On Wed, 2011-03-16 at 21:36 +1100, NeilBrown wrote: > Hi Trond, > > I'm currently trying to track down the cause of some very > odd behaviour in readdir in openSUSE 11.4 (2.6.37.3 based). > > I think it might be caused by xdr_copy_to_scratch not quite > behaving correctly. > > In particular, when it has to copy into the scratch buffer > it only copies 'nbytes' bytes - which sounds reasonable but > isn't. It should copy XDR_QUADLEN(nbytes) words. > > In particular, nfs3_decode_dirent contains: > > p = xdr_inline_decode(xdr, entry->len + 8); > if (unlikely(!p)) > goto out_overflow; > entry->name = (const char *) p; > p += XDR_QUADLEN(entry->len); > entry->prev_cookie = entry->cookie; > p = xdr_decode_hyper(p, &entry->cookie); > > > where the cookie needs all of those last few bytes which > we would only get by rounding nbytes up to a multiple of 4. > > > I haven't developed or tested a fix yet, but as it is clearly a bug, > I thought I would let you know before I call it a night. Hi Neil, I'm not sure I 100% agree that is a bug in xdr_copy_to_scratch(). I think it is rather an artifact of the fact that xdr_inline_decode takes a byte argument instead of a word argument, which means that combining buffer lengths needs to done with care: my fault :-(. To illustrate what I mean, consider the following snippet of xdr: p = xdr_decode_inline(xdr, len1 + len2); name1 = p; p += XDR_QUADLEN(len1); name2 = p; No matter what we do in xdr_decode_inline(), there is no way we can determine the true value of XDR_QUADLEN(len1)<<2 + XDR_QUADLEN(len2)<<2 if len1 and len2 are arbitrary buffer lengths. So I'd argue that while we could fix this issue in xdr_copy_to_scratch for this particular case, in reality nfs3_decode_dirent should not be asking for a buffer of (entry->len + 8) bytes when it knows there is an alignment issue due to the fact that the 8 cookie bytes follows the string. Cheers Trond -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@xxxxxxxxxx www.netapp.com -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
