getdirentries cookies usage outside of UFS

[Kirk McKusick]

> Date: Fri, 11 Apr 2014 21:03:57 -0500
> From: Bryan Drewery <bdrewery at freebsd.org>
> To: freebsd-fs at freebsd.org
> Subject: getdirentries cookies usage outside of UFS
> 
> Recently I was working on a compat syscall for sys_getdirentries() that
> converts between our dirent and the FreeBSD dirent struct. We had never
> tried using this on TMPFS and when we did ran into weird issues (hence
> my recent commits to TMPFS to clarify some of the getdirentries() code).
> We were not using cookies, so I referenced the Linux compat module
> (linux_file.c getdents_common())
> 
> I ran across this code:
> 
>>     /*
>>      * When using cookies, the vfs has the option of reading from
>>      * a different offset than that supplied (UFS truncates the
>>      * offset to a block boundary to make sure that it never reads
>>      * partway through a directory entry, even if the directory
>>      * has been compacted).
>>      */
>>     while (len > 0 && ncookies > 0 && *cookiep <= off) {
>>             bdp = (struct dirent *) inp;
>>             len -= bdp->d_reclen;
>>             inp += bdp->d_reclen;
>>             cookiep++;
>>             ncookies--;
>>     }=20
> 
> 
> At first it looked innocuous but then it occurred to me it was the root
> of the issue I was having as it was eating my cookies based on their
> value, despite tmpfs cookies being random hash values that have no
> sequential relation. So I looked at how NFS was handling the same code
> and found this lovely hack from r216691:
> 
>> not_zfs = strcmp(vp->v_mount->mnt_vfc->vfc_name, "zfs");
> =2E..
>>  while (cpos < cend && ncookies > 0 &&
>>      (dp->d_fileno == 0 || dp->d_type == DT_WHT ||
>>       (not_zfs != 0 && ((u_quad_t)(*cookiep)) <= toff))) {
>>          cpos += dp->d_reclen;
>>          dp = (struct dirent *)cpos;
>>          cookiep++;
>>          ncookies--;
>>  }
> 
> I ended up doing the opposite, only running the code if getting dirents
> from "ufs".
> 
> So there's multiple issue here.
> 
> 1. NFS is broken on TMPFS. I can see why it's gone so long unnoticed,
> why would you do that? Still probably worth fixing.
> 
> 2. Linux and SVR4 getdirentries() are both broken on TMPFS/ZFS. I am
> surprised Linux+ZFS has not been noticed by now. I am aware the SVR4 is
> full of other bugs too. I ran across many just reviewing the
> getdirentries code alone.
> 
> Do any other file systems besides UFS do this offset/cookie
> truncation/rewind? If UFS is the only one it may be acceptable to change
> this zfs check to !ufs and add it to the other modules. If we don't like
> that, or there are potentially other file systems doing this too, how
> about adding a flag to somewhere to indicate the file system has
> monotonically increasing offsets and needs this rewind support. I'm not
> sure where that is best done, struct vfsconf?
> 
> Regards,
> Bryan Drewery

This code is specific to UFS. I concur with your fix of making
it conditionl on UFS. I feel guilty for putting that code in
unconditionally in the first place. In my defense it was 1982
and UFS was the only filesystem :-)

	Kirk McKusick