netdev - Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import

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Message-ID: <20200923144025.GL3421308@ZenIV.linux.org.uk>
Date:   Wed, 23 Sep 2020 15:40:25 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        David Laight <David.Laight@...lab.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-aio@...ck.org, io-uring@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        netdev@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and
 import_iovec

On Wed, Sep 23, 2020 at 03:16:54PM +0100, Al Viro wrote:
> On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote:
> 
> > +struct iovec *iovec_from_user(const struct iovec __user *uvec,
> > +		unsigned long nr_segs, unsigned long fast_segs,
> 
> Hmm...  For fast_segs unsigned long had always been ridiculous
> (4G struct iovec on caller stack frame?), but that got me wondering about
> nr_segs and I wish I'd thought of that when introducing import_iovec().
> 
> The thing is, import_iovec() takes unsigned int there.  Which is fine
> (hell, the maximal value that can be accepted in 1024), except that
> we do pass unsigned long syscall argument to it in some places.
> 
> E.g. vfs_readv() quietly truncates vlen to 32 bits, and vlen can
> come unchanged through sys_readv() -> do_readv() -> vfs_readv().
> With unsigned long passed by syscall glue.
> 
> AFAICS, passing 4G+1 as the third argument to readv(2) on 64bit box
> will be quietly treated as 1 these days.  Which would be fine, except
> that before "switch {compat_,}do_readv_writev() to {compat_,}import_iovec()"
> it used to fail with -EINVAL.
> 
> Userland, BTW, describes readv(2) iovcnt as int; process_vm_readv(),
> OTOH, has these counts unsigned long from the userland POV...
> 
> I suppose we ought to switch import_iovec() to unsigned long for nr_segs ;-/
> Strictly speaking that had been a userland ABI change, even though nothing
> except regression tests checking for expected errors would've been likely
> to notice.  And it looks like no regression tests covered that one...
> 
> Linus, does that qualify for your "if no userland has noticed the change,
> it's not a breakage"?

Egads...  We have sys_readv() with unsigned long for file descriptor, since
1.3.31 when it had been introduced.  And originally it did comparison with
NR_OPEN right in sys_readv().  Then in 2.1.60 it had been switched to
fget(), which used to take unsigned long at that point.  And in 2.1.90pre1
it went unsigned int, so non-zero upper 32 bits in readv(2) first argument
ceased to cause EBADF...

Of course, libc had it as int fd all along.