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Date:   Wed, 15 Jan 2020 09:53:27 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Eugene Syromiatnikov <esyr@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, Jeff Moyer <jmoyer@...hat.com>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [PATCH] io_uring: fix compat for IORING_REGISTER_FILES_UPDATE

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.

-- 
Jens Axboe

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