358 lines
14 KiB
Markdown
358 lines
14 KiB
Markdown
Turbo NSS
|
|
---------
|
|
|
|
Turbonss is a plugin for GNU Name Service Switch (NSS) functionality of GNU C
|
|
Library (glibc). Turbonss implements lookup for `user` and `passwd` database
|
|
entries (i.e. system users, groups, and group memberships). It's main goal is
|
|
performance, with focus on making [`id(1)`][id] run as fast as possible.
|
|
|
|
Turbonss is optimized for reading. If the data changes in any way, the whole
|
|
file will need to be regenerated (and tooling only supports only full
|
|
generation). It was created, and best suited, for environments that have a
|
|
central user & group database which then needs to be distributed to many
|
|
servers/services.
|
|
|
|
To understand more about name service switch, start with
|
|
[`nsswitch.conf(5)`][nsswitch].
|
|
|
|
Design & constraints
|
|
--------------------
|
|
|
|
To be fast, the user/group database (later: DB) has to be small
|
|
([background][data-oriented-design]). It encodes user & group information in a
|
|
way that minimizes the DB size, and reduces jumping across the DB ("chasing
|
|
pointers and thrashing CPU cache").
|
|
|
|
To understand how this is done efficiently, let's analyze the
|
|
[`getpwnam_r(3)`][getpwnam_r] in high level. This API call accepts a username
|
|
and returns the following user information:
|
|
|
|
```
|
|
struct passwd {
|
|
char *pw_name; /* username */
|
|
char *pw_passwd; /* user password */
|
|
uid_t pw_uid; /* user ID */
|
|
gid_t pw_gid; /* group ID */
|
|
char *pw_gecos; /* user information */
|
|
char *pw_dir; /* home directory */
|
|
char *pw_shell; /* shell program */
|
|
};
|
|
```
|
|
|
|
Turbonss, among others, implements this call, and takes the following steps to
|
|
resolve a username to a `struct passwd*`:
|
|
|
|
- Open the DB (using `mmap`) and interpret it's first 40 bytes as a `struct
|
|
Header`. The header stores offsets to the sections of the file. This needs to
|
|
be done once, when the NSS library is loaded (or on the first call).
|
|
- Hash the username using a perfect hash function. Perfect hash function
|
|
returns a number `n ∈ [0,N-1]`, where N is the total number of users.
|
|
- Jump to the `n`'th location in the `idx_name2user` section (by pointer
|
|
arithmetic), which contains the index `i` to the user's information.
|
|
- Jump to the location `i` of section `Users` (again, using pointer arithmetic)
|
|
which stores the full user information.
|
|
- Decode the user information (which is all in a continuous memory block) and
|
|
return it to the caller.
|
|
|
|
In total, that's one hash for the username (~150ns), two pointer jumps within
|
|
the group file (to sections `idx_name2user` and `Users`), and, now that the
|
|
user record is found, `memcpy` for each field.
|
|
|
|
The turbonss DB file is be `mmap`-ed, making it simple to implement pointer
|
|
arithmetic and jumping across the file. This also reduces memory usage,
|
|
especially across multiple concurrent invocations of the `id` command. The
|
|
consumed heap space for each separate turbonss instance will be minimal.
|
|
|
|
Tight packing places some constraints on the underlying data:
|
|
|
|
- Maximum database size: 4GB.
|
|
- Permitted length of username and groupname: 1-32 bytes.
|
|
- Permitted length of shell and home: 1-64 bytes.
|
|
- Permitted comment ("gecos") length: 0-255 bytes.
|
|
- User name, groupname and gecos must be utf8-encoded.
|
|
|
|
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. They
|
|
work just like regular submodules, except all the refs of the submodules are in
|
|
this repository. Repeat after me: all the submodules are in this repository.
|
|
So if you have a copy of this repo, dependencies will not disappear.
|
|
|
|
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 for the same payload.
|
|
|
|
To better reason about the trade-offs, it is useful to understand how `id(1)`
|
|
is implemented, in rough terms:
|
|
- lookup user by name.
|
|
- get all additional gids (an array attached to a member).
|
|
- for each additional gid, get the group information (`struct group*`).
|
|
|
|
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
|
|
read amplification. Therefore, if `argv[0] == "id"`, our implementation of
|
|
`getgrid(3)` returns the `struct group*` without the members. This speeds up
|
|
`id` by about 10x on a known NSS implementation.
|
|
|
|
Relatedly, because `getgrid(3)` does not need the group members, the group
|
|
members are stored in a different DB sectoin, making the `Groups` section
|
|
smaller, thus more CPU-cache-friendly in the hot path.
|
|
|
|
Indices
|
|
-------
|
|
|
|
Now that we've sketched the implementation of `id(3)`, it's clearer to
|
|
understand which operations need to be fast; in order of importance:
|
|
|
|
1. lookup gid -> group info (this is on hot path in id) without members.
