Turbo NSS --------- Glibc nss library for passwd and group. Checking out and building ------------------------- ``` $ git clone --recursive https://git.sr.ht/~motiejus/turbonss ``` Alternatively, if you forgot `--recursive`: ``` $ git submodule update --init ``` And run tests: ``` $ zig build test ``` Other commands will be documented as they are implemented. This project uses [git subtrac][git-subtrac] for managing dependencies. remarks on `id(1)` ------------------ A known implementation runs id(1) at ~250 rps sequentially on ~20k users and ~10k groups. Our target is 10k id/s. `id(1)` works as follows: - lookup user by name. - get all additional gids (an array attached to a member). - for each additional gid, get the group name. Assuming a member is in ~100 groups on average, that's 1M group lookups per second. We need to convert gid to a group index, and group index to a group gid/name quickly. Caveat: `struct group` contains an array of pointers to names of group members (`char **gr_mem`). However, `id` does not use that information, resulting in a significant read amplification. Therefore, if `argv[0] == "id"`, `getgrid(3)` will return group without the members. This speeds up `id` by about 10x on a known NSS implementation. Indices ------- The following operations need to be fast, in order of importance: 1. lookup gid -> group (this is on hot path in id) with or without members (2 separate calls). 2. lookup uid -> user. 3. lookup groupname -> group. 4. lookup username -> user. 5. (optional) iterate users using a defined order (`getent passwd`). 6. (optional) iterate groups using a defined order (`getent group`). First 4 can use perfect hashing like [cmph][cmph]: it hashes a list of bytes to a sequential list of integers. Perfect hashing algorithms require some space, and take some time to calculate ("hashing duration"). I've tested BDZ, which hashes [][]u8 to a sequential list of integers (not preserving order) and CHM, which does the same, but preserves order. BDZ accepts an argument 3 <= b <= 10. BDZ: tried b=3, b=7 (default), and b=10. * BDZ algorithm requires (900KB, 338KB, 306KB, respectively) for 1M values. * Latency to resolve 1M keys: (170ms, 180ms, 230ms). * Packed vs non-packed latency differences are not meaningful. CHM retains order, however, 1M keys weigh 8MB. 10k keys are ~20x larger with CHM than with BDZ, eliminating the benefit of preserved ordering. Turbonss header --------------- The turbonss header looks like this: ``` OFFSET TYPE NAME DESCRIPTION 0 [4]u8 magic always 0xf09fa4b7 4 u8 version now `0` 5 u16 bom 0x1234 7 u2 padding u6 num_shells see "SHELLS" section. 8 u32 num_users number of passwd entries 12 u32 num_groups number of group entries 16 u32 offset_cmph_gid2group 20 u32 offset_cmph_uid2user 24 u32 offset_cmph_groupname2group 28 u32 offset_cmph_username2user 32 u32 offset_groupmembers 36 u32 offset_additional_gids ``` `magic` is 0xf09fa4b7, and `version` must be `0`. All integers are native-endian. `bom` is a byte-order-mark. It must resolve to `0x1234` (4460). If that's not true, the file is consumed in a different endianness than it was created at. Turbonss files cannot be moved across different-endianness computers. If that happens, turbonss will refuse to read the file. Offsets are indices to further sections of the file, with zero being the first block (pointing to the `magic` field). As all blobs are 64-byte aligned, the offsets are always pointing to the beginning of an 64-byte "block". Therefore, all `offset_*` values could be `u26`. As `u32` is easier to visualize with xxd, and the header block fits to 64 bytes anyway, we are keeping them as u32 now. Primitive types: ``` const Group = struct { gid: u32, // index to a separate structure with a list of members. The memberlist is // always 2^5-byte aligned, this is an index there. members_offset: u27, groupname_len: u5, // a groupname_len-sized string groupname []u8; } const User = struct { uid: u32, gid: u32, // pointer to a separate structure that contains a list of gids additional_gids_offset: u29, // shell is a different story, documented elsewhere. shell_here: u1, shell_len_or_place: u6, home_len: u6, username_pos: u1, username_len: u5, gecos_len: u8, // a variable-sized array that will be stored immediately after this // struct. stringdata []u8; } ``` Complete file structure ----------------------- ``` OFFSET Section SIZE DESCRIPTION 0<<6 Header 1<<6 documented above *<<6 []Group num_groups * sizeof(Group) *<<6 []User num_users * sizeof(User) *<<6 []u32 num_groups * sizeof(u32) *<<6 []u32 num_users * sizeof(u32) *<<6 Shells unknown documented in "SHELLS" *<<6 cmph_gid2group unknown offset by offset_cmph_gid2group *<<6 cmph_uid2user unknown offset by offset_cmph_gid2group *<<6 cmph_groupname2group unknown offset by offset_cmph_groupname2group *<<6 cmph_username2user unknown offset by offset_cmph_username2user *<<6 groupmembers unknown list of group members for each group *<<6 additional_gids unknown list of gids (group membership) for each member ``` TODO explain: - shells - `additional_gids` - `groupmembers` [git-subtrac]: https://github.com/apenwarr/git-subtrac/ [cmph]: http://cmph.sourceforge.net/