>On Tue, Apr 26, 2005 at 03:00:10AM -0700, Andrew Morton wrote:
>> Not as thick as mine! Could someone please explain in small words
what's
>> wrong with an suid mount helper?
>
>Nothing per-se. What makes it bad is the context of a userland
filesystem
>where the actual filesystem operations in the mounted filesystem happen
>in context of a non-privileged user.
How did the fact that the file access system calls involve user-controlled
code come into this? I thought the FUSE kernel code already shielded the
system from said code to everyone's satisfaction.
We've been talking, rather, about the namespace changes. The exact same
issue exists with a non-userspace filesystem where the user controls the
filesystem contents. For example, a filesystem on a user-supplied CD. A
system administrator -- personally or through his setuid proxy -- might
want to mount this CD for the benefit of some users/processes/whatever but
not add it to the global namespace.
The issue of private mounts (mount = namespace change) would be good to
resolve separately from any problem with bringing user space code into the
kernel.
BTW, since Miklos said "mount helper" and others have said "mount
wrapper," I think some of us may not be familiar with mount helpers. It's
irrelevant to this discussion, but: util-linux 'mount' has a little-known
feature wherein it can run a filesystem-type-specific program in a child
process to do some of the mount function. A "mount wrapper" would be the
opposite -- a filesystem-type-specific program that runs the generic
'mount' program in a child process.
--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems
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