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Date:   Tue, 21 Jan 2020 02:51:46 +0300
From:   "Dmitry V. Levin" <ldv@altlinux.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Eugene Syromiatnikov <esyr@redhat.com>,
        linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [PATCH] io_uring: fix compat for IORING_REGISTER_FILES_UPDATE
Message-ID: <20200120235146.GA12351@altlinux.org>
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On Wed, Jan 15, 2020 at 09:53:27AM -0700, Jens Axboe wrote:
> On 1/15/20 9:50 AM, Eugene Syromiatnikov wrote:
> > On Wed, Jan 15, 2020 at 09:41:58AM -0700, Jens Axboe wrote:
> >> On 1/15/20 9:35 AM, Eugene Syromiatnikov wrote:
> >>> fds field of struct io_uring_files_update is problematic with regards
> >>> to compat user space, as pointer size is different in 32-bit, 32-on-64-bit,
> >>> and 64-bit user space.  In order to avoid custom handling of compat in
> >>> the syscall implementation, make fds __u64 and use u64_to_user_ptr in
> >>> order to retrieve it.  Also, align the field naturally and check that
> >>> no garbage is passed there.
> >>
> >> Good point, it's an s32 pointer so won't align nicely. But how about
> >> just having it be:
> >>
> >> struct io_uring_files_update {
> >> 	__u32 offset;
> >> 	__u32 resv;
> >> 	__s32 *fds;
> >> };
> >>
> >> which should align nicely on both 32 and 64-bit?
> > 
> > The issue is that 32-bit user space would pass a 12-byte structure with
> > a 4-byte pointer in it to the 64-bit kernel, that, in turn, would treat it
> > as a 8-byte value (which might sometimes work on little-endian architectures,
> > if there are happen to be zeroes after the pointer, but will be always broken
> > on big-endian ones). __u64 is used in order to avoid special compat wrapper;
> > see, for example, __u64 usage in btrfs or BPF for similar purposes.
> 
> Ah yes, I'm an idiot, apparently not enough coffee yet. We'd need it in
> a union for this to work. I'll just go with yours, it'll work just fine.
> I will fold it in, I need to make some updates and rebase anyway.

I see the patch has missed v5.5-rc7.
Jens, please make sure a fix is merged before v5.5 is out.
Thanks,


-- 
ldv