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File Descriptor Management
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<li><a class=" active" href="../../en-US/tasks/Tasks-Descriptors.html">File Descriptor Management</a></li>
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<div class="col-xs-12 col-sm-9 col-md-9 main">
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<div class="page-header">
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<h2>File Descriptor Management</h2>
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</div>
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<div id="preamble">
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<div class="sectionbody">
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<div class="paragraph">
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<p>File descriptors underlie all input/output mechanisms offered by
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the system. They are used to implementation the <code>FILE
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*</code>-based functions found in
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<code><stdio.h></code>, and all the file and network
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communication facilities provided by the Python and Java
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environments are eventually implemented in them.</p>
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</div>
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<div class="paragraph">
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<p>File descriptors are small, non-negative integers in userspace,
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and are backed on the kernel side with complicated data structures
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which can sometimes grow very large.</p>
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</div>
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</div>
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</div>
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<div class="sect1">
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<h2 id="closing-descriptors"><a class="anchor" href="#closing-descriptors"></a>Closing Descriptors</h2>
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<div class="sectionbody">
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<div class="paragraph">
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<p>If a descriptor is no longer used by a program and is not closed
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explicitly, its number cannot be reused (which is problematic in
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itself, see <a href="#sect-Defensive_Coding-Tasks-Descriptors-Limit">Dealing with the <code>select</code> Limit</a>), and
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the kernel resources are not freed. Therefore, it is important
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to close all descriptors at the earliest point in time
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possible, but not earlier.</p>
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</div>
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<div class="sect2">
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<h3 id="error-handling-during-descriptor-close"><a class="anchor" href="#error-handling-during-descriptor-close"></a>Error Handling during Descriptor Close</h3>
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<div class="paragraph">
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<p>The <code>close</code> system call is always
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successful in the sense that the passed file descriptor is
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never valid after the function has been called. However,
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<code>close</code> still can return an error, for
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example if there was a file system failure. But this error is
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not very useful because the absence of an error does not mean
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that all caches have been emptied and previous writes have
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been made durable. Programs which need such guarantees must
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open files with <code>O_SYNC</code> or use
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<code>fsync</code> or <code>fdatasync</code>, and
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may also have to <code>fsync</code> the directory
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containing the file.</p>
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</div>
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</div>
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<div class="sect2">
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<h3 id="closing-descriptors-and-race-conditions"><a class="anchor" href="#closing-descriptors-and-race-conditions"></a>Closing Descriptors and Race Conditions</h3>
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<div class="paragraph">
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<p>Unlike process IDs, which are recycle only gradually, the
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kernel always allocates the lowest unused file descriptor when
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a new descriptor is created. This means that in a
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multi-threaded program which constantly opens and closes file
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descriptors, descriptors are reused very quickly. Unless
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descriptor closing and other operations on the same file
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descriptor are synchronized (typically, using a mutex), there
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will be race conditons and I/O operations will be applied to
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the wrong file descriptor.</p>
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</div>
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<div class="paragraph">
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<p>Sometimes, it is necessary to close a file descriptor
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concurrently, while another thread might be about to use it in
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a system call. In order to support this, a program needs to
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create a single special file descriptor, one on which all I/O
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operations fail. One way to achieve this is to use
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<code>socketpair</code>, close one of the descriptors,
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and call <code>shutdown(fd, SHUTRDWR)</code> on the
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other.</p>
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</div>
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<div class="paragraph">
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<p>When a descriptor is closed concurrently, the program does not
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call <code>close</code> on the descriptor. Instead it
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program uses <code>dup2</code> to replace the
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descriptor to be closed with the dummy descriptor created
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earlier. This way, the kernel will not reuse the descriptor,
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but it will carry out all other steps associated with calling
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a descriptor (for instance, if the descriptor refers to a
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stream socket, the peer will be notified).</p>
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</div>
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<div class="paragraph">
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<p>This is just a sketch, and many details are missing.
