11 files changed, 166 insertions, 147 deletions

diff --git a/RELNOTES b/RELNOTES
index a14200a0f..e00eaee00 100644
--- a/RELNOTES
+++ b/RELNOTES
@@ -6,7 +6,7 @@ firejail (0.9.45) baseline; urgency=low
     allows a full bypass of seccomp filter; problem reported by Lizzie Dixon
     (CVE-2017-5206)
   * security: overwrite /etc/resolv.conf found by Martin Carpenter (CVE-2016-10118)
-  * secuirty: TOCTOU exploit for --get and --put found by Daniel Hodson
+  * security: TOCTOU exploit for --get and --put found by Daniel Hodson
   * security: invalid environment exploit found by Martin Carpenter (CVE-2016-10122)
   * security: split most of networking code in a separate executable
   * security: split seccomp filter code configuration in a separate executable
diff --git a/etc/disable-common.inc b/etc/disable-common.inc
index 187d26c83..5a281a91f 100644
--- a/etc/disable-common.inc
+++ b/etc/disable-common.inc
@@ -84,6 +84,7 @@ read-only ${HOME}/.profile
 read-only ${HOME}/.antigen
 read-only ${HOME}/.bash_login
 read-only ${HOME}/.bashrc
+read-only ${HOME}/.bash_aliases
 read-only ${HOME}/.bash_profile
 read-only ${HOME}/.bash_logout
 read-only ${HOME}/.zsh.d
@@ -104,6 +105,9 @@ read-only ${HOME}/.caffrc
 read-only ${HOME}/.dotfiles
 read-only ${HOME}/dotfiles
 read-only ${HOME}/.mailcap
+read-only ${HOME}/.muttrc
+read-only ${HOME}/.mutt/muttrc
+read-only ${HOME}/.msmtprc
 read-only ${HOME}/.exrc
 read-only ${HOME}/_exrc
 read-only ${HOME}/.vimrc
diff --git a/etc/evolution.profile b/etc/evolution.profile
index ab6dd7a4a..1707e562b 100644
--- a/etc/evolution.profile
+++ b/etc/evolution.profile
@@ -6,6 +6,9 @@ noblacklist ~/.pki
 noblacklist ~/.pki/nssdb
 noblacklist ~/.gnupg
 
+noblacklist /var/spool/mail
+noblacklist /var/mail
+
 include /etc/firejail/disable-common.inc
 include /etc/firejail/disable-programs.inc
 include /etc/firejail/disable-devel.inc
diff --git a/src/firejail/firejail.h b/src/firejail/firejail.h
index 36cf47435..586cfd65e 100644
--- a/src/firejail/firejail.h
+++ b/src/firejail/firejail.h
@@ -403,7 +403,7 @@ char *fs_check_overlay_dir(const char *subdirname, int allow_reuse);
 void fs_overlayfs(void);
 // chroot into an existing directory; mount exiting /dev and update /etc/resolv.conf
 void fs_chroot(const char *rootdir);
-int fs_check_chroot_dir(const char *rootdir);
+void fs_check_chroot_dir(const char *rootdir);
 
 // profile.c
 // find and read the profile specified by name from dir directory
@@ -450,6 +450,8 @@ void logmsg(const char *msg);
 void logargs(int argc, char **argv) ;
 void logerr(const char *msg);
 int copy_file(const char *srcname, const char *destname, uid_t uid, gid_t gid, mode_t mode);
+void copy_file_as_user(const char *srcname, const char *destname, uid_t uid, gid_t gid, mode_t mode);
+void touch_file_as_user(const char *fname, uid_t uid, gid_t gid, mode_t mode);
 int is_dir(const char *fname);
 int is_link(const char *fname);
 char *line_remove_spaces(const char *buf);
diff --git a/src/firejail/fs.c b/src/firejail/fs.c
index e2fc09533..d7764accd 100644
--- a/src/firejail/fs.c
+++ b/src/firejail/fs.c
@@ -994,20 +994,25 @@ void fs_overlayfs(void) {
 
 #ifdef HAVE_CHROOT              
 // return 1 if error
-int fs_check_chroot_dir(const char *rootdir) {
+void fs_check_chroot_dir(const char *rootdir) {
         EUID_ASSERT();
         assert(rootdir);
         struct stat s;
         char *name;
 
