fix(userspace/libsinsp): fix a possible source of use-after-free.

Basically, `scap_get_fdlist()` called in the sinsp parsers loop to manage SCM_RIGHTS flag for recvmsg,
updates the pointers in the thread fdtable.
After #1637, doing so invalidates `event` m_fdinfo field, that is a pointer to a now freed data.

Signed-off-by: Federico Di Pierro <nierro92@gmail.com>

userspace/libsinsp/fdinfo.cpp

@@ -283,6 +283,8 @@ sinsp_fdinfo* sinsp_fdtable::find(int64_t fd)
 
 sinsp_fdinfo* sinsp_fdtable::add(int64_t fd, std::unique_ptr<sinsp_fdinfo> fdinfo)
 {
+	fdinfo->m_fd = fd;
+
 	//
 	// Look for the FD in the table
 	//

userspace/libsinsp/fdinfo.h

@@ -432,6 +432,7 @@ class SINSP_PUBLIC sinsp_fdinfo
 	uint32_t m_mount_id = 0;
 	uint64_t m_ino = 0;
 	int64_t m_pid = 0; // only if fd is a pidfd
+	int64_t m_fd = -1;
 };
 
 /*@}*/

userspace/libsinsp/parsers.cpp

@@ -4172,13 +4172,26 @@ void sinsp_parser::parse_rw_exit(sinsp_evt *evt)
 
 						m_inspector->m_thread_manager->thread_to_scap(*evt->get_tinfo(), &scap_tinfo);
 
+						// Store current fd; it might get changed by scap_get_fdlist below.
+						int64_t fd = -1;
+						if (evt->get_fd_info())
+						{
+							fd = evt->get_fd_info()->m_fd;
+						}
+
 						// Get the new fds. The callbacks we have registered populate the fd table
 						// with the new file descriptors.
 						if (scap_get_fdlist(m_inspector->get_scap_platform(), &scap_tinfo, error) != SCAP_SUCCESS)
 						{
 							libsinsp_logger()->format(sinsp_logger::SEV_DEBUG, "scap_get_fdlist failed: %s, proc table will not be updated with new fds.",
 									error);
 						}
+
+						// Force refresh event fdinfo
+						if (fd != -1)
+						{
+							evt->set_fd_info(evt->get_tinfo()->get_fd(fd));
+						}
 					}
 				}
 			}

userspace/libsinsp/threadinfo.cpp

@@ -262,6 +262,7 @@ void sinsp_threadinfo::add_fd_from_scap(scap_fdinfo *fdi)
 	newfdi->m_type = fdi->type;
 	newfdi->m_flags = sinsp_fdinfo::FLAGS_FROM_PROC;
 	newfdi->m_ino = fdi->ino;
+	newfdi->m_fd = fdi->fd;
 
 	switch(newfdi->m_type)
 	{
@@ -1316,6 +1317,7 @@ static void fd_to_scap(scap_fdinfo *dst, sinsp_fdinfo* src)
 {
 	dst->type = src->m_type;
 	dst->ino = src->m_ino;
+	dst->fd = src->m_fd;
 
 	switch(dst->type)
 	{
@@ -1992,7 +1994,6 @@ void sinsp_thread_manager::dump_threads_to_file(scap_dumper_t* dumper)
 				//
 				// Populate the fd info
 				//
-				scfdinfo->fd = fd;
 				fd_to_scap(scfdinfo, &info);
 
 				//