Re: (repost) Confirmation of methods for calculating requestedpathname.

[Alexey Dobriyan]

On Tue, Sep 02, 2008 at 01:31:50PM +0900, Kentaro Takeda wrote:
> Al, could you answer the following question?
> 
> 
> The current Linux kernel is not designed to pass vfsmount parameter
> that is crucial for pathname-based security including AppArmor and
> TOMOYO Linux, to LSM. Though both projects have been proposing
> patches to calculate pathname, none of them have been accepted as
> you know.
> 
> To find the reason for NACK, we examined past proposals and the
> threads. And we came to understand that you oppose accessing vfsmount
> inside vfs helper functions. Is our understanding correct?
> 
> If our understanding is correct, we would like to propose a new
> method that does not require modifications to vfs helper functions.
> Attached patch is a trial of this method.
> 
> vfs helper functions are surrounded by mnt_want_write() and
> mnt_drop_write() pairs which receive "struct vfsmount" parameter
> since 2.6.26. So, by remembering the absolute pathname of "struct
> vfsmount" of the moment, LSM module can calculate an absolute
> pathname of the given "struct dentry" parameter inside vfs_*
> functions, without passing "struct vfsmount" parameter to vfs_*
> functions.
> 
> This approach doesn't access vfsmount inside vfs helper functions,
> and modification of existing kernel is only in task_struct and
> mnt_want/drop_write().

I don't know what Al Viro will say, but this is incredibly ugly.
Even more ugly than previous patches.

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