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Message-ID: <CAGW2f1HnA2tcShZZaUd+dP2vwPyT0QemeuiwK4UGq7hVc3icyw@mail.gmail.com> Date: Thu, 5 Sep 2013 01:25:46 -0400 From: jon ernst <jonernst07@...il.com> To: Felipe Monteiro de Carvalho <felipemonteiro.carvalho@...il.com>, "Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org Subject: Re: Which features should I implement in my ext4 reader? On Wed, Sep 4, 2013 at 1:35 AM, Zheng Liu <gnehzuil.liu@...il.com> wrote: > > On Mon, Sep 02, 2013 at 02:35:31PM +0200, Felipe Monteiro de Carvalho wrote: > > Hello, > > > > Ok, I think I got it almost finished =) Just 1 last wierd thing. I > > dumped my inode and I found that when the extended attribute is in the > > space after the inode, then the attribute data starts 3 bytes before > > the end of this area! But in the wiki nothing is said about this. It > > was supposed to start at the end of the section ... not 3 bytes before > > the end of the section > > > > Here are the 3 zeroes in the end: > > > > 7760 4444 4444 4444 4444 4444 4444 0a00 0000 DDDDDDDDDDDD.... > > > > Any idea what are those 3 zeroes in the end? I simply hardcoded in my > > application that there exists 3 zeroes in the end, but its really > > strange as this does not make any sense to waste 3 bytes and I didn't > > see any such a thing in the wiki nor the xattr.h header... > > > > Also wierd: This 3 last bytes do no appear when extended attributes > > are stored in a separate block. > > Could you please tell me how to reproduce this problem? I try to create > a file with inline data, but I couldn't reproduce your problem. > > Thanks, > - Zheng > I did reproduce this one. what I did is: use pu branch e2fsprogs to make ext4 image, mount it, cat "AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDDDDDDDD" to a file. Then use debugfs to check corresponding block. I got almost same result as Felipe's. kernel 3.8.0-27 But I don't quite understand why does Felipe suspect those padding 0s? What's wrong with it? Pardon my silly question. Jon > > > > > Full set of dumps: > > > > debugfs: stat smallfile120.txt > > Inode: 16 Type: regular Mode: 0644 Flags: 0x10000000 > > Generation: 230499867 Version: 0x00000000:00000001 > > User: 0 Group: 0 Size: 121 > > File ACL: 0 Directory ACL: 0 > > Links: 1 Blockcount: 0 > > Fragment: Address: 0 Number: 0 Size: 0 > > ctime: 0x52149464:04e2df1c -- Wed Aug 21 06:20:20 2013 > > atime: 0x52177658:c0a4e548 -- Fri Aug 23 10:48:56 2013 > > mtime: 0x52149464:04e2df1c -- Wed Aug 21 06:20:20 2013 > > crtime: 0x52149464:04e2df1c -- Wed Aug 21 06:20:20 2013 > > Size of extra inode fields: 28 > > Extended attributes stored in inode body: > > data = "CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD\012" (61) > > INLINE DATA: > > The size of inline data: 121 > > > > More detailed information about the file with extended attribute: > > > > debugfs: imap smallfile120.txt > > Inode 16 is part of block group 0 > > located at block 65, offset 0x0f00 > > > > So we know that it is located in the block 0000065 with offset 0x0f00 > > = 0740 octal > > > > debugfs: block_dump 65 > > ..... > > * > > 7400 a481 0000 7900 0000 5876 1752 6494 1452 ....y...Xv.Rd..R > > 7420 6494 1452 0000 0000 0000 0100 0000 0000 d..R............ > > 7440 0000 0010 0100 0000 4141 4141 4141 4141 ........AAAAAAAA > > 7460 4141 4141 4141 4141 4141 4141 4141 4141 AAAAAAAAAAAAAAAA > > 7500 4141 4141 4141 4242 4242 4242 4242 4242 AAAAAABBBBBBBBBB > > 7520 4242 4242 4242 4242 4242 4242 4242 4242 BBBBBBBBBBBBBBBB > > 7540 4242 4242 1b26 bd0d 0000 0000 0000 0000 BBBB.&.......... > > 7560 0000 0000 0000 0000 0000 0000 0000 0000 ................ > > 7600 1c00 0000 1cdf e204 1cdf e204 48e5 a4c0 ............H... > > 7620 6494 1452 1cdf e204 0000 0000 0000 02ea d..R............ > > 7640 0407 2000 0000 0000 3d00 0000 0000 0000 .. .....=....... > > 7660 6461 7461 0000 0000 0000 0000 0000 0000 data............ > > 7700 4343 4343 4343 4343 4343 4343 4343 4343 CCCCCCCCCCCCCCCC > > 7720 4343 4343 4343 4343 4343 4343 4343 4444 CCCCCCCCCCCCCCDD > > 7740 4444 4444 4444 4444 4444 4444 4444 4444 DDDDDDDDDDDDDDDD > > 7760 4444 4444 4444 4444 4444 4444 0a00 0000 DDDDDDDDDDDD.... > > > > And thats how I got to this place. > > > > thanks, > > > > Felipe Monteiro de Carvalho > > > > On Mon, Sep 2, 2013 at 2:07 PM, Zheng Liu <gnehzuil.liu@...il.com> wrote: > > > Hello, > > > > > > Sorry for my late reply. > > > > > > On Mon, Sep 02, 2013 at 11:40:15AM +0200, Felipe Monteiro de Carvalho wrote: > > >> Hello, > > >> > > >> Ok, I found the extra data over 60 bytes ... it is in an extended > > >> attribute called "data" in the space after the inode. The wiki says it > > >> should be called "system.data" =( > > >> > > >> Also the wiki says here > > >> https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Extended_Attributes > > >> that "Extended attributes, when >>>not<<<< stored after the inode, > > >> have a header ext4_xattr_ibody_header that is 4 bytes long:" > > >> > > >> Which in my oppinion is false, it should be: "Extended attributes, > > >> when stored after the inode, have a header ext4_xattr_ibody_header > > >> that is 4 bytes long:" without the "not" > > >> > > >> But I still am not sure of one thing: How can I know how many entries > > >> of extended attributes there are? Nothing in the wiki explains this =( > > > > > > You need to traverse the all entries to get this value. > > > > > >> > > >> Otherwise I cannot differentiate between yet another entry and simply > > >> data from an extended attribute... > > > > > > You can use attribute name and name index to differentiate these > > > entries. You can read the comment at the front of fs/ext4/xattr.c and > > > it might help you to understand extended attribute. > > > > > > Regards, > > > - Zheng > > > > > > > > -- > > Felipe Monteiro de Carvalho > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > > the body of a message to majordomo@...r.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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