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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe