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

[chenxiaosong (A)]

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.