+        if (strcmp(rootdir, "/tmp") == 0 || strcmp(rootdir, "/var/tmp") == 0) {
+                fprintf(stderr, "Error: invalid chroot directory\n");
+                exit(1);
+        }
+
         // rootdir has to be owned by root
         if (stat(rootdir, &s) != 0) {
                 fprintf(stderr, "Error: cannot find chroot directory\n");
-                return 1;
+                exit(1);
         }
         if (s.st_uid != 0) {
                 fprintf(stderr, "Error: chroot directory should be owned by root\n");
-                return 1;
+                exit(1);
         }
 
         // check /dev
@@ -1015,7 +1020,11 @@ int fs_check_chroot_dir(const char *rootdir) {
                 errExit("asprintf");
         if (stat(name, &s) == -1) {
                 fprintf(stderr, "Error: cannot find /dev in chroot directory\n");
-                return 1;
+                exit(1);
+        }
+        if (s.st_uid != 0) {
+                fprintf(stderr, "Error: chroot /dev directory should be owned by root\n");
+                exit(1);
         }
         free(name);
 
@@ -1024,7 +1033,11 @@ int fs_check_chroot_dir(const char *rootdir) {
                 errExit("asprintf");
         if (stat(name, &s) == -1) {
                 fprintf(stderr, "Error: cannot find /var/tmp in chroot directory\n");
-                return 1;
+                exit(1);
+        }
+        if (s.st_uid != 0) {
+                fprintf(stderr, "Error: chroot /var/tmp directory should be owned by root\n");
+                exit(1);
         }
         free(name);
         
@@ -1033,7 +1046,11 @@ int fs_check_chroot_dir(const char *rootdir) {
                 errExit("asprintf");
         if (stat(name, &s) == -1) {
                 fprintf(stderr, "Error: cannot find /proc in chroot directory\n");
-                return 1;
+                exit(1);
+        }
+        if (s.st_uid != 0) {
+                fprintf(stderr, "Error: chroot /proc directory should be owned by root\n");
+                exit(1);
         }
         free(name);
         
@@ -1042,18 +1059,41 @@ int fs_check_chroot_dir(const char *rootdir) {
                 errExit("asprintf");
         if (stat(name, &s) == -1) {
                 fprintf(stderr, "Error: cannot find /tmp in chroot directory\n");
-                return 1;
+                exit(1);
+        }
+        if (s.st_uid != 0) {
+                fprintf(stderr, "Error: chroot /tmp directory should be owned by root\n");
+                exit(1);
         }
         free(name);
 
-        // check /bin/bash
-//      if (asprintf(&name, "%s/bin/bash", rootdir) == -1)
-//              errExit("asprintf");
-//      if (stat(name, &s) == -1) {
-//              fprintf(stderr, "Error: cannot find /bin/bash in chroot directory\n");
-//              return 1;
-//      }
-//      free(name);
+        // check /etc
+        if (asprintf(&name, "%s/etc", rootdir) == -1)
+                errExit("asprintf");
+        if (stat(name, &s) == -1) {
+                fprintf(stderr, "Error: cannot find /etc in chroot directory\n");
+                exit(1);
+        }
+        if (s.st_uid != 0) {
+                fprintf(stderr, "Error: chroot /etc directory should be owned by root\n");
+                exit(1);
+        }
+        free(name);
+
+        // check /etc/resolv.conf
+        if (asprintf(&name, "%s/etc/resolv.conf", rootdir) == -1)
+                errExit("asprintf");
+        if (stat(name, &s) == 0) {
+                if (s.st_uid != 0) {
+                        fprintf(stderr, "Error: chroot /etc/resolv.conf should be owned by root\n");
+                        exit(1);
+                }
+        }
+        if (is_link(name)) {
+                fprintf(stderr, "Error: invalid %s file\n", name);
+                exit(1);
+        }
+        free(name);
 
         // check x11 socket directory
         if (getenv("FIREJAIL_X11")) {
@@ -1063,12 +1103,14 @@ int fs_check_chroot_dir(const char *rootdir) {
                         errExit("asprintf");
                 if (stat(name, &s) == -1) {
                         fprintf(stderr, "Error: cannot find /tmp/.X11-unix in chroot directory\n");
-                        return 1;
+                        exit(1);
+                }
+                if (s.st_uid != 0) {
+                        fprintf(stderr, "Error: chroot /tmp/.X11-unix directory should be owned by root\n");
+                        exit(1);
                 }
                 free(name);
         }
-        
-        return 0;       
 }
 
