erts/kernel: sendfile no longer uses async threads

This has been done because a slow client attack is possible if the
async thread pool is used. The scenario is:

Client does a request for a file and then slowly receives the file one
byte at a time. This will eventually fill the async thread pool with blocking
sendfile operations and thus starving the vm of all file operations.

If you still want to use the async threads pool for sendfile an option to
enable it has been introduced.

1 files changed, 22 insertions, 11 deletions

diff --git a/erts/emulator/drivers/common/efile_drv.c b/erts/emulator/drivers/common/efile_drv.c
index 8de578d8b7..23c5cde7ed 100644
--- a/erts/emulator/drivers/common/efile_drv.c
+++ b/erts/emulator/drivers/common/efile_drv.c
@@ -168,7 +168,7 @@ dt_private *get_dt_private(int);
 
 
 #ifdef USE_THREADS
-#define IF_THRDS if (sys_info.async_threads > 0)
+#define THRDS_AVAILABLE (sys_info.async_threads > 0)
 #ifdef HARDDEBUG /* HARDDEBUG in io.c is expected too */
 #define TRACE_DRIVER fprintf(stderr, "Efile: ")
 #else
@@ -178,24 +178,26 @@ dt_private *get_dt_private(int);
 #define MUTEX_LOCK(m)    do { IF_THRDS { TRACE_DRIVER; driver_pdl_lock(m);   } } while (0)
 #define MUTEX_UNLOCK(m)  do { IF_THRDS { TRACE_DRIVER; driver_pdl_unlock(m); } } while (0)
 #else
-#define IF_THRDS if (0)
+#define THRDS_AVAILABLE (0)
 #define MUTEX_INIT(m, p)
 #define MUTEX_LOCK(m)
 #define MUTEX_UNLOCK(m)
 #endif
+#define IF_THRDS if (THRDS_AVAILABLE)
 
 
+#define SENDFILE_FLGS_USE_THREADS (1 << 0)
 /**
  * On DARWIN sendfile can deadlock with close if called in
  * different threads. So until Apple fixes so that sendfile
  * is not buggy we disable usage of the async pool for
  * DARWIN. The testcase t_sendfile_crashduring reproduces
- * this error when using +A 10.
+ * this error when using +A 10 and enabling SENDFILE_FLGS_USE_THREADS.
  */
 #if defined(__APPLE__) && defined(__MACH__)
-#define USE_THRDS_FOR_SENDFILE 0
+#define USE_THRDS_FOR_SENDFILE(DATA) 0
 #else
-#define USE_THRDS_FOR_SENDFILE (sys_info.async_threads > 0)
+#define USE_THRDS_FOR_SENDFILE(DATA) (DATA->flags & SENDFILE_FLGS_USE_THREADS)
 #endif /* defined(__APPLE__) && defined(__MACH__) */
 
 
@@ -301,7 +303,7 @@ static void file_stop_select(ErlDrvEvent event, void* _);
 enum e_timer {timer_idle, timer_again, timer_write};
 #ifdef HAVE_SENDFILE
 enum e_sendfile {sending, not_sending};
-static void free_sendfile(void *data);
+#define SENDFILE_USE_THREADS (1 << 0)
 #endif /* HAVE_SENDFILE */
 
 struct t_data;
@@ -1933,7 +1935,7 @@ static void invoke_sendfile(void *data)
 
     d->c.sendfile.written += nbytes;
 
- if (result == 1 || (result == 0 && USE_THRDS_FOR_SENDFILE)) {
+    if (result == 1 || (result == 0 && USE_THRDS_FOR_SENDFILE(d))) {
       d->result_ok = 0;
     } else if (result == 0 && (d->errInfo.posix_errno == EAGAIN
 				 || d->errInfo.posix_errno == EINTR)) {
@@ -1950,7 +1952,7 @@ static void invoke_sendfile(void *data)
 
 static void free_sendfile(void *data) {
     struct t_data *d = (struct t_data *)data;
-    if (USE_THRDS_FOR_SENDFILE) {
+    if (USE_THRDS_FOR_SENDFILE(d)) {
 	SET_NONBLOCKING(d->c.sendfile.out_fd);
     } else {
 	MUTEX_LOCK(d->c.sendfile.q_mtx);
@@ -4123,8 +4125,16 @@ file_outputv(ErlDrvData e, ErlIOVec *ev) {
 	    goto done;
 	}
 
-	if (hd_len != 0 || tl_len != 0 || flags != 0) {
-	    /* We do not allow header, trailers and/or flags right now */
+	if (hd_len != 0 || tl_len != 0) {
+	    /* We do not allow header, trailers */
+	    reply_posix_error(desc, EINVAL);
+	    goto done;
+	}
+
+	
+	if (flags & SENDFILE_FLGS_USE_THREADS && !THRDS_AVAILABLE) {
+	    /* We do not allow use_threads flag on a system where
+	       no threads are available. */
 	    reply_posix_error(desc, EINVAL);
 	    goto done;
 	}
@@ -4134,6 +4144,7 @@ file_outputv(ErlDrvData e, ErlIOVec *ev) {
 	d->command = command;
 	d->invoke = invoke_sendfile;
 	d->free = free_sendfile;
+	d->flags = flags;
 	d->level = 2;
 
 	d->c.sendfile.out_fd = (int) out_fd;
@@ -4153,7 +4164,7 @@ file_outputv(ErlDrvData e, ErlIOVec *ev) {
 
 	d->c.sendfile.nbytes = nbytes;
 
-	if (USE_THRDS_FOR_SENDFILE) {
+	if (USE_THRDS_FOR_SENDFILE(d)) {
 	    SET_BLOCKING(d->c.sendfile.out_fd);
 	} else {
 	    /**