Re: [PATCH] NFS: allow name_to_handle_at() to work for Amazon EFS.

[Amir Goldstein]

On Thu, Dec 7, 2017 at 5:20 AM, NeilBrown <neilb@xxxxxxxx> wrote:
> On Wed, Dec 06 2017, Linus Torvalds wrote:
>
>> On Thu, Nov 30, 2017 at 12:56 PM, NeilBrown <neilb@xxxxxxxx> wrote:
>>>
>>> -/* limit the handle size to NFSv4 handle size now */
>>> -#define MAX_HANDLE_SZ 128
>>> +/* Must be larger than NFSv4 file handle, but small
>>> + * enough for an on-stack allocation. overlayfs doesn't
>>> + * want this too close to 255.
>>> + */
>>> +#define MAX_HANDLE_SZ 200
>>
>> This really smells for so many reasons.
>>
>> Also, that really is starting to be a fairly big stack allocation, and
>> it seems to be used in exactly one place (show_mark_fhandle), which
>> makes me go "why is that on the stack anyway?".
>>
>> Could we just allocate a buffer at open time or something?
>>
>>                Linus
>
> "open time" would be when /proc/X/fdinfo/Y was opened in
> seq_fdinfo_open(), and allocating a file_handle there seems a bit odd.
>
> We can allocate in fs/notify/fdinfo.c:show_fdinfo() which is
> the earliest 'notify' specific code to run.  There is no
> opportunity to return an error but GFP_KERNEL allocations under 1 page
> never fail..
>
> This patch allocates a single buffer for all inodes reported for a given
> inotify fdinfo, and if the allocation files, the filehandle is silently
> left blank.  More surgery would be needed to be able to return an error.
>
> Is that at all suitable?
>
> Thanks,
> NeilBrown
>
> From: NeilBrown <neilb@xxxxxxxx>
> Subject: fs/notify: don't put file handle buffer on stack.
>
> A file handle buffer is not tiny, and could need to be larger in future,
> so it isn't safe to allocate one on the stack.  Instead, we need to
> kmalloc().
>
> There is no way to return an error status from a ->show_fdinfo()
> function, so if the kmalloc fails, we silently exclude the filehandle
> from the output.  As it is at the end of line, this shouldn't
> upset parsing too much.

It shouldn't upset parsing because that would be the same out
output as without CONFIG_EXPORTFS. AFAIK this information
is used by CRUI.

>
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
>
> diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
> index d478629c728b..20d863b9ae16 100644
> --- a/fs/notify/fdinfo.c
> +++ b/fs/notify/fdinfo.c
> @@ -23,56 +23,58 @@
>
>  static void show_fdinfo(struct seq_file *m, struct file *f,
>                         void (*show)(struct seq_file *m,
> -                                    struct fsnotify_mark *mark))
> +                                    struct fsnotify_mark *mark,
> +                                    struct fid *fh))
>  {
>         struct fsnotify_group *group = f->private_data;
>         struct fsnotify_mark *mark;
> +       struct fid *fh = kmalloc(MAX_HANDLE_SZ, GFP_KERNEL);
>
>         mutex_lock(&group->mark_mutex);
>         list_for_each_entry(mark, &group->marks_list, g_list) {
> -               show(m, mark);
> +               show(m, mark, fh);
>                 if (seq_has_overflowed(m))
>                         break;
>         }
>         mutex_unlock(&group->mark_mutex);
> +       kfree(fh);
>  }
>
>  #if defined(CONFIG_EXPORTFS)
> -static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
> +static void show_mark_fhandle(struct seq_file *m, struct inode *inode,
> +                             struct fid *fhbuf)
>  {
> -       struct {
> -               struct file_handle handle;
> -               u8 pad[MAX_HANDLE_SZ];
> -       } f;
>         int size, ret, i;
> +       unsigned char *bytes;
>
> -       f.handle.handle_bytes = sizeof(f.pad);
> -       size = f.handle.handle_bytes >> 2;
> +       if (!fhbuf)
> +               return;
> +       size = MAX_HANDLE_SZ >> 2;
>
> -       ret = exportfs_encode_inode_fh(inode, (struct fid *)f.handle.f_handle, &size, 0);
> +       ret = exportfs_encode_inode_fh(inode, fhbuf, &size, 0);
>         if ((ret == FILEID_INVALID) || (ret < 0)) {
>                 WARN_ONCE(1, "Can't encode file handler for inotify: %d\n", ret);

This WARN_ONCE is out of order. It is perfectly valid for inotify/fanotify
to watch over fs that doesn't support exportfs. Care to clean it up?
Perhaps a pr_warn_ratelimited() for either !fhbuf or can't encode?

Cheers,
Amir.