Re: [PATCH] filesetup: do not ask O_RDWR for read-only workload

[Vincent Fu <vincentfu@gmail.com>]

On 2/6/23 14:37, Jens Axboe wrote:
> On 2/5/23 11:52?PM, Kanchan Joshi wrote:
>> On Fri, Feb 03, 2023 at 10:26:15AM -0500, Vincent Fu wrote:
>>> On 2/3/23 09:14, Jens Axboe wrote:
>>>> On 2/3/23 7:08?AM, Vincent Fu wrote:
>>>>> On 2/3/23 07:46, Jens Axboe wrote:
>>>>>> On Feb 3, 2023, at 5:35 AM, Kanchan Joshi <joshi.k@samsung.com> wrote:
>>>>>>>
>>>>>>> ?Use O_RDONLY flag when read is requested on char-type files.
>>>>>>>
>>>>>>> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
>>>>>>> ---
>>>>>>>
>>>>>>> Before this patch: unexpected permission-denial for unprivileged-user.
>>>>>>>
>>>>>>> $ ls -l /dev/ng0n1
>>>>>>> cr--r--r-- 1 root root 242, 0 Feb  3 16:30 /dev/ng0n1
>>>>>>>
>>>>>>> $ ./fio -iodepth=1 -rw=randread -ioengine=io_uring_cmd -bs=4k -numjobs=1
>>>>>>> -size=4k -cmd_type=nvme -filename=/dev/ng0n1 -name=t
>>>>>>> t: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T)
>>>>>>> 4096B-4096B, ioengine=io_uring_cmd, iodepth=1
>>>>>>> fio-3.33-71-g7d7a
>>>>>>> Starting 1 process
>>>>>>> fio: pid=131312, err=13/file:filesetup.c:805, func=open(/dev/ng0n1),
>>>>>>> error=Permission denied
>>>>>>>
>>>>>>> filesetup.c | 5 +----
>>>>>>> 1 file changed, 1 insertion(+), 4 deletions(-)
>>>>>>>
>>>>>>> diff --git a/filesetup.c b/filesetup.c
>>>>>>> index 1d3cc5ad..d77b8ba4 100644
>>>>>>> --- a/filesetup.c
>>>>>>> +++ b/filesetup.c
>>>>>>> @@ -768,10 +768,7 @@ open_again:
>>>>>>>          else
>>>>>>>              from_hash = file_lookup_open(f, flags);
>>>>>>>      } else if (td_read(td)) {
>>>>>>> -        if (f->filetype == FIO_TYPE_CHAR && !read_only)
>>>>>>> -            flags |= O_RDWR;
>>>>>>> -        else
>>>>>>> -            flags |= O_RDONLY;
>>>>>>> +        flags |= O_RDONLY;
>>>>>>
>>>>>> This will break sg like interfaces, where a read is done by writing the command to the char device.
>>>>>>
>>>>>> ?
>>>>>> Jens Axboe
>>>>>>
>>>>>
>>>>> Kanchan, does it work if you run fio with the --readonly option?
>>>>
>>>> I think we should just make it work. Conceptually, the patch is obviously
>>>> fine, it just happens to break some oddball cases like sg/bsg. But maybe
>>>> we just add a fio ioengine flag for that, like FIO_RO_NEEDS_RW_OPEN or
>>>> something, where the engine can tell us if it needs a writeable open
>>>> even or a read-only workload.
>>>>
>>>> Then generic_file_open() can use O_RDONLY for td_read(), except if the
>>>> engine has FIO_RO_NEEDS_RW_OPEN set.
>>>>
>>>
>>> Kanchan, can you try the diff below? It's also available at https://protect2.fireeye.com/v1/url?k=37173803-566c928b-3716b34c-74fe4860018a-aac6167af74310c3&q=1&e=3e678cf7-7d3f-487a-8a94-a9f0b41ca778&u=https%3A%2F%2Fgithub.com%2Fvincentkfu%2Ffio%2Ftree%2Frwopen
>>
>> Tried, this works, thanks!
>> The patch looks good. Seems this flag can be added to libzbc engine too.
>> That also treats the the char-type open in the same way.
> 
> Thanks for testing - I've added it for libzbc as well.
> 

The new flag only affects generic_file_open() but the libzbc ioengine 
has its own file open function. So the flag has no effect.

Moreover libzbc uses the SG_IO ioctl even for character devices. So the 
code in libzbc_open_dev() (copied from generic_file_open) that opens 
character devices with O_RDWR is actually unnecessary.

I've committed some patches to tidy things up.

Vincent