 // chroot into an existing directory; mount exiting /dev and update /etc/resolv.conf
@@ -1129,7 +1171,7 @@ void fs_chroot(const char *rootdir) {
                         fprintf(stderr, "Error: invalid %s file\n", fname);
                         exit(1);
                 }
-                if (copy_file("/etc/resolv.conf", fname, 0, 0, 0644) == -1)
+                if (copy_file("/etc/resolv.conf", fname, 0, 0, 0644) == -1) // root needed
                         fprintf(stderr, "Warning: /etc/resolv.conf not initialized\n");
         }
         
diff --git a/src/firejail/fs_home.c b/src/firejail/fs_home.c
index 4de082b06..8a52314ed 100644
--- a/src/firejail/fs_home.c
+++ b/src/firejail/fs_home.c
@@ -42,19 +42,17 @@ static void skel(const char *homedir, uid_t u, gid_t g) {
                 // don't copy it if we already have the file
                 if (stat(fname, &s) == 0)
                         return;
+                if (is_link(fname)) { // stat on dangling symlinks fails, try again using lstat
+                        fprintf(stderr, "Error: invalid %s file\n", fname);
+                        exit(1);
+                }
                 if (stat("/etc/skel/.zshrc", &s) == 0) {
-                        if (copy_file("/etc/skel/.zshrc", fname, u, g, 0644) == 0) {
-                                fs_logger("clone /etc/skel/.zshrc");
-                        }
+                        copy_file_as_user("/etc/skel/.zshrc", fname, u, g, 0644); // regular user
+                        fs_logger("clone /etc/skel/.zshrc");
                 }
-                else { // 
-                        FILE *fp = fopen(fname, "w");
-                        if (fp) {
-                                fprintf(fp, "\n");
-                                SET_PERMS_STREAM(fp, u, g, S_IRUSR | S_IWUSR);
-                                fclose(fp);
-                                fs_logger2("touch", fname);
-                        }
+                else {
+                        touch_file_as_user(fname, u, g, 0644);
+                        fs_logger2("touch", fname);
                 }
                 free(fname);
         }
@@ -64,23 +62,21 @@ static void skel(const char *homedir, uid_t u, gid_t g) {
                 if (asprintf(&fname, "%s/.cshrc", homedir) == -1)
                         errExit("asprintf");
                 struct stat s;
+                
                 // don't copy it if we already have the file
                 if (stat(fname, &s) == 0)
                         return;
+                if (is_link(fname)) { // stat on dangling symlinks fails, try again using lstat
+                        fprintf(stderr, "Error: invalid %s file\n", fname);
+                        exit(1);
+                }
                 if (stat("/etc/skel/.cshrc", &s) == 0) {
-                        if (copy_file("/etc/skel/.cshrc", fname, u, g, 0644) == 0) {
-                                fs_logger("clone /etc/skel/.cshrc");
-                        }
+                        copy_file_as_user("/etc/skel/.cshrc", fname, u, g, 0644); // regular user
+                        fs_logger("clone /etc/skel/.cshrc");
                 }
-                else { // 
-                        /* coverity[toctou] */
-                        FILE *fp = fopen(fname, "w");
-                        if (fp) {
-                                fprintf(fp, "\n");
-                                SET_PERMS_STREAM(fp, u, g, S_IRUSR | S_IWUSR);
-                                fclose(fp);
-                                fs_logger2("touch", fname);
-                        }
+                else {
+                        touch_file_as_user(fname, u, g, 0644);
+                        fs_logger2("touch", fname);
                 }
                 free(fname);
         }
@@ -93,10 +89,13 @@ static void skel(const char *homedir, uid_t u, gid_t g) {
                 // don't copy it if we already have the file
                 if (stat(fname, &s) == 0) 
                         return;
+                if (is_link(fname)) { // stat on dangling symlinks fails, try again using lstat
+                        fprintf(stderr, "Error: invalid %s file\n", fname);
+                        exit(1);
+                }
                 if (stat("/etc/skel/.bashrc", &s) == 0) {
-                        if (copy_file("/etc/skel/.bashrc", fname, u, g, 0644) == 0) {
-                                fs_logger("clone /etc/skel/.bashrc");
-                        }
+                        copy_file_as_user("/etc/skel/.bashrc", fname, u, g, 0644); // regular user
+                        fs_logger("clone /etc/skel/.bashrc");
                 }
                 free(fname);
         }
@@ -106,7 +105,7 @@ static int store_xauthority(void) {
         // put a copy of .Xauthority in XAUTHORITY_FILE
         char *src;
         char *dest = RUN_XAUTHORITY_FILE;
-        // create an empty file 
+        // create an empty file as root, and change ownership to user
         FILE *fp = fopen(dest, "w");
         if (fp) {
                 fprintf(fp, "\n");
@@ -124,27 +123,8 @@ static int store_xauthority(void) {
                         return 0;
                 }
 