|
|
2. lookup uid -> user.
|
|
3. lookup groupname -> group.
|
|
4. lookup username -> user.
|
|
|
|
These indices can use perfect hashing like [bdz from cmph][cmph]: a perfect
|
|
hash 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, preserves order. BDZ accepts an
|
|
optional argument `3 <= b <= 10`.
|
|
|
|
* BDZ algorithm requires (b=3, 900KB, b=7, 338KB, b=10, 306KB) for 1M values.
|
|
* Latency to resolve 1M keys: (170ms, 180ms, 230ms, respectively).
|
|
* 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: we can just
|
|
have a separate index.
|
|
|
|
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
|
|
u8 num_shells max value: 63. Padding is strange on little endian.
|
|
8 u32 num_users number of passwd entries
|
|
12 u32 num_groups number of group entries
|
|
16 u32 offset_bdz_uid2user
|
|
20 u32 offset_bdz_groupname2group
|
|
24 u32 offset_bdz_name2user
|
|
28 u32 offset_idx offset to the first idx_ section
|
|
32 u32 offset_groups
|
|
36 u32 offset_users
|
|
40 u32 offset_groupmembers
|
|
44 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.
|
|
|
|
Sections whose lengths can be calculated do not have a corresponding `offset_*`
|
|
header field. For example, `bdz_gid2group` comes immediately after the header,
|
|
and `idx_groupname2group` comes after `idx_gid2group`, whose offset is
|
|
`offset_idx`, and size can be calculated.
|
|
|
|
`num_shells` would fit to u6; however, we would need 2 bits of padding (all
|
|
other fields are byte-aligned). If we instead do `u2` followed by `u6`, the
|
|
byte would look very unusual on a little-endian architecture. Therefore we will
|
|
just refuse loading the file if the number of shells exceeds 63.
|
|
|
|
Primitive types
|
|
---------------
|
|
|
|
`User` and `Group` entries are sorted by name, ordered by their unicode
|
|
codepoints. They are byte-aligned (8bits). All `User` and `Group` entries are
|
|
referred by their byte offset in the `Users` and `Groups` section relative to
|
|
the beginning of the section.
|
|
|
|
```
|
|
const Group = struct {
|
|
gid: u32,
|
|
// index to a separate structure with a list of members. The memberlist is
|
|
// always 2^5-byte aligned (32b), 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: bool,
|
|
shell_len_or_idx: u6,
|
|
home_len: u6,
|
|
name_is_a_suffix: bool,
|
|
name_len: u5,
|
|
gecos_len: u8,
|
|
// a variable-sized array that will be stored immediately after this
|
|
// struct.
|
|
stringdata []u8;
|
|
}
|
|
```
|
|
|
|
`stringdata` contains a few string entries:
|
|
- home.
|
|
- name.
|
|
- gecos.
|
|
- shell (optional).
|
|
|
|
First byte of the home is stored right after the `gecos_len` field, and it's
|
|
length is `home_len`. The same logic applies to all the `stringdata` fields:
|
|
there is a way to calculate their relative position from the length of the
|
|
fields before them.
|
|
|
|
Additionally, two optimizations for special fields are made:
|
|
- shells are often shared across different users, see the "Shells" section.
|
|
- name is frequently a suffix of the home. For example, `/home/motiejus`
|
|
and `motiejus`. In which case storing both name and home strings is
|
|
wasteful. For that cases, name has two options:
|
|
1. `name_is_a_suffix=true`: name is a suffix of the home dir. In that
|
|
case, the name starts at the `home_len - name_len`'th
|
|
byte of the home, and ends at the same place as the home.
|
|
2. `name_is_a_suffix=false`: name is stored separately. In that case,
|
|
it begins one byte after home, and it's length is
|
|
`name_len`.
|
|
|
|
Shells
|
|
------
|
|
|
|
Normally there is a limited number of shells even in the huge user databases. A
|
|
few examples: `/bin/bash`, `/usr/bin/nologin`, `/bin/zsh` among others.
|
|
Therefore, "shells" have an optimization: they can be pointed by in the
|
|
external list, or reside among the user's data.
|
|
|
|
63 most popular shells (i.e. referred to by at least two User entries) are
|
|
stored externally in "Shells" area. The less popular ones are stored with
|
|
userdata.
|
|
|
|
There are two "Shells" areas: the index and the blob. The index is a list of
|
|
structs which point to a location in the "blob" area:
|
|
|
|
```
|
|
const ShellIndex = struct {
|
|
offset: u10,
|
|
len: u6,
|
|
};
|
|
```
|
|
|
|
In the user's struct the `shell_here=true` bit signifies that the shell is
