356 lines
14 KiB
Markdown
356 lines
14 KiB
Markdown
Turbo NSS
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---------
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Turbonss is a plugin for GNU Name Service Switch (NSS) functionality of GNU C
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Library (glibc). Turbonss implements lookup for `user` and `passwd` database
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entries (i.e. system users, groups, and group memberships). It's main goal is
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performance, with focus on making [`id(1)`][id] run as fast as possible.
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Turbonss is optimized for reading. If the data changes in any way, the whole
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file will need to be regenerated (and tooling only supports only full
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generation). It was created, and best suited, for environments that have a
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central user & group database which then needs to be distributed to many
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servers/services.
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To understand more about name service switch, start with
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[`nsswitch.conf(5)`][nsswitch].
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Design & constraints
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--------------------
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To be fast, the user/group database (later: DB) has to be small
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([background][data-oriented-design]). It encodes user & group information in a
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way that minimizes the DB size, and reduces jumping across the DB ("chasing
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pointers and thrashing CPU cache").
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To understand how this is done efficiently, let's analyze the
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[`getpwnam_r(3)`][getpwnam_r] in high level. This API call accepts a username
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and returns the following user information:
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```
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struct passwd {
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char *pw_name; /* username */
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char *pw_passwd; /* user password */
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uid_t pw_uid; /* user ID */
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gid_t pw_gid; /* group ID */
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char *pw_gecos; /* user information */
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char *pw_dir; /* home directory */
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char *pw_shell; /* shell program */
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};
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```
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Turbonss, among others, implements this call, and takes the following steps to
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resolve a username to a `struct passwd*`:
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- Open the DB (using `mmap`) and interpret it's first 40 bytes as a `struct
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Header`. The header stores offsets to the sections of the file. This needs to
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be done once, when the NSS library is loaded (or on the first call).
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- Hash the username using a perfect hash function. Perfect hash function
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returns a number `n ∈ [0,N-1]`, where N is the total number of users.
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- Jump to the `n`'th location in the `idx_username2user` section (by pointer
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arithmetic), which contains the index `i` to the user's information.
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- Jump to the location `i` of section `Users` (again, using pointer arithmetic)
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which stores the full user information.
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- Decode the user information (which is all in a continuous memory block) and
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return it to the caller.
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In total, that's one hash for the username (~150ns), two pointer jumps within
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the group file (to sections `idx_username2user` and `Users`), and, now that the
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user record is found, `memcpy` for each field.
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The turbonss DB file is be `mmap`-ed, making it simple to implement pointer
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arithmetic and jumping across the file. This also reduces memory usage,
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especially across multiple concurrent invocations of the `id` command. The
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consumed heap space for each separate turbonss instance will be minimal.
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Tight packing places some constraints on the underlying data:
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- Maximum database size: 4GB.
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- Permitted length of username and groupname: 1-32 bytes.
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- Permitted length of shell and homedir: 1-64 bytes.
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- Permitted comment ("gecos") length: 0-255 bytes.
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- Username, groupname and gecos must be utf8-encoded.
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Checking out and building
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-------------------------
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```
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$ git clone --recursive https://git.sr.ht/~motiejus/turbonss
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```
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Alternatively, if you forgot `--recursive`:
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```
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$ git submodule update --init
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```
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And run tests:
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```
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$ zig build test
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```
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Other commands will be documented as they are implemented.
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This project uses [git subtrac][git-subtrac] for managing dependencies.
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remarks on `id(1)`
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------------------
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A known implementation runs id(1) at ~250 rps sequentially on ~20k users and
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~10k groups. Our target is 10k id/s for the same payload.
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To better reason about the trade-offs, it is useful to understand how `id(1)`
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is implemented, in rough terms:
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- lookup user by name.
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- get all additional gids (an array attached to a member).
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- for each additional gid, get the group information (`struct group*`).
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Assuming a member is in ~100 groups on average, that's 1M group lookups per
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second. We need to convert gid to a group index, and group index to a group
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gid/name quickly.
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Caveat: `struct group` contains an array of pointers to names of group members
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(`char **gr_mem`). However, `id` does not use that information, resulting in
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read amplification. Therefore, if `argv[0] == "id"`, our implementation of
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`getgrid(3)` returns the `struct group*` without the members. This speeds up
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`id` by about 10x on a known NSS implementation.
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Relatedly, because `getgrid(3)` does not need the group members, the group
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members are stored in a different DB sectoin, making the `Groups` section
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smaller, thus more CPU-cache-friendly in the hot path.
