linux-kernel - Re: Bisected regression: iterate_fd() selinux change affects flash plugin

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Date:	Fri, 16 Nov 2012 14:58:46 -0500
From:	Eric Paris <eparis@...isplace.org>
To:	Pavel Roskin <proski@....org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	SE-Linux <selinux@...ho.nsa.gov>
Subject: Re: Bisected regression: iterate_fd() selinux change affects flash plugin

On Mon, Nov 12, 2012 at 11:57 AM, Pavel Roskin <proski@....org> wrote:
> Quoting Eric Paris <eparis@...isplace.org>:
>
>> OMG this +1 -1 stuff is nuts...

Ping, Al.

int iterate_fd(struct files_struct *files, unsigned n,
[snip]
        while (!res && n < fdt->max_fds) {
                file = rcu_dereference_check_fdtable(files, fdt->fd[n++]);
                if (file)
                        res = f(p, file, n);
        }
        spin_unlock(&files->file_lock);
        return res;

So we increment n (the file descriptor number) in the dereference,
then pass that (wrong) number to f().

Every single f() (including SELinux, the cause of this bug) returns
fd+1 (so now we are up by 2).  Then all of the users of iterate fd
actually use fd-1 (which is wrong)

Why not have iterate_fd return -ENOENT on no entries and stop all of
the stupid games?  We fix the real bug (the above function should do
the n++ after the f() call, and the interface is sane to design
against...

-Eric
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