DragonFlyBSD

:Hey,
:
:I just noticed that we can produce stacked mounts of procfs and linprocfs. Probably not what we want:
:
:% mount|grep procfs
:procfs on /proc (procfs, local)
:procfs on /proc (procfs, local)
:
:I tried looking into where to prevent this, probably in the vfsmount routine. However I'm a bit out of the loop concerning the namecache, so I'm not sure when I need to lock/release what.
:
:Nevertheless I think we should prevent this behavior.
:
:cheers
: simon

Yah, that wouldn't be good.  procfs likes to destroy nodes dynamically
    and that could lead to unmountable filesystems.

The easiest solution is to add a kernel mount flag (MNTK_*) to the
    mount structure that tells the kernel not to allow mounts under a
    particular mount (e.g. procfs), and just have the kernel check it when
    a mount is attempted.  Probably not more then 10 lines of code if you
    would like to have a go at it.

-Matt

[snip]

The easiest solution is to add a kernel mount flag (MNTK_*) to the
mount structure that tells the kernel not to allow mounts under a
particular mount (e.g. procfs), and just have the kernel check it when
a mount is attempted. Probably not more then 10 lines of code if you
would like to have a go at it.

Something along these lines?

Index: dfly/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c ===================================================================
--- dfly.orig/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c 2006-12-23
01:27:02.000000000 0100
++ dfly/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c 2007-09-01
14:29:38.000000000 +0200
@ -84,6 +84,7 @
}

mp->mnt_flag |= MNT_LOCAL;
+    mp->mnt_kern_flag |= MNTK_NOSTKMNT;
     mp->mnt_data = 0;
     vfs_getnewfsid(mp);

Index: dfly/src/sys/kern/vfs_syscalls.c ===================================================================
--- dfly.orig/src/sys/kern/vfs_syscalls.c 2007-08-13 19:43:55.000000000 0200
++ dfly/src/sys/kern/vfs_syscalls.c 2007-09-01 14:20:53.000000000 0200
@ -248,6 +248,11 @
vput(vp);
return (ENOTDIR);
}
if (vp->v_mount->mnt_kern_flag & MNTK_NOSTKMNT) {
+ cache_drop(&nch);
+ vput(vp);
+ return (EPERM);
+ }
if ((error = copyinstr(uap->type, fstypename, MFSNAMELEN, NULL)) != 0) {
cache_drop(&nch);
vput(vp);
Index: dfly/src/sys/sys/mount.h ===================================================================
--- dfly.orig/src/sys/sys/mount.h 2006-10-27 06:56:33.000000000 0200
++ dfly/src/sys/sys/mount.h 2007-09-01 14:50:36.000000000 +0200
@ -243,6 +243,8 @ * dounmount() is still waiting to lock the mountpoint. This allows * the filesystem to cancel operations that might otherwise deadlock * with the unmount attempt (used by NFS).
+
+ * MNTK_NOSTKMNT prevents mounting another filesystem inside the flagged one.
*/
#define MNTK_UNMOUNTF 0x00000001 / forced unmount in progress /
#define MNTK_NCALIASED 0x00800000 / namecached aliased /
@ -250,6 +252,7 @
#define MNTK_MWAIT 0x02000000 / waiting for unmount to finish /
#define MNTK_WANTRDWR 0x04000000 / upgrade to read/write requested /
#define MNTK_FSMID 0x08000000 / getattr supports FSMIDs /
+#define MNTK_NOSTKMNT 0x10000000 / no stacked mount point allowed */ ===================================================================
--- dfly.orig/src/sys/vfs/procfs/procfs_vfsops.c 2006-12-23
01:41:30.000000000 0100
++ dfly/src/sys/vfs/procfs/procfs_vfsops.c 2007-09-01 14:23:43.000000000 +0200
@ -81,6 +81,7 @
}

mp->mnt_flag |= MNT_LOCAL;
+    mp->mnt_kern_flag |= MNTK_NOSTKMNT;
     mp->mnt_data = 0;
     vfs_getnewfsid(mp);

Index: dfly/src/lib/libc/sys/mount.2 ===================================================================
--- dfly.orig/src/lib/libc/sys/mount.2 2007-07-14 23:48:15.000000000 0200
++ dfly/src/lib/libc/sys/mount.2 2007-09-01 16:08:00.000000000 0200
@ -187,7 +187,10 @
.Bl tag -width Er
.It Bq Er EPERM
The caller is neither the super-user nor the owner of
.Ar dir .
.Ar dir ,
or
.Ar dir
+belongs to a filesystem that does not support stacked mounts.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or the entire length of a path name exceeded 1023 characters.

:Something along these lines?
:
:Index: dfly/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c
:===================================================================
:--- dfly.orig/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c 2006-12-23
:01:27:02.000000000 0100
:++ dfly/src/sys/emulation/linux/i386/linprocfs/linprocfs_vfsops.c 2007-09-01
:14:29:38.000000000 +0200
:@ -84,6 +84,7 @
: }

That looks good.  If you tested it and it works, I'd say its commitable.

-Matt

I tested it roughly this way:

mount -t procfs proc /proc -> ok
mount -t procfs proc /proc -> ko
mount -t procfs proc /proc/1 ->ko
mount -t procfs proc /mnt -> ok

Then I repeated for linprocfs.

Cheers,
Nicolas

Committed!

-Matt