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Indices
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-------
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Now that we've sketched the implementation of `id(3)`, it's clearer to
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understand which operations need to be fast; in order of importance:
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1. lookup gid -> group info (this is on hot path in id) without members.
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2. lookup uid -> user.
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3. lookup groupname -> group.
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4. lookup username -> user.
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These indices can use perfect hashing like [cmph][cmph]: a perfect hash hashes
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a list of bytes to a sequential list of integers. Perfect hashing algorithms
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require some space, and take some time to calculate ("hashing duration"). I've
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tested BDZ, which hashes [][]u8 to a sequential list of integers (not
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preserving order) and CHM, preserves order. BDZ accepts an optional argument `3
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<= b <= 10`.
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* BDZ algorithm requires (b=3, 900KB, b=7, 338KB, b=10, 306KB) for 1M values.
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* Latency to resolve 1M keys: (170ms, 180ms, 230ms, respectively).
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* Packed vs non-packed latency differences are not meaningful.
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CHM retains order, however, 1M keys weigh 8MB. 10k keys are ~20x larger with
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CHM than with BDZ, eliminating the benefit of preserved ordering: we can just
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have a separate index.
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Turbonss header
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---------------
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The turbonss header looks like this:
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```
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OFFSET TYPE NAME DESCRIPTION
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0 [4]u8 magic always 0xf09fa4b7
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4 u8 version now `0`
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5 u16 bom 0x1234
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u8 num_shells max value: 63. Padding is strange on little endian.
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8 u32 num_users number of passwd entries
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12 u32 num_groups number of group entries
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16 u32 offset_cmph_uid2user
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20 u32 offset_cmph_groupname2group
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24 u32 offset_cmph_username2user
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28 u32 offset_idx offset to the first idx_ section
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32 u32 offset_groups
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36 u32 offset_users
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40 u32 offset_groupmembers
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44 u32 offset_additional_gids
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```
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`magic` is 0xf09fa4b7, and `version` must be `0`. All integers are
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native-endian. `bom` is a byte-order-mark. It must resolve to `0x1234` (4460).
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If that's not true, the file is consumed in a different endianness than it was
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created at. Turbonss files cannot be moved across different-endianness
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computers. If that happens, turbonss will refuse to read the file.
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Offsets are indices to further sections of the file, with zero being the first
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block (pointing to the `magic` field). As all blobs are 64-byte aligned, the
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offsets are always pointing to the beginning of an 64-byte "block". Therefore,
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all `offset_*` values could be `u26`. As `u32` is easier to visualize with xxd,
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and the header block fits to 64 bytes anyway, we are keeping them as u32 now.
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Sections whose lengths can be calculated do not have a corresponding `offset_*`
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header field. For example, `cmph_gid2group` comes immediately after the header,
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and `idx_groupname2group` comes after `idx_gid2group`, whose offset is
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`offset_idx`, and size can be calculated.
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`num_shells` would fit to u6; however, we would need 2 bits of padding (all
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other fields are byte-aligned). If we instead do `u2` followed by `u6`, the
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byte would look very unusual on a little-endian architecture. Therefore we will
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just refuse loading the file if the number of shells exceeds 63.
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Primitive types
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---------------
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`User` and `Group` entries are sorted by name, ordered by their unicode
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codepoints. They are byte-aligned (8bits). All `User` and `Group` entries are
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referred by their byte offset in the `Users` and `Groups` section relative to
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the beginning of the section.
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```
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const Group = struct {
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gid: u32,
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// index to a separate structure with a list of members. The memberlist is
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// always 2^5-byte aligned (32b), this is an index there.
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members_offset: u27,
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groupname_len: u5,
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// a groupname_len-sized string
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groupname []u8;
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}
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const User = struct {
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uid: u32,
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gid: u32,
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// pointer to a separate structure that contains a list of gids
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additional_gids_offset: u29,
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// shell is a different story, documented elsewhere.
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shell_here: u1,
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shell_len_or_place: u6,
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homedir_len: u6,
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username_is_a_suffix: u1,
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username_offset_or_len: u5,
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gecos_len: u8,
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// a variable-sized array that will be stored immediately after this
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// struct.
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stringdata []u8;
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}
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```
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`stringdata` contains a few string entries:
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- homedir.
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- username.
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- gecos.
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- shell (optional).
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First byte of the homedir is stored right after the `gecos_len` field, and it's
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length is `homedir_len`. The same logic applies to all the `stringdata` fields:
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there is a way to calculate their relative position from the length of the
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fields before them.