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Additional data structures are needed to determine when it is
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safe to really close the descriptor, and proper locking is
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required for that.</p>
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</div>
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</div>
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<div class="sect2">
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<h3 id="lingering-state-after-close"><a class="anchor" href="#lingering-state-after-close"></a>Lingering State after Close</h3>
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<div class="paragraph">
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<p>By default, closing a stream socket returns immediately, and
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the kernel will try to send the data in the background. This
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means that it is impossible to implement accurate accounting
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of network-related resource utilization from userspace.</p>
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</div>
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<div class="paragraph">
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<p>The <code>SO_LINGER</code> socket option alters the
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behavior of <code>close</code>, so that it will return
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only after the lingering data has been processed, either by
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sending it to the peer successfully, or by discarding it after
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the configured timeout. However, there is no interface which
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could perform this operation in the background, so a separate
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userspace thread is needed for each <code>close</code>
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call, causing scalability issues.</p>
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</div>
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<div class="paragraph">
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<p>Currently, there is no application-level countermeasure which
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applies universally. Mitigation is possible with
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<strong class="application">iptables</strong> (the
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<code>connlimit</code> match type in particular) and
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specialized filtering devices for denial-of-service network
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traffic.</p>
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</div>
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<div class="paragraph">
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<p>These problems are not related to the
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<code>TIME_WAIT</code> state commonly seen in
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<strong class="application">netstat</strong> output. The kernel
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automatically expires such sockets if necessary.</p>
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</div>
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</div>
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</div>
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</div>
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<div class="sect1">
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<h2 id="sect-Defensive_Coding-Tasks-Descriptors-Child_Processes"><a class="anchor" href="#sect-Defensive_Coding-Tasks-Descriptors-Child_Processes"></a>Preventing File Descriptor Leaks to Child Processes</h2>
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<div class="sectionbody">
|
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<div class="paragraph">
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<p>Child processes created with <code>fork</code> share
|
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the initial set of file descriptors with their parent
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process. By default, file descriptors are also preserved if
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a new process image is created with <code>execve</code>
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|
(or any of the other functions such as <code>system</code>
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or <code>posix_spawn</code>).</p>
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</div>
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<div class="paragraph">
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<p>Usually, this behavior is not desirable. There are two ways to
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turn it off, that is, to prevent new process images from
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inheriting the file descriptors in the parent process:</p>
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</div>
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<div class="ulist">
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<ul>
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<li>
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<p>Set the close-on-exec flag on all newly created file
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descriptors. Traditionally, this flag is controlled by the
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<code>FD_CLOEXEC</code> flag, using
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<code>F_GETFD</code> and <code>F_SETFD</code>
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operations of the <code>fcntl</code> function.</p>
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<div class="paragraph">
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<p>However, in a multi-threaded process, there is a race
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condition: a subprocess could have been created between the
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time the descriptor was created and the
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<code>FD_CLOEXEC</code> was set. Therefore, many system
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calls which create descriptors (such as
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<code>open</code> and <code>openat</code>)
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now accept the <code>O_CLOEXEC</code> flag
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(<code>SOCK_CLOEXEC</code> for
|
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<code>socket</code> and
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<code>socketpair</code>), which cause the
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<code>FD_CLOEXEC</code> flag to be set for the file
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descriptor in an atomic fashion. In addition, a few new
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systems calls were introduced, such as
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<code>pipe2</code> and <code>dup3</code>.</p>
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</div>
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<div class="paragraph">
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<p>The downside of this approach is that every descriptor needs
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to receive special treatment at the time of creation,
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otherwise it is not completely effective.</p>
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</div>
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</li>
|
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<li>
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<p>After calling <code>fork</code>, but before creating
|
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a new process image with <code>execve</code>, all
|
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file descriptors which the child process will not need are
|
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closed.</p>
|
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<div class="paragraph">
|
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<p>Traditionally, this was implemented as a loop over file
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descriptors ranging from <code>3</code> to
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<code>255</code> and later <code>1023</code>.
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But this is only an approximation because it is possible to
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create file descriptors outside this range easily (see <a href="#sect-Defensive_Coding-Tasks-Descriptors-Limit">Dealing with the <code>select</code> Limit</a>).
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Another approach reads <code>/proc/self/fd</code>
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and closes the unexpected descriptors listed there, but this
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approach is much slower.</p>
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</div>
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</li>
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</ul>
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</div>
|
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<div class="paragraph">
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<p>At present, environments which care about file descriptor
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leakage implement the second approach. OpenJDK 6 and 7
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are among them.</p>
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</div>
|
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</div>
|
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</div>
|
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<div class="sect1">
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<h2 id="sect-Defensive_Coding-Tasks-Descriptors-Limit"><a class="anchor" href="#sect-Defensive_Coding-Tasks-Descriptors-Limit"></a>Dealing with the <code>select</code> Limit</h2>
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<div class="sectionbody">
|
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<div class="paragraph">
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<p>By default, a user is allowed to open only 1024 files in a
|
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single process, but the system administrator can easily change
|
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this limit (which is necessary for busy network servers).
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However, there is another restriction which is more difficult to
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overcome.</p>
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</div>
|
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<div class="paragraph">
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<p>The <code>select</code> function only supports a
|
|
maximum of <code>FD_SETSIZE</code> file descriptors
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(that is, the maximum permitted value for a file descriptor
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|
is <code>FD_SETSIZE - 1</code>, usually 1023.) If a
|
|
process opens many files, descriptors may exceed such
|
|
limits. It is impossible to query such descriptors using
|
|
<code>select</code>.</p>
|
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</div>
|
|
<div class="paragraph">
|
|
<p>If a library which creates many file descriptors is used in
|
|
the same process as a library which uses
|
|
<code>select</code>, at least one of them needs to
|
|
be changed.
|
|
Calls to <code>select</code> can be replaced with
|
|
calls to <code>poll</code> or another event handling
|
|
mechanism. Replacing the <code>select</code> function
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|
is the recommended approach.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Alternatively, the library with high descriptor usage can
|
|
relocate descriptors above the <code>FD_SETSIZE</code>
|
|
limit using the following procedure.</p>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>Create the file descriptor <code>fd</code> as
|
|
usual, preferably with the <code>O_CLOEXEC</code>
|
|
flag.</p>
|
|
</li>
|
|
<li>
|
|
<p>Before doing anything else with the descriptor
|
|
<code>fd</code>, invoke:</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> int newfd = fcntl(fd, F_DUPFD_CLOEXEC, (long)FD_SETSIZE);</pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>Check that <code>newfd</code> result is
|
|
non-negative, otherwise close <code>fd</code> and
|
|
report an error, and return.</p>
|
|
</li>
|
|
<li>
|
|
<p>Close <code>fd</code> and continue to use
|
|
<code>newfd</code>.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
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<div class="paragraph">
|
|
<p>The new descriptor has been allocated above the
|
|
<code>FD_SETSIZE</code>. Even though this algorithm
|
|
is racy in the sense that the <code>FD_SETSIZE</code>
|
|
first descriptors could fill up, a very high degree of
|
|
physical parallelism is required before this becomes a problem.</p>
|
|
</div>
|
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</div>
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