-                pid_t child = fork();
-                if (child < 0)
-                        errExit("fork");
-                if (child == 0) {
-                        // drop privileges
-                        drop_privs(0);
-        
-                        // copy, set permissions and ownership
-                        int rv = copy_file(src, dest, getuid(), getgid(), 0600);
-                        if (rv)
-                                fprintf(stderr, "Warning: cannot transfer .Xauthority in private home directory\n");
-                        else {
-                                fs_logger2("clone", dest);
-                        }
-#ifdef HAVE_GCOV
-                        __gcov_flush();
-#endif
-                        _exit(0);
-                }
-                // wait for the child to finish
-                waitpid(child, NULL, 0);
+                copy_file_as_user(src, dest, getuid(), getgid(), 0600); // regular user
+                fs_logger2("clone", dest);
                 return 1; // file copied
         }
         
@@ -152,9 +132,10 @@ static int store_xauthority(void) {
 }
 
 static int store_asoundrc(void) {
+        // put a copy of .Xauthority in XAUTHORITY_FILE
         char *src;
         char *dest = RUN_ASOUNDRC_FILE;
-        // create an empty file 
+        // create an empty file as root, and change ownership to user
         FILE *fp = fopen(dest, "w");
         if (fp) {
                 fprintf(fp, "\n");
@@ -182,27 +163,8 @@ static int store_asoundrc(void) {
                         free(rp);
                 }
 
-                pid_t child = fork();
-                if (child < 0)
-                        errExit("fork");
-                if (child == 0) {
-                        // drop privileges
-                        drop_privs(0);
-        
-                        // copy, set permissions and ownership
-                        int rv = copy_file(src, dest, getuid(), getgid(), 0644);
-                        if (rv)
-                                fprintf(stderr, "Warning: cannot transfer .asoundrc in private home directory\n");
-                        else {
-                                fs_logger2("clone", dest);
-                        }
-#ifdef HAVE_GCOV
-                        __gcov_flush();
-#endif
-                        _exit(0);
-                }
-                // wait for the child to finish
-                waitpid(child, NULL, 0);
+                copy_file_as_user(src, dest, getuid(), getgid(), 0644); // regular user
+                fs_logger2("clone", dest);
                 return 1; // file copied
         }
         
@@ -222,27 +184,8 @@ static void copy_xauthority(void) {
                 exit(1);
         }
 
-        pid_t child = fork();
-        if (child < 0)
-                errExit("fork");
-        if (child == 0) {
-                // drop privileges
-                drop_privs(0);
-
-                // copy, set permissions and ownership
-                int rv = copy_file(src, dest, getuid(), getgid(), S_IRUSR | S_IWUSR);
-                if (rv)
-                        fprintf(stderr, "Warning: cannot transfer .Xauthority in private home directory\n");
-                else {
-                        fs_logger2("clone", dest);
-                }
-#ifdef HAVE_GCOV
-                __gcov_flush();
-#endif
-                _exit(0);
-        }
-        // wait for the child to finish
-        waitpid(child, NULL, 0);
+        copy_file_as_user(src, dest, getuid(), getgid(), S_IRUSR | S_IWUSR); // regular user
+        fs_logger2("clone", dest);
         
         // delete the temporary file
         unlink(src);
@@ -261,27 +204,8 @@ static void copy_asoundrc(void) {
                 exit(1);
         }
 