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Additionally, two optimizations for special fields are made:
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- shells are often shared across different users, see the "Shells" section.
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- username is frequently a suffix of the homedir. For example, `/home/motiejus`
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and `motiejus`. In which case storing both username and homedir strings is
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wasteful. For that cases, username has two options:
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1. `username_is_a_suffix=true`: username is a suffix of the home dir. In that
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case, the username starts at the `username_offset_or_len`'th byte of the
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homedir, and ends at the same place as the homedir.
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2. `username_is_a_suffix=false`: username is stored separately. In that case,
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it begins one byte after homedir, and it's length is
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`username_offset_or_len`.
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Shells
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------
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Normally there is a limited number of shells even in the huge user databases. A
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few examples: `/bin/bash`, `/usr/bin/nologin`, `/bin/zsh` among others.
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Therefore, "shells" have an optimization: they can be pointed by in the
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external list, or reside among the user's data.
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63 most popular shells (i.e. referred to by at least two User entries) are
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stored externally in "Shells" area. The less popular ones are stored with
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userdata.
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There are two "Shells" areas: the index and the blob. The index is a list of
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structs which point to a location in the "blob" area:
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```
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const ShellIndex = struct {
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offset: u10,
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len: u6,
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};
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```
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In the user's struct the `shell_here=true` bit signifies that the shell is
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stored with userdata. `false` means it is stored in the `Shells` section. If
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the shell is stored "here", it is the first element in `stringdata`, and it's
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length is `shell_len_or_place`. If it is stored externally, the latter variable
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points to it's index in the ShellIndex area.
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Shells in the external storage are sorted by their weight, which is
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`length*frequency`.
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Variable-length integers (varints)
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----------------------------------
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Varint is an efficiently encoded integer (packed for small values). Same as
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[protocol buffer varints][varint], except the largest possible value is `u64`.
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They compress integers well.
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`groupmembers`, `additional_gids`
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---------------------------------
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`groupmembers` and `additional_gids` store group and user memberships
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respectively: for each group, a list of pointers (offsets) to User records, and
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for each user — a list of pointers to Group records. These fields are always
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used in their entirety — not necessitating random access, thus suitable for
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tight packing.
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An entry of `groupmembers` and `additional_gids` looks like this piece of
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pseudo-code:
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```
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const PackedList = struct {
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length: varint,
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members: []varint
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}
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const Groupmembers = PackedList;
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const AdditionalGids = PackedList;
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```
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An entry in `members` field points to the offset into a respective `User` or
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`Group` entry (number of bytes relative to the first entry of the type).
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`members` in `PackedList` is sorted by the name (`username` or `groupname`) of
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the record it is pointing to.
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A packed list is a list of varints.
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Complete file structure
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-----------------------
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`idx_*` sections are of type `[]PackedIntArray(u29)` and are pointing to the
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respective `Groups` and `Users` entries (from the beginning of the respective
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section). Since User and Group records are 8-byte aligned, 3 bits are saved
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from every element.
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Each section is padded to 64 bytes.
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```
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SECTION SIZE DESCRIPTION
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Header 48 see "Turbonss header" section
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cmph_gid2group ? gid->group cmph
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cmph_uid2user ? uid->user cmph
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cmph_groupname2group ? groupname->group cmph
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cmph_username2user ? username->user cmph
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idx_gid2group len(group)*4*29/32 cmph->offset gid2group
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idx_groupname2group len(group)*4*29/32 cmph->offset groupname2group
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idx_uid2user len(user)*4*29/32 cmph->offset uid2user
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idx_username2user len(user)*4*29/32 cmph->offset username2user
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ShellIndex len(shells)*2 Shell index array
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ShellBlob <= 4032 Shell data blob (max 63*64 bytes)
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Groups ? packed Group entries (8b padding)
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Users ? packed User entries (8b padding)
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groupmembers ? per-group memberlist (32b padding)
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additional_gids ? per-user grouplist (8b padding)
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```
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[git-subtrac]: https://github.com/apenwarr/git-subtrac/
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[cmph]: http://cmph.sourceforge.net/
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[id]: https://linux.die.net/man/1/id
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[nsswitch]: https://linux.die.net/man/5/nsswitch.conf
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[data-oriented-design]: https://media.handmade-seattle.com/practical-data-oriented-design/
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[getpwnam_r]: https://linux.die.net/man/3/getpwnam_r
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[varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
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