Re: [PATCH -next 2/2] NFSv4: fix open failure with O_ACCMODE flag

["chenxiaosong (A)" <chenxiaosong2@huawei.com>]

在 2022/3/29 21:05, Trond Myklebust 写道:
> No. This will not fit the definition of open(2) in the manpage.
> 
>         Linux reserves the special, nonstandard access mode 3  (binary  11)  in
>         flags  to mean: check for read and write permission on the file and re‐
>         turn a file descriptor that can't be used for reading or writing.  This
>         nonstandard  access mode is used by some Linux drivers to return a file
>         descriptor that is to be used only for device-specific ioctl(2)  opera‐
>         tions.
 > Your patch will now cause FMODE_READ and FMODE_WRITE to be set on the
 > file, allowing the file descriptor to be usable for I/O.

Reproducer:
```
   1. mount -t nfs -o vers=4.2 $server_ip:/ /mnt/
   2. fd = open("/mnt/file", O_ACCMODE|O_DIRECT|O_CREAT) = 3
   3. close(fd)
   4. fd = open("/mnt/file", O_ACCMODE|O_DIRECT) = -1
```

When firstly open with O_ACCMODE|O_DIRECT flags:
```c
   path_openat
     open_last_lookups
       lookup_open
         atomic_open
           nfs_atomic_open
             create_nfs_open_context
               f_mode = flags_to_mode
               alloc_nfs_open_context(..., f_mode, ...)
                 ctx->mode = f_mode // FMODE_READ|FMODE_WRITE
```

When secondly open with O_ACCMODE|O_DIRECT flags:
```c
   path_openat
     do_open
       vfs_open
         do_dentry_open
           nfs4_file_open
             f_mode = filp->f_mode | flags_to_mode(openflags)
             alloc_nfs_open_context(..., f_mode, ...)
               ctx->mode = f_mode // FMODE_READ|FMODE_WRITE
```

Before merging this patch, when firstly open, we does not set FMODE_READ 
and FMODE_WRITE to file mode of client, FMODE_READ and FMODE_WRITE just 
be set to context mode.

After merging this patch, when secondly open, I just do the same thing, 
file mode of client will not have FMODE_READ and FMODE_WRITE bits, file 
descriptor can't be used for reading or writing.