[PATCH] tap: use close_range() when forking scripts and helper

["Clément Léger" <cleger@rivosinc.com>]

Since commit 03e471c41d8b ("qemu_init: increase NOFILE soft limit on
POSIX"), the maximum number of file descriptors that can be opened are
raised to nofile.rlim_max. On recent debian distro, this yield a maximum
of 1073741816 file descriptors. Now, when forking to start
qemu-bridge-helper, this actually calls close() on the full possible file
descriptor range (more precisely [3 - sysconf(_SC_OPEN_MAX)]) which
takes a considerable amount of time. Use close_range() which only
requires to be called twice and factorize it in a separate function for
both call sites.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
---
 net/tap.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/net/tap.c b/net/tap.c
index 51f7aec39d..6f5bf06bb5 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -385,6 +385,17 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
     return s;
 }
 
+static void fork_close_all_fds_except(int fd)
+{
+    int open_max = sysconf(_SC_OPEN_MAX);
+
+    if (fd > 3)
+        close_range(3, fd - 1, 0);
+
+    if (fd < open_max)
+        close_range(fd + 1, open_max, 0);
+}
+
 static void launch_script(const char *setup_script, const char *ifname,
                           int fd, Error **errp)
 {
@@ -400,13 +411,8 @@ static void launch_script(const char *setup_script, const char *ifname,
         return;
     }
     if (pid == 0) {
-        int open_max = sysconf(_SC_OPEN_MAX), i;
+        fork_close_all_fds_except(fd);
 
-        for (i = 3; i < open_max; i++) {
-            if (i != fd) {
-                close(i);
-            }
-        }
         parg = args;
         *parg++ = (char *)setup_script;
         *parg++ = (char *)ifname;
@@ -490,16 +496,11 @@ static int net_bridge_run_helper(const char *helper, const char *bridge,
         return -1;
     }
     if (pid == 0) {
-        int open_max = sysconf(_SC_OPEN_MAX), i;
         char *fd_buf = NULL;
         char *br_buf = NULL;
         char *helper_cmd = NULL;
 
-        for (i = 3; i < open_max; i++) {
-            if (i != sv[1]) {
-                close(i);
-            }
-        }
+        fork_close_all_fds_except(sv[1]);
 
         fd_buf = g_strdup_printf("%s%d", "--fd=", sv[1]);
 
-- 
2.45.2