linux-kernel - Re: [fuse-devel] FS block count, size and seek offset?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87645lr69g.fsf@informatik.uni-tuebingen.de>
Date:	Mon, 18 Jun 2007 10:55:39 +0200
From:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
To:	"David Brown" <dmlb2000@...il.com>
Cc:	fuse-devel@...ts.sourceforge.net,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [fuse-devel] FS block count, size and seek offset?

"David Brown" <dmlb2000@...il.com> writes:

> I was looking at various file systems and how they return
> stat.st_blocks and stat.st_size for directories and had some questions
> on how a fuse filesystem is supposed to implement readdir with the
> seek offset when trying to union two directories together.
>
> Is the offset a byte offset into the DIR *dp? or is it the struct
> dirent size (which is relative based on the name of the file) into the
> dir pointer?

I think it is totaly your call what you store in it. Only requirement
is that you never pass 0 to the filler function.

off_t is 64bit so storing DIR* in it should be no problem. Or a
pointer to

struct UnionDIR {
  DIR *dp;
  struct UnionDIR *next;
}

> Also, if you want to be accurate when you stat a directory that's
> unioned in the fuse file system how many blocks should one return?
> Since each filesystem seems to return different values for size and
> number of blocks for directories. I know I could just say that its not
> supported with my filesystem built using fuse... but I'd like to at
> least try to be accurate.

You could add them up and round it to some common block size (if they
differ). But I don't think it maters and nothing uses that info.

What is more important is the link count. For example find uses the
link count to know how many subdirs a directory has. Once it found
that many it assumes there are no more dirs and saves on stat() calls.

> Is it accurate to assume that the size or number of blocks returned
> from a stat will be used to pass a seek offset?
>
> When does fuse use the seek offset?

Afaik never. The offset is only stored for the next readdir call but
never used inside fuse.

> These are the number of blocks and size on an empty dir.
> ext3
> size 4096 nblocks 8
> reiserfs
> size 48 nblocks 0
> jfs
> size 1 nblocks 0
> xfs
> size 6 nblocks 0
>
> Any help to figure out how to union two directories and return correct
> values would be helpful.

Why don't you use the existing fuse-unionfs?

> Thanks,
> - David Brown
>
> P.S. maybe a posix filesystem interface manual would be good?

MfG
        Goswin
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/