src/utils/closefrom.cpp

@@ -0,0 +1,218 @@
+#ifndef lint
+static char rcsid[] = "@(#$Id: $ (C) 2009 J.F.Dockes";
+#endif
+/*
+ * Close all file descriptors above a given value.
+ *
+ * A Unix execXX() call used to execute another program does not close open 
+ * file descriptors by default.
+ *
+ * The only descriptors closed are those on which the FD_CLOEXEC flag was
+ * set. FD_CLOEXEC is not easily usable on files opened by external
+ * libraries.
+ *
+ * There are many reasons for closing file descriptors before
+ * an exec (security,  pipe control, the possibility that a bug will trigger
+ * an unwanted write, etc.)
+ *
+ * A process has currently no POSIX way to determine the list of open file 
+ * descriptors or at least the highest value. Closing all files (except a few),
+ * thus implies performing a close() system call on each entry up to the 
+ * maximum, which can be both relatively difficult to determine, and quite
+ * high (ie: several thousands), incurring a non-negligible cost.
+ *
+ * A number of systems have non-portable support for mitigating or solving
+ * this problem. 
+ * 
+ * This module supplies a portable interface to this functionality.
+ *
+ * The initial data on system interfaces was obtained from:
+ * http://stackoverflow.com/questions/899038/\
+ *   getting-the-highest-allocated-file-descriptor
+ *
+ * System interfaces:
+ *  FreeBSD:
+ *   - Has a closefrom() system call as of release 7.x around Sep 2009
+ *   - Has a /dev/fd, directory which lists the current process' open
+ *     descriptors. Only descriptors 0, 1, 2 are shown except if
+ *     fdescfs is mounted which it is not by default
+ *
+ * Interface:
+ * int libclf_closefrom(fd)
+ *  @param fd All open file descriptors with equal or higher numeric 
+ *       values will be closed. fd needs not be a valid descriptor.
+ *  @return 0 for success, -1 for error.
+ */
+#ifndef TEST_CLOSEFROM
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/param.h>
+
+/* #define DEBUG_CLOSEFROM*/
+#ifdef DEBUG_CLOSEFROM
+#define DPRINT(X) fprintf X
+#else
+#define DPRINT(X)
+#endif
+
+/* Note: sudo has a closefrom implementation, needs a lot of autoconfig, but
+ * we could use it instead. It's quite close to this though */
+
+/*************************************************************************/
+
+/* closefrom() exists on Solaris, netbsd and openbsd, but someone will
+ * have to provide me the appropriate macro to test */
+#if (defined(__FreeBSD__) && __FreeBSD_version >= 702104)
+/* Use closefrom() system call */
+int libclf_closefrom(int fd0)
+{
+    DPRINT((stderr, "libclf_closefrom: using closefrom(2)\n"));
+    closefrom(fd0);
+    return 0;
+}
+
+/*************************************************************************/
+#elif defined(F_CLOSEM)
+
+/* Use fcntl(fd, F_CLOSEM) */
+
+int libclf_closefrom(int fd0)
+{
+    DPRINT((stderr, "libclf_closefrom: using fcntl(F_CLOSEM)\n"));
+    // We need a valid descriptor for this to work. Try to dup stdin, else
+    // go wild
+    if (fcntl(0, F_GETFL) != -1) {
+        if (fd0 != 0)
+            dup2(0, fd0);
+    } else {
+        int fd = open("/etc/group", 0); // yes i am a unix man
+        if (fd >= 0 && fd != fd0) {
+            dup2(fd, fd0);
+            close(fd);
+        }
+    }
+    return fcntl(fd0, F_CLOSEM, 0);
+}
+
+/*************************************************************************/
+#elif defined(linux)
+
+/* Use /proc/self/fd directory */
+#include <sys/types.h>
+#include <dirent.h>
+
+int libclf_closefrom(int fd0)
+{
+    DIR *dirp;
+    struct dirent *ent;
+
+    DPRINT((stderr, "libclf_closefrom: using /proc\n"));
+    dirp = opendir("/proc/self/fd");
+    if (dirp == 0)
+        return -1;
+
+    while ((ent = readdir(dirp)) != 0) {
+        int fd;
+        if (!strcmp(ent->d_name, ".") || !strcmp(ent->d_name, "..")) 
+            continue;
+
+        if (sscanf(ent->d_name, "%d", &fd) == 1 && fd >= fd0 && 
+            fd != dirfd(dirp)) {
+            close(fd);
+        }
+    }
+    closedir(dirp);
+    return 0;
+}
+
+/*************************************************************************/
+#else 
+/* System has no native support for this functionality whatsoever.
+ *
+ * Close all descriptors up to compiled/configured maximum.
+ * The caller will usually have an idea of a reasonable maximum, else
+ * we retrieve a value from the system.
+ */
+
+static int closefrom_maxfd = -1;
+
+void libclf_setmaxfd(int max)
+{
+    closefrom_maxfd = max;
+}
+
+int libclf_closefrom(int fd0)
+{
+    int i, maxfd = closefrom_maxfd;
+
+    if (maxfd < 0) {
+#ifdef _SC_OPEN_MAX
+        maxfd = sysconf(_SC_OPEN_MAX);
+        DPRINT((stderr, "Maxfd is %d after sysconf()\n", maxfd));
+#else
+        maxfd = getdtablesize();
+        DPRINT((stderr, "Maxfd is %d after getdtablesize()\n", maxfd));
+#endif 
+    }
+    if (maxfd < 0)
+	maxfd = OPEN_MAX;
+
+    DPRINT((stderr, "libclf_closefrom: using loop to %d\n", maxfd));
+
+    for (i = fd0; i < maxfd; i++) {
+        (void)close(i);
+    }
+    return 0;
+}
+#endif
+
+
+#else /* TEST_CLOSEFROM */
+
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+#include "closefrom.h"
+
+int main(int argc, char **argv)
+{
+    int i;
+
+    int fd0 = open("/etc/group", 0);
+    if (fd0 < 0) {
+        perror("open /etc/group");
+        exit(1);
+    }
+    
+    if (dup2(fd0, 11) < 0) {
+        perror("dup2->11");
+        exit(1);
+    }
+    if (dup2(fd0, 19) < 0) {
+        perror("dup2->19");
+        exit(1);
+    }
+    if (dup2(fd0, 99)< 0) {
+        perror("dup2->99 (ok)");
+    }
+    if (dup2(fd0, 999) < 0) {
+        perror("dup3->999 (ok)");
+    }
+
+    libclf_closefrom(11);
+    for (i = 0; i < 10000; i++) {
+        if (fcntl(i, F_GETFL) != -1) {
+            fprintf(stderr, "Descriptor %d is still open", i);
+            if (i < 11)
+                fprintf(stderr, " (OK)\n");
+            else
+                fprintf(stderr, " (BAD)\n");
+        }
+    }
+    exit(0);
+}
+
+#endif

src/utils/closefrom.h

@@ -0,0 +1,8 @@
+#ifndef _closefrom_h_included_
+#define _closefrom_h_included_
+/* @(#$Id: $  (C) 2004 J.F.Dockes */
+
+/* Close all descriptors >=  fd */
+extern int libclf_closefrom(int fd);
+
+#endif /* _closefrom_h_included_ */