Hi Andreas, I had looked in __ext4fs_dirhash(). Yes, it does reference the seed - and create a default if none is there at the filesystem level - but it doesn't appear to use it, in that function. hinfo is populated in the function - hash, minor-hash, seed - but it never uses the seed to manipulate the hash. Are you saying that it is at a higher level? i.e. __ext4fs_dirhash() is the *first* step, and there is further processing to get the actual hash? I did walk up the stack, but couldn't figure out. Thanks for stepping in Avi On Sun, Oct 3, 2021 at 7:43 PM Andreas Dilger <adilger@xxxxxxxxx> wrote: > > On Oct 3, 2021, at 06:47, Avi Deitcher <avi@xxxxxxxxxxxx> wrote: > > > > I can narrow down the question further. In my live sample, one of the > > entries in the tree is for a directory named "dir155". > > > > If I run "dx_hash dir155", I get: > > > > # debugfs -R "dx_hash dir155" /var/lib/file.img > > debugfs 1.46.2 (28-Feb-2021) > > Hash of dir155 is 0x16279534 (minor 0x0) > > > > If I look in the tree with "htree_dump", I get: > > > > # debugfs -R "htree_dump /testdir" /var/lib/file.img > > debugfs 1.46.2 (28-Feb-2021) > > .... > > Entry #0: Hash 0x00000000, block 1 > > Reading directory block 1, phys 6459 > > 168 0x00d11d98-b9b6b16b (16) dir155 332 0x009edafe-77de7d72 (16) dir319 > > > > That hash for dir155 does not match what dx_hash gave. If I try to > > take the code from fs/ext4/hash.c and build a small program to > > calculate the hash, I get: > > > > $ ./md4 dir155 > > MD4: d90278a1 25182ac7 a02e56be c3f30f04 > > hash: 0x25182ac6 > > minor: 0xa02e56be > > > > Clearly that isn't what is in the tree. What basic am I missing? > > One important factor is that the directory hash has an initial seed > to prevent pathological cases where the user can construct thousands > of directory entries that have a hash collision. > > Looking at the code explains this in the comment for __ext4fs_dirhash(). > The seed itself comes from sbi->s_hash_seed and is stored in the > per-directory hinfo.seed to be used when counting the filename hash. > In theory there could be a per-directory hash, but it appears to be a > constant for the whole filesystem. > > Cheers, Andreas > > > > >> On Fri, Oct 1, 2021 at 2:49 PM Avi Deitcher <avi@xxxxxxxxxxxx> wrote: > >> > >> Hi, > >> > >> I have been trying to understand the algorithm used for the "half-md4" > >> in htree-structured directories. Going through the code (and trying > >> not to get into reverse engineering), it looks like it is part of md4 > >> but not entirely? Yet any subset I take doesn't quite line up with > >> what I see in an actual sample. > >> > >> What is the algorithm it is using to turn an entry of, e.g., "file125" > >> into the appropriate hash. I did run a live sample, and try to get > >> some form of correlation between the actual md4 hash (16 bytes) of the > >> above to the actual entry (4 bytes) shown by debugfs, without much > >> luck. > >> > >> What basic thing am I missing? > >> > >> Separately, how does the seed play into it? > >> > >> Thanks > >> Avi > > > > > > > > -- > > Avi Deitcher > > avi@xxxxxxxxxxxx > > Follow me http://twitter.com/avideitcher > > Read me http://blog.atomicinc.com -- Avi Deitcher avi@xxxxxxxxxxxx Follow me http://twitter.com/avideitcher Read me http://blog.atomicinc.com
