accept4() breaks Linux 2.6.31 compatibility

[ryao]

I was informed in IRC that ARM systems often use older kernels that lack accept4() support. The only invocation of accept4() in the tree is in ./src/udev/udev-ctrl.c. Based on the kernel documentation, it looks like we can replace the call to accept4() with a call to accept(), call fcntl to set SOCK_NONBLOCK and use O_CLOEXEC on open().

[ryao]

Commit ff2c503 introduced accept4. We need to rework this to fix it.

[ryao]

Commit b51f834 is intended to fix this. It needs testing before we can merge it to master. I will come back to this after some more pressing issues that prevent eudev from being a drop-in replacement for Gentoo's sys-fs/udev-171-r9 package are addressed.

[ryao]

I have backported this patch to udev 171 and verified that it works on Linux 3.6.6:

diff --git a/libudev/libudev-ctrl.c b/libudev/libudev-ctrl.c
index e0ec2fa..9233d0c 100644
--- a/libudev/libudev-ctrl.c
+++ b/libudev/libudev-ctrl.c
@@ -186,7 +186,7 @@ struct udev_ctrl_connection *udev_ctrl_get_connection(struct udev_ctrl *uctrl)
    conn->refcount = 1;
    conn->uctrl = uctrl;

-   conn->sock = accept4(uctrl->sock, NULL, NULL, SOCK_CLOEXEC|SOCK_NONBLOCK);
+   conn->sock = accept(uctrl->sock, NULL, NULL);
    if (conn->sock < 0) {
        if (errno != EINTR)
            err(uctrl->udev, "unable to receive ctrl connection: %m\n");

I have asked steev for confirmation that this works on Linux 2.6.31. I will merge it to master afterward.

[ryao]

As an additional note, this regression occurred in udev 169.

[ryao]

It has been pointed out to me that Linux decided against supporting inheritance on the file descriptors involved, which requires that we invoke fcntl. I have written f04fc412682a98f9da9f8e9b353ac3cbbef66a77 to address that by introducing a fallback path and updated the backported patch:

diff --git a/src/udev/udev-ctrl.c b/src/udev/udev-ctrl.c
index a235912..01747b1 100644
--- a/libudev/libudev-ctrl.c
+++ b/libudev/libudev-ctrl.c
@@ -189,6 +189,18 @@ struct udev_ctrl_connection *udev_ctrl_get_connection(struct udev_ctrl *uctrl)
         conn->uctrl = uctrl;

         conn->sock = accept4(uctrl->sock, NULL, NULL, SOCK_CLOEXEC|SOCK_NONBLOCK);
+
+        /* Fallback path when accept4() is unavailable */
+        if ( conn->sock < 0 && (errno == ENOSYS || errno == ENOTSUP) )
+        {
+                conn->sock = accept(uctrl->sock, NULL, NULL);
+
+                if (conn->sock >= 0) {
+                        fcntl(conn->sock, F_SETFL, O_NONBLOCK);
+                        fcntl(conn->sock, F_SETFD, FD_CLOEXEC);
+                }
+        }
+
         if (conn->sock < 0) {
                 if (errno != EINTR)
                         log_error("unable to receive ctrl connection: %m\n");

[lu-zero]

On Mon, Nov 19, 2012 at 7:15 AM, Richard Yao notifications@github.com wrote:

It has been pointed out to me that Linux decided against supporting inheritance on the file descriptors involved, which requires that we invoke fcntl. I have written f04fc41 to address that by introducing a fallback path and updated the backported patch:

Looks fine to me.

[ryao]

This is still waiting on feedback from steev.

[steev]

Requires including linux/signalfd.h, not sure if that's a big deal to others or not.

[ryao]

This was merged into eudev in time for the 1_beta1 tag. I was inclined to keep this issue open until it was backported to udev-171, but that belongs in the Gentoo bug tracker.

[jshafer817]

Thanks!!! I think this will do the trick!