On Wed, Jan 8, 2020 at 1:34 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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
