Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 9 Jan 2020 16:08:16 -0800 | Subject | Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks |
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On Wed, Jan 8, 2020 at 1:34 PM Al Viro <viro@zeniv.linux.org.uk> wrote: > > The point is, we'd never followed mounts on /proc/self/cwd et.al. > I hadn't checked 2.0, but 2.1.100 ('97, before any changes from me) > is that way.
Hmm. If that's the case, maybe they should be marked implicitly as O_PATH when opened?
> Actually, scratch that - 2.0 behaves the same way > (mountpoint crossing is done in iget() there; is that Minix influence > or straight from the Lions' book?)
I don't think I ever had access to Lions' - I've _seen_ a printout of it later, and obviously maybe others did,
More likely it's from Maurice Bach: the Design of the Unix Operating System. I'm pretty sure that's where a lot of the FS layer stuff came from. Certainly the bad old buffer head interfaces, and quite likely the iget() stuff too.
> 0.10: forward traversal in iget(), back traversal in fs/namei.c:find_entry()
Whee, you _really_ went back in time.
So I did too.
And looking at that code in iget(), I doubt it came from anywhere. Christ. It's just looping over a fixed-size array, both when finding the inode, and finding the superblock.
Cute, but unbelievably stupid. It was a more innocent time.
In other words, I think you can chalk it up to just me, because blaming anybody else for that garbage would be very very unfair indeed ;)
> How would your proposal deal with access("/proc/self/fd/42/foo", MAY_READ) > vs. faccessat(42, "foo", MAY_READ)?
I think that in a perfect world, the O_PATH'ness of '42' would be the deciding factor. Wouldn't those be the best and most consistent semantics?
And then 'cwd'/'root' always have the O_PATH behavior.
> The latter would trigger automount, > the former would not... Or would you extend that to "traverse mounts > upon following procfs links, if the file in question had been opened with > O_PATH"?
Exactly.
But you know what? I do not believe this is all that important, and I doubt it will matter to anybody.
So what matters most is what makes the most sense to the VFS layer, and what makes the most sense to _you_.
Because my reaction from this thread is that not only have you thought about this issue and followed the history a whole lot more than I would ever have done, it's also that I trust you to DTRT.
I think it would be good to have some self-consistency, but at the same time clearly we already don't really, and our behavior here has subtly changed over the years (and not so subtly - if you go back sufficiently far, /proc behavior wrt file descriptors has had both "dup()" behavior and "make a new file descriptor with the same inode" behavior, afaik).
Linus
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