On Tue, 2022-03-29 at 21:44 +0800, chenxiaosong (A) wrote:
> 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.
I see. OK, I'll probably not apply this for the merge window (since I'm
pretty much queued up to send the pull request at this point), but it
might go in as a bug fix in rc1.
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx
