答复: [PATCH v2] e2fsprogs: Check device id in advance to skip fake device name

答复: [PATCH v2] e2fsprogs: Check device id in advance to skip fake device name

[Guiyao]

Hi Theodore,

Thanks for your response and your rewriting.

Actually, we found some insane system administrators, they not only do something like "mount -t tmpfs /dev/sdb /tmp ", but also they do " ln -s /dev/sdb abc ", then "resize2fs abc xxx". :(

So we have to add the fixing code in both sides of "name matched" and "name not matched".

For the compiling issue, it's my fault in previous patch, and added the macro in a wrong line.

So, I rewrote it again, and please give more advise. Thank you in advance.


diff --git a/lib/ext2fs/ismounted.c b/lib/ext2fs/ismounted.c
index 6cd497d..729769e 100644
--- a/lib/ext2fs/ismounted.c
+++ b/lib/ext2fs/ismounted.c
@@ -97,6 +97,9 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
                   int *mount_flags, char *mtpt, int mtlen)
 {
    struct mntent   *mnt;
+#ifndef __GNU__
+   struct stat dir_st_buf;
+#endif  /* __GNU__ */
    struct stat st_buf;
    errcode_t   retval = 0;
    dev_t       file_dev=0, file_rdev=0;
@@ -128,13 +131,32 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
    while ((mnt = getmntent (f)) != NULL) {
        if (mnt->mnt_fsname[0] != '/')
            continue;
-       if (strcmp(file, mnt->mnt_fsname) == 0)
+#ifndef __GNU__
+       if (stat(mnt->mnt_dir, &dir_st_buf) != 0)
+           continue;
+#endif
+       if (strcmp(file, mnt->mnt_fsname) == 0) {
+#ifndef __GNU__
+           if (file_rdev && (file_rdev != dir_st_buf.st_dev)) {
+#ifdef DEBUG
+               printf("Bogus entry in %s!  "
+                      "(%s does not exist)\n",
+                      mtab_file, mnt->mnt_dir);
+#endif /* DEBUG */
+               continue;
+           }
+#endif /* __GNU__ */
            break;
+       }
+
        if (stat(mnt->mnt_fsname, &st_buf) == 0) {
            if (ext2fsP_is_disk_device(st_buf.st_mode)) {
 #ifndef __GNU__
-               if (file_rdev && (file_rdev == st_buf.st_rdev))
-                   break;
+               if (file_rdev && (file_rdev == st_buf.st_rdev)) {
+                   if (file_rdev == dir_st_buf.st_dev)
+                       break;
+               }
+
                if (check_loop_mounted(mnt->mnt_fsname,
                        st_buf.st_rdev, file_dev,
                        file_ino) == 1)
@@ -168,32 +190,6 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
 #endif /* __GNU__ */
        goto errout;
    }
-#ifndef __GNU__ /* The GNU hurd is deficient; what else is new? */
-   /* Validate the entry in case /etc/mtab is out of date */
-   /*
-    * We need to be paranoid, because some broken distributions
-    * (read: Slackware) don't initialize /etc/mtab before checking
-    * all of the non-root filesystems on the disk.
-    */
-   if (stat(mnt->mnt_dir, &st_buf) < 0) {
-       retval = errno;
-       if (retval == ENOENT) {
-#ifdef DEBUG
-           printf("Bogus entry in %s!  (%s does not exist)\n",
-                  mtab_file, mnt->mnt_dir);
-#endif /* DEBUG */
-           retval = 0;
-       }
-       goto errout;
-   }
-   if (file_rdev && (st_buf.st_dev != file_rdev)) {
-#ifdef DEBUG
-       printf("Bogus entry in %s!  (%s not mounted on %s)\n",
-              mtab_file, file, mnt->mnt_dir);
-#endif /* DEBUG */
-       goto errout;
-   }
-#endif /* __GNU__ */
    *mount_flags = EXT2_MF_MOUNTED;

 #ifdef MNTOPT_RO
-- 
1.8.3.1

Re: 答复: [PATCH v2] e2fsprogs: Check device id in advance to skip fake device name

[Theodore Y. Ts'o]

On Mon, Nov 11, 2019 at 02:43:46PM +0000, Guiyao wrote:
> 
> Actually, we found some insane system administrators, they not only do something like "mount -t tmpfs /dev/sdb /tmp ", but also they do " ln -s /dev/sdb abc ", then "resize2fs abc xxx". :(

So I don't consider ourselves necessarily obligated to twist ourselves
into knots for insane system administrators.  :-)

Did you test the patch that I sent out?  It handles that case already:

% grep /dev/loop /proc/mounts
/dev/loop0 /mnt2 tmpfs rw,relatime 0 0
/dev/loop0 /mnt ext4 rw,relatime 0 0
% ln -s /dev/loop0 abc
% ./tst_ismounted abc
Device abc reports flags 11
abc is apparently in use.
abc is mounted.
abc is mounted on /mnt2.

> So we have to add the fixing code in both sides of "name matched" and "name not matched".
> 
> For the compiling issue, it's my fault in previous patch, and added the macro in a wrong line.
> 
> So, I rewrote it again, and please give more advise. Thank you in advance.

Given that I have a patch which I've already tested, and which is a
substantial clean up in terms of removing #ifdef cases and number of
lines of code:

 lib/ext2fs/ismounted.c | 39 ++++++++++++---------------------------
  1 file changed, 12 insertions(+), 27 deletions(-)
  
I'm inclined to stick with mine.

But here's the quick review.

>  {
>     struct mntent   *mnt;
> +#ifndef __GNU__
> +   struct stat dir_st_buf;
> +#endif  /* __GNU__ */

Lots of extra #ifdef/#ifndef is undesirable.  As it turns out, it
isn't necessary to have a separate dir_st_buf at all.

> @@ -128,13 +131,32 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
>     while ((mnt = getmntent (f)) != NULL) {
>         if (mnt->mnt_fsname[0] != '/')
>             continue;
> -       if (strcmp(file, mnt->mnt_fsname) == 0)
> +#ifndef __GNU__
> +       if (stat(mnt->mnt_dir, &dir_st_buf) != 0)
> +           continue;
> +#endif
> +       if (strcmp(file, mnt->mnt_fsname) == 0) {
> +#ifndef __GNU__
> +           if (file_rdev && (file_rdev != dir_st_buf.st_dev)) {

This doesn't need to be under #ifndef __GNU__.  In the GNU hurd case,
file_rdev will be zero, so the compiler will remove the if statement
for us, without needing an additional #ifndef __GNU__ test.

>         if (stat(mnt->mnt_fsname, &st_buf) == 0) {
>             if (ext2fsP_is_disk_device(st_buf.st_mode)) {
>  #ifndef __GNU__
> -               if (file_rdev && (file_rdev == st_buf.st_rdev))
> -                   break;
> +               if (file_rdev && (file_rdev == st_buf.st_rdev)) {
> +                   if (file_rdev == dir_st_buf.st_dev)
> +                       break;
> +               }
> +

The reason why this isn't necessary is because we're using stat, and
stat follows symlinks.  So when you do "ln -s /dev/sdb abc", and then
we stat abc, st_buf.st_rdev contains the device node of /dev/sbc, not
the symbolic link of abc.  So adding a check for dir_st_buf.st_dev is
not needed.

Cheers,

					- Ted