[9fans] ramfs bug

[Richard Miller <9fans@hamnavoe.com>]

Deleting an open file on ramfs(4) can have surpising results.

Here's how it's supposed to behave. When a file is removed, the
Ram struct which represents the in-memory file has its busy flag
reset, so a subsequent i/o will see that it's been deleted.

cpu% ramfs
cpu% echo this is x >/tmp/x
cpu% { sleep 10; cat } </tmp/x &
cpu% rm /tmp/x
cpu% cat: error reading <stdin>: file does not exist

However, before the i/o takes place, it's possible for another
file to be created and re-use the same Ram struct, setting
the busy flag again. The process which had the deleted file open
is now doing i/o to a different file, without any checks on its
permissions, exclusive use flag, etc.

cpu% ramfs
cpu% echo this is x >/tmp/x
cpu% { sleep 10; cat } </tmp/x &
cpu% rm /tmp/x; echo actually this is y >/tmp/y
cpu% actually this is y

A simple fix is to add a reference count to the Ram struct, to
keep track of how many Fids point to it, and only allow a Ram to
be re-used if both busy==0 and ref==0. The new 9legacy patch
ramfs-refcount makes this change.

Note to 9front users: don't worry, your ramfs is a completely
different program using the 9p(2) library, which doesn't have
this flaw.


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