-        pid_t child = fork();
-        if (child < 0)
-                errExit("fork");
-        if (child == 0) {
-                // drop privileges
-                drop_privs(0);
-
-                // copy, set permissions and ownership
-                int rv = copy_file(src, dest, getuid(), getgid(), S_IRUSR | S_IWUSR);
-                if (rv)
-                        fprintf(stderr, "Warning: cannot transfer .asoundrc in private home directory\n");
-                else {
-                        fs_logger2("clone", dest);
-                }
-#ifdef HAVE_GCOV
-                __gcov_flush();
-#endif
-                _exit(0);
-        }
-        // wait for the child to finish
-        waitpid(child, NULL, 0);
+        copy_file_as_user(src, dest, getuid(), getgid(), S_IRUSR | S_IWUSR); // regular user
+        fs_logger2("clone", dest);
 
         // delete the temporary file
         unlink(src);
diff --git a/src/firejail/ls.c b/src/firejail/ls.c
index 77eb35f97..1af56751a 100644
--- a/src/firejail/ls.c
+++ b/src/firejail/ls.c
@@ -336,7 +336,7 @@ void sandboxfs(int op, pid_t pid, const char *path1, const char *path2) {
                         drop_privs(0);
                         
                         // copy the file
-                        if (copy_file(src_fname, tmp_fname, getuid(), getgid(), 0600))
+                        if (copy_file(src_fname, tmp_fname, getuid(), getgid(), 0600)) // already a regular user
                                 _exit(1);
 #ifdef HAVE_GCOV
                         __gcov_flush();
@@ -362,7 +362,7 @@ void sandboxfs(int op, pid_t pid, const char *path1, const char *path2) {
                         drop_privs(0);
                         
                         // copy the file
-                        if (copy_file(tmp_fname, dest_fname, getuid(), getgid(), 0600))
+                        if (copy_file(tmp_fname, dest_fname, getuid(), getgid(), 0600)) // already a regular user
                                 _exit(1);
 #ifdef HAVE_GCOV
                         __gcov_flush();
@@ -411,7 +411,7 @@ void sandboxfs(int op, pid_t pid, const char *path1, const char *path2) {
                         drop_privs(0);
                         
                         // copy the file
-                        if (copy_file(src_fname, tmp_fname, getuid(), getgid(), 0600))
+                        if (copy_file(src_fname, tmp_fname, getuid(), getgid(), 0600)) // already a regular user
                                 _exit(1);
 #ifdef HAVE_GCOV
                         __gcov_flush();
@@ -443,7 +443,7 @@ void sandboxfs(int op, pid_t pid, const char *path1, const char *path2) {
                         drop_privs(0);
                         
                         // copy the file
-                        if (copy_file(tmp_fname, dest_fname, getuid(), getgid(), 0600))
+                        if (copy_file(tmp_fname, dest_fname, getuid(), getgid(), 0600)) // already a regular user
                                 _exit(1);
 #ifdef HAVE_GCOV
                         __gcov_flush();
diff --git a/src/firejail/main.c b/src/firejail/main.c
index 3a347b3d9..84bf5e8e6 100644
--- a/src/firejail/main.c
+++ b/src/firejail/main.c
@@ -1468,13 +1468,10 @@ int main(int argc, char **argv) {
                                         fprintf(stderr, "Error: invalid chroot directory\n");
                                         exit(1);
                                 }
-                                free(rpath);
+                                cfg.chrootdir = rpath;
                                 
                                 // check chroot directory structure
-                                if (fs_check_chroot_dir(cfg.chrootdir)) {
-                                        fprintf(stderr, "Error: invalid chroot\n");
-                                        exit(1);
-                                }
+                                fs_check_chroot_dir(cfg.chrootdir);
                         }
                         else
                                 exit_err_feature("chroot");
diff --git a/src/firejail/preproc.c b/src/firejail/preproc.c
index d2db7d3dd..e17f39caa 100644
--- a/src/firejail/preproc.c
+++ b/src/firejail/preproc.c
@@ -76,12 +76,12 @@ void preproc_mount_mnt_dir(void) {
                 fs_logger2("tmpfs", RUN_MNT_DIR);
                 
