Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for_address

On Mon, 29 Jan 2024 at 11:24, Linus Torvalds
[off-list ref] wrote:

So the patch was completely broken. Here's the one that should
actually compile (although still not actually *tested*).

Note that this fixes the d_instantiate() ordering wrt initializing the inode.

But as I look up the call chain, I see many more fundamental mistakes.

Steven - the reason you think that the VFS doesn't have documentation
is that we *do* have tons of documentation, but it's of the kind "Here
is what you should do".

It is *not* of the kind that says "You messed up and did something
else, and how do you recover from it?".

So the fundamental bug I now find is that eventfs_root_lookup() gets a
target dentry, and for some unfathomable reason it then does

        ret = simple_lookup(dir, dentry, flags);

on it. Which is *completely* broken, because what "simple_lookup()"
does is just say "oh, you didn't have a dentry of this kind before, so
clearly a lookup must be a non-existent file". Remember: this is for
'tmpfs' kinds of filesystems where the dentry cache cotnains *ALL*
files.

For the tracefs kind of filesystem, it's TOTALLY BOGUS. What the
"simple_lookup()" will do is just a plain

        d_add(dentry, NULL);

and nothing else. And guess what *that* does? It basically
instantiates a negative dentry, telling all other lookups that the
path does not exist.

So if you have two concurrent lookups, one will do that
simple_lookup(), and the other will then - depending on timing -
either see the negative dentry and return -ENOENT, or - if it comes in
a bit later - see the new inode that then later gets added by the
first lookup with d_instantiate().

See? That simple_lookup() is not just unnecessary, but it's also
actively completely WRONG. Because it instantiates a NULL pointer,
other processes that race with the lookup may now end up saying "that
file doesn't exist", even though it should.

Basically, you can't use *any* of the "simple" filesystem helpers.
Because they are all designed for that "the dentry tree is all there
is" case.

                Linus