[Libguestfs] [PATCH v3] daemon: Use bind mount to overwrite /etc/resolv.conf

Implement bind mount to overwrite /etc/resolv.conf instead of the current method of rename followed by copy. The bind mount method works even if the file is designated as immutable Fixes: https://redhat.atlassian.net/browse/RHEL-178556 Signed-off-by: Srihari Parimi <sparimi(a)redhat.com&gt; --- daemon/sh.c | 97 ++++++++++++++++++++++++++--------------------------- 1 file changed, 47 insertions(+), 50 deletions(-) diff --git a/daemon/sh.c b/daemon/sh.c index 74e003410..9f6c8de78 100644 --- a/daemon/sh.c +++ b/daemon/sh.c @@ -48,8 +48,8 @@ struct bind_state { struct resolver_state { bool mounted; + bool sysroot_etc_resolv_conf_exists; char *sysroot_etc_resolv_conf; - char *sysroot_etc_resolv_conf_old; }; /* While running the command, bind-mount /dev, /proc, /sys @@ -150,10 +150,12 @@ free_bind_state (struct bind_state *bs) static int set_up_etc_resolv_conf (struct resolver_state *rs) { + const char *src_etc_resolv = "/etc/resolv.conf"; struct stat statbuf; - CLEANUP_FREE char *buf = NULL; + int r, fd; - rs->sysroot_etc_resolv_conf_old = NULL; + /* assume that /sysroot/etc/resolv.conf file exists */ + rs->sysroot_etc_resolv_conf_exists = true; rs->sysroot_etc_resolv_conf = sysroot_path ("/etc/resolv.conf"); @@ -161,50 +163,52 @@ set_up_etc_resolv_conf (struct resolver_state *rs) reply_with_perror ("malloc"); goto error; } - - /* If /etc/resolv.conf exists, rename it to the backup file. Note - * that on Ubuntu it's a dangling symlink. + /* If /sysroot/etc/resolv.conf does not exist, create one for mount + * bind to succeed. lstat() call handles dangling links */ - if (lstat (rs->sysroot_etc_resolv_conf, &statbuf) == 0) { - /* Make a random name for the backup file. */ - if (asprintf (&buf, "%s/etc/XXXXXXXX", sysroot) == -1) { - reply_with_perror ("asprintf"); - goto error; - } - if (random_name (buf) == -1) { - reply_with_perror ("random_name"); - goto error; + if (lstat (rs->sysroot_etc_resolv_conf, &statbuf) == -1) { + if (errno != ENOENT) { + reply_with_perror ("lstat: %s", rs->sysroot_etc_resolv_conf); + return -1; } - rs->sysroot_etc_resolv_conf_old = strdup (buf); - if (!rs->sysroot_etc_resolv_conf_old) { - reply_with_perror ("strdup"); - goto error; + /* create empty file */ + fd = open (rs->sysroot_etc_resolv_conf, + O_WRONLY|O_CREAT|O_NOCTTY|O_CLOEXEC, 0644); + if (fd == -1) { + reply_with_perror ("open: %s", rs->sysroot_etc_resolv_conf); + return -1; } - - if (verbose) - fprintf (stderr, "renaming %s to %s\n", rs->sysroot_etc_resolv_conf, - rs->sysroot_etc_resolv_conf_old); - - if (rename (rs->sysroot_etc_resolv_conf, - rs->sysroot_etc_resolv_conf_old) == -1) { - reply_with_perror ("rename: %s to %s", rs->sysroot_etc_resolv_conf, - rs->sysroot_etc_resolv_conf_old); - goto error; + if (close(fd) == -1) { + reply_with_perror ("close: %s", rs->sysroot_etc_resolv_conf); + return -1; } + /* /sysroot/etc/resolv.conf file did not exist, we created it. + * We remove it using this marker flag after umount. + */ + rs->sysroot_etc_resolv_conf_exists = false; } - - /* Now that the guest's <sysroot>/etc/resolv.conf is out the way, we - * can create our own copy of the appliance /etc/resolv.conf. - */ - ignore_value (command (NULL, NULL, "cp", "/etc/resolv.conf", - rs->sysroot_etc_resolv_conf, NULL)); - + if (verbose) + fprintf (stderr, "mount %s to %s\n", src_etc_resolv, + rs->sysroot_etc_resolv_conf); + /* mount bind read only */ + r = command (NULL, NULL, "mount", "--bind", "-o ro", src_etc_resolv, + rs->sysroot_etc_resolv_conf, NULL); + if (r == -1) { + reply_with_perror ("mount: %s to %s failed\n", + src_etc_resolv, rs->sysroot_etc_resolv_conf); + goto error; + } + if (verbose) + fprintf (stderr, "mount %s to %s done\n", src_etc_resolv, + rs->sysroot_etc_resolv_conf); rs->mounted = true; return 0; error: + if (rs->sysroot_etc_resolv_conf_exists == false) { + unlink (rs->sysroot_etc_resolv_conf); + } free (rs->sysroot_etc_resolv_conf); - free (rs->sysroot_etc_resolv_conf_old); return -1; } @@ -212,20 +216,13 @@ static void free_resolver_state (struct resolver_state *rs) { if (rs->mounted) { - unlink (rs->sysroot_etc_resolv_conf); - - if (rs->sysroot_etc_resolv_conf_old) { - if (verbose) - fprintf (stderr, "renaming %s to %s\n", rs->sysroot_etc_resolv_conf_old, - rs->sysroot_etc_resolv_conf); - - if (rename (rs->sysroot_etc_resolv_conf_old, - rs->sysroot_etc_resolv_conf) == -1) - perror ("error: could not restore /etc/resolv.conf"); - - free (rs->sysroot_etc_resolv_conf_old); + if (rs->sysroot_etc_resolv_conf_exists == false) { + /* do not want to leave the file which we created */ + unlink (rs->sysroot_etc_resolv_conf); } - + if (verbose) + fprintf (stderr, "umount %s\n", rs->sysroot_etc_resolv_conf); + umount_ignore_fail (rs->sysroot_etc_resolv_conf); free (rs->sysroot_etc_resolv_conf); rs->mounted = false; } -- 2.52.0