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

[Kanchan Joshi <joshi.k@samsung.com>]

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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.

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