Date: Fri, 24 Apr 2015 21:39:31 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: Jilles Tjoelker <jilles@stack.nl> Cc: Julian Elischer <julian@freebsd.org>, freebsd-current@freebsd.org, John Baldwin <jhb@freebsd.org> Subject: Re: readdir/telldir/seekdir problem (i think) Message-ID: <326462676.25571625.1429925971889.JavaMail.root@uoguelph.ca> In-Reply-To: <20150424215249.GA96554@stack.nl>
next in thread | previous in thread | raw e-mail | index | archive | help
Jilles Tjoelker wrote: > On Fri, Apr 24, 2015 at 04:28:12PM -0400, John Baldwin wrote: > > Yes, this isn't at all safe. There's no guarantee whatsoever that > > the offset on the directory fd that isn't something returned by > > getdirentries has any meaning. In particular, the size of the > > directory entry in a random filesystem might be a different size > > than the structure returned by getdirentries (since it converts > > things into a FS-independent format). > > > This might work for UFS by accident, but this is probably why ZFS > > doesn't work. > > > However, this might be properly fixed by the thing that ino64 is > > doing where each directory entry returned by getdirentries gives > > you a seek offset that you _can_ directly seek to (as opposed to > > seeking to the start of the block and then walking forward N > > entries until you get an inter-block entry that is the same). > > The ino64 branch only reserves space for d_off and does not use it in > any way. This is appropriate since actually using d_off is a major > feature addition. > Well, at some point ino64 will need to define a new getdirentries(2) syscall and I believe this new syscall can have different/additional arguments. I'd suggest that the new gtedirentries(2) syscall should return a flag to indicate that the underlying file system is filling in d_off. Then the libc functions can use d_off if it it available. (They will still need to "work" at least as well as they do now if the file system doesn't support d_off. The old getdirentries(2) syscall will be returning the old/current "struct dirent" which doesn't have the field anyhow.) Another bit of fun is that the argument for seekdir()/telldir() is a long and ends up 32bits for some arches. d_off is 64bits, since that is what some file systems require. Maybe the library code can only use d_off if it is a 64bit arch and the file system is filling it in. (Or maybe the library can keep track of 32<->64bit mappings for the offsets. I haven't looked at the libc functions for a while, so I can't remember what they keep track of.) rick > A proper d_off would still be useful even if UFS's readdir keeps > masking > off the offset so a directory read always starts at the beginning of > a > 512-byte directory block, since this allows more distinct offset > values > than safely using getdirentries()'s *basep. With d_off, one outer > loop > must read at least one directory block to avoid spinning > indefinitely, > while using getdirentries()'s *basep requires reading the whole > getdirentries() buffer. > > Some Linux filesystems go further and provide a unique d_off for each > entry. > > Another idea would be to store the last d_ino instead of dd_loc into > the > struct ddloc. On seekdir(), this would seek to loc_seek as before and > skip entries until that d_ino is found, or to the start of the buffer > if > not found (and possibly return some entries again that should not be > returned, but Samba copes with that). > > -- > Jilles Tjoelker > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to > "freebsd-current-unsubscribe@freebsd.org" >
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?326462676.25571625.1429925971889.JavaMail.root>