linux-kernel - Re: [V9fs-developer] [PATCH kernel] 9p/trans

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Date:   Wed, 29 Jul 2020 08:38:22 +0200
From:   Greg Kurz <groug@...d.org>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        v9fs-developer@...ts.sourceforge.net,
        Latchesar Ionkov <lucho@...kov.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Eric Van Hensbergen <ericvh@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [V9fs-developer] [PATCH kernel] 9p/trans_fd: Check file mode at
 opening

On Wed, 29 Jul 2020 08:14:49 +0200
Dominique Martinet <asmadeus@...ewreck.org> wrote:

> Greg Kurz wrote on Tue, Jul 28, 2020:
> > > The "fd" transport layer uses 2 file descriptors passed externally
> > > and calls kernel_write()/kernel_read() on these. If files were opened
> > > without FMODE_WRITE/FMODE_READ, WARN_ON_ONCE() will fire.
> 
> There already is a fix in linux-next as a39c46067c84 ("net/9p: validate
> fds in p9_fd_open")
> 
> > > This adds file mode checking in p9_fd_open; this returns -EBADF to
> > > preserve the original behavior.
> > 
> > So this would cause open() to fail with EBADF, which might look a bit

Oops... this seems to rather end up in mount(). :)

> > weird to userspace since it didn't pass an fd... Is this to have a
> > different error than -EIO that is returned when either rfd or wfd
> > doesn't point to an open file descriptor ? If yes, why do we care ?
> 
> FWIW the solution taken just returns EIO as it would if an invalid fd
> was given, but since it did pass an fd EBADF actually makes sense to me?
> 

POSIX says:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

[EBADF]
Bad file descriptor. A file descriptor argument is out of range, refers to
no open file, or a read (write) request is made to a file that is only open
for writing (reading).

It seems that EBADF would be appropriate for both the existing and the
new error path.

> However to the second question I'm not sure I care :)
> 
> > > Found by syzkaller.
> 
> I'm starting to understand where David comment came from the other day,
> I guess it's still time to change my mind and submit to linus now I've
> had time to test it...
>