Re: silent semantic changes with reiser4

[Linus Torvalds]

On Thu, 26 Aug 2004, Jamie Lokier wrote:
>
>     run_setuid_program
> 
>         -> calls pwd
>            pwd opens("."), (".."), ("../..") etc.

Ehh.. Not only does pwd not do that..

(hint: there's a getcwd() system call)

>         -> the setuid program thus ends up opening a device or fifo,
>            when it does pwd's path walk.  Yes it could use the getcwd
>            syscall, but some programs do their own path walk.

.. but even if it did that, it should use O_DIRECTORY when it did so. If 
it doesn't, it's broken.

So no, it would _not_ open the device or fifo when it did so.

The fact is, anything that expects to open a directory should already be 
opening it with O_DIRECTORY.

That said, ".." and "." are special already inside the kernel, and it 
migth be worth making them automatically imply O_DIRECTORY, since nothing 
else makes sense anyway. That would fix the case where somebody uses ".." 
_without_ using O_DIRECTORY. 

> It also fits the container idea very well:
> 
>         /dev/hda/part1/ <- the filesystem inside partition 1

I don't think you can do that. The kernel has no idea how to mount the
filesystem.

If it's already mounted somewhere else, that's a different issue.  
Although it might be mounted in several places (as a bind mount) with
different writability, I guess, so even then it might be "interesting".

		Linus
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