virCommand: use procfs to learn opened FDs

When spawning a child process, between fork() and exec() we close
all file descriptors and keep only those the caller wants us to
pass onto the child. The problem is how we do that. Currently, we
get the limit of opened files and then iterate through each one
of them and either close() it or make it survive exec(). This
approach is suboptimal (although, not that much in default
configurations where the limit is pretty low - 1024). We have
/proc where we can learn what FDs we hold open and thus we can
selectively close only those.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/util/vircommand.c

@@ -418,27 +418,97 @@ virExecCommon(virCommandPtr cmd, gid_t *groups, int ngroups)
     return ret;
 }
 
+# ifdef __linux__
+/* On Linux, we can utilize procfs and read the table of opened
+ * FDs and selectively close only those FDs we don't want to pass
+ * onto child process (well, the one we will exec soon since this
+ * is called from the child). */
+static int
+virCommandMassCloseGetFDsLinux(virCommandPtr cmd ATTRIBUTE_UNUSED,
+                               virBitmapPtr fds)
+{
+    DIR *dp = NULL;
+    struct dirent *entry;
+    const char *dirName = "/proc/self/fd";
+    int rc;
+    int ret = -1;
+
+    if (virDirOpen(&dp, dirName) < 0)
+        return -1;
+
+    while ((rc = virDirRead(dp, &entry, dirName)) > 0) {
+        int fd;
+
+        if (virStrToLong_i(entry->d_name, NULL, 10, &fd) < 0) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("unable to parse FD: %s"),
+                           entry->d_name);
+            goto cleanup;
+        }
+
+        if (virBitmapSetBit(fds, fd) < 0) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("unable to set FD as open: %d"),
+                           fd);
+            goto cleanup;
+        }
+    }
+
+    if (rc < 0)
+        goto cleanup;
+
+    ret = 0;
+ cleanup:
+    VIR_DIR_CLOSE(dp);
+    return ret;
+}
+
+# else /* !__linux__ */
+
+static int
+virCommandMassCloseGetFDsGeneric(virCommandPtr cmd ATTRIBUTE_UNUSED,
+                                 virBitmapPtr fds)
+{
+    virBitmapSetAll(fds);
+    return 0;
+}
+# endif /* !__linux__ */
+
 static int
 virCommandMassClose(virCommandPtr cmd,
                     int childin,
                     int childout,
                     int childerr)
 {
+    VIR_AUTOPTR(virBitmap) fds = NULL;
     int openmax = sysconf(_SC_OPEN_MAX);
-    int fd;
-    int tmpfd;
+    int fd = -1;
 
-    if (openmax < 0) {
-        virReportSystemError(errno,  "%s",
-                             _("sysconf(_SC_OPEN_MAX) failed"));
+    /* In general, it is not safe to call malloc() between fork() and exec()
+     * because the child might have forked at the worst possible time, i.e.
+     * when another thread was in malloc() and thus held its lock. That is to
+     * say, POSIX does not mandate malloc() to be async-safe. Fortunately,
+     * glibc developers are aware of this and made malloc() async-safe.
+     * Therefore we can safely allocate memory here (and transitively call
+     * opendir/readdir) without a deadlock. */
+
+    if (!(fds = virBitmapNew(openmax)))
         return -1;
-    }
 
-    for (fd = 3; fd < openmax; fd++) {
+# ifdef __linux__
+    if (virCommandMassCloseGetFDsLinux(cmd, fds) < 0)
+        return -1;
+# else
+    if (virCommandMassCloseGetFDsGeneric(cmd, fds) < 0)
+        return -1;
+# endif
+
+    fd = virBitmapNextSetBit(fds, 2);
+    for (; fd >= 0; fd = virBitmapNextSetBit(fds, fd)) {
         if (fd == childin || fd == childout || fd == childerr)
             continue;
         if (!virCommandFDIsSet(cmd, fd)) {
-            tmpfd = fd;
+            int tmpfd = fd;
             VIR_MASS_CLOSE(tmpfd);
         } else if (virSetInherit(fd, true) < 0) {
             virReportSystemError(errno, _("failed to preserve fd %d"), fd);