                 //copy defaultl seccomp files
-                copy_file(PATH_SECCOMP_I386, RUN_SECCOMP_I386, getuid(), getgid(), 0644);
-                copy_file(PATH_SECCOMP_AMD64, RUN_SECCOMP_AMD64, getuid(), getgid(), 0644);
+                copy_file(PATH_SECCOMP_I386, RUN_SECCOMP_I386, getuid(), getgid(), 0644); // root needed
+                copy_file(PATH_SECCOMP_AMD64, RUN_SECCOMP_AMD64, getuid(), getgid(), 0644); // root needed
                 if (arg_allow_debuggers)
-                        copy_file(PATH_SECCOMP_DEFAULT_DEBUG, RUN_SECCOMP_CFG, getuid(), getgid(), 0644);
+                        copy_file(PATH_SECCOMP_DEFAULT_DEBUG, RUN_SECCOMP_CFG, getuid(), getgid(), 0644); // root needed
                 else
-                        copy_file(PATH_SECCOMP_DEFAULT, RUN_SECCOMP_CFG, getuid(), getgid(), 0644);
+                        copy_file(PATH_SECCOMP_DEFAULT, RUN_SECCOMP_CFG, getuid(), getgid(), 0644); // root needed
                         
                 // as root, create an empty RUN_SECCOMP_PROTOCOL file
                 create_empty_file_as_root(RUN_SECCOMP_PROTOCOL, 0644);
diff --git a/src/firejail/pulseaudio.c b/src/firejail/pulseaudio.c
index 14a7f03dd..4ec84ec61 100644
--- a/src/firejail/pulseaudio.c
+++ b/src/firejail/pulseaudio.c
@@ -22,6 +22,7 @@
 #include <sys/stat.h>
 #include <sys/mount.h>
 #include <dirent.h>
+#include <sys/wait.h>
 
 static void disable_file(const char *path, const char *file) {
         assert(file);
@@ -113,7 +114,7 @@ void pulseaudio_init(void) {
         char *pulsecfg = NULL;
         if (asprintf(&pulsecfg, "%s/client.conf", RUN_PULSE_DIR) == -1)
                 errExit("asprintf");
-        if (copy_file("/etc/pulse/client.conf", pulsecfg, -1, -1, 0644))
+        if (copy_file("/etc/pulse/client.conf", pulsecfg, -1, -1, 0644)) // root needed
                 errExit("copy_file");
         FILE *fp = fopen(pulsecfg, "a+");
         if (!fp)
diff --git a/src/firejail/util.c b/src/firejail/util.c
index 5b94aa288..763e6b58b 100644
--- a/src/firejail/util.c
+++ b/src/firejail/util.c
@@ -28,6 +28,7 @@
 #include <grp.h>
 #include <sys/ioctl.h>
 #include <termios.h>
+#include <sys/wait.h>
 
 #define MAX_GROUPS 1024
 // drop privileges
@@ -218,6 +219,51 @@ int copy_file(const char *srcname, const char *destname, uid_t uid, gid_t gid, m
         return 0;
 }
 
+// return -1 if error, 0 if no error
+void copy_file_as_user(const char *srcname, const char *destname, uid_t uid, gid_t gid, mode_t mode) {
+        pid_t child = fork();
+        if (child < 0)
+                errExit("fork");
+        if (child == 0) {
+                // drop privileges
+                drop_privs(0);
+
+                // copy, set permissions and ownership
+                int rv = copy_file(srcname, destname, uid, gid, mode); // already a regular user
+                if (rv)
+                        fprintf(stderr, "Warning: cannot transfer .Xauthority in private home directory\n");
+#ifdef HAVE_GCOV
+                __gcov_flush();
+#endif
+                _exit(0);
+        }
+        // wait for the child to finish
+        waitpid(child, NULL, 0);
+}
+
+// return -1 if error, 0 if no error
+void touch_file_as_user(const char *fname, uid_t uid, gid_t gid, mode_t mode) {
+        pid_t child = fork();
+        if (child < 0)
+                errExit("fork");
+        if (child == 0) {
+                // drop privileges
+                drop_privs(0);
+
+                FILE *fp = fopen(fname, "w");
+                if (fp) {
+                        fprintf(fp, "\n");
+                        SET_PERMS_STREAM(fp, uid, gid, mode);
+                        fclose(fp);
+                }
+#ifdef HAVE_GCOV
+                __gcov_flush();
+#endif
+                _exit(0);
+        }
+        // wait for the child to finish
+        waitpid(child, NULL, 0);
+}
 
 // return 1 if the file is a directory
 int is_dir(const char *fname) {