_t tv_sec;
diff --git a/pcap.h b/pcap.h
index 9795f2e..2aeb53e 100644
--- a/pcap.h
+++ b/pcap.h
@@ -6,6 +6,8 @@
 #ifndef PCAP_H
 #define PCAP_H
 
+extern int pcap_fd;
+
 void pcap(const char *pkt, size_t l2len);
 void pcap_multiple(const struct iovec *iov, size_t frame_parts, unsigned int n,
 		   size_t offset);
diff --git a/tap.c b/tap.c
index 6db5d88..cddc164 100644
--- a/tap.c
+++ b/tap.c
@@ -1117,8 +1117,10 @@ void tap_sock_reset(struct ctx *c)
 {
 	info("Client connection closed%s", c->one_off ? ", exiting" : "");
 
-	if (c->one_off)
+	if (c->one_off) {
+		fsync_pcap_and_log();
 		_exit(EXIT_SUCCESS);
+	}
 
 	/* Close the connected socket, wait for a new connection */
 	epoll_del(c, c->fd_tap);
diff --git a/util.c b/util.c
index 3e93d47..c492f90 100644
--- a/util.c
+++ b/util.c
@@ -34,6 +34,7 @@
 #include "passt.h"
 #include "packet.h"
 #include "log.h"
+#include "pcap.h"
 #ifdef HAS_GETRANDOM
 #include 
 #endif
@@ -1045,3 +1046,17 @@ void abort_with_msg(const char *fmt, ...)
 	 */
 	abort();
 }
+
+/**
+ * fsync_pcap_and_log() - Flush pcap and log files as needed
+ *
+ * #syscalls fsync
+ */
+void fsync_pcap_and_log(void)
+{
+	if (pcap_fd != -1 && fsync(pcap_fd))
+		warn_perror("Failed to flush pcap file, it might be truncated");
+
+	if (log_file != -1)
+		(void)fsync(log_file);
+}
diff --git a/util.h b/util.h
index b5b1b31..2a8c38f 100644
--- a/util.h
+++ b/util.h
@@ -226,6 +226,7 @@ int read_all_buf(int fd, void *buf, size_t len);
 int read_remainder(int fd, const struct iovec *iov, size_t cnt, size_t skip);
 void close_open_files(int argc, char **argv);
 bool snprintf_check(char *str, size_t size, const char *format, ...);
+void fsync_pcap_and_log(void);
 
 /**
  * af_name() - Return name of an address family
-- 
2.43.0

    

Stefano Brivio

David Gibson

tags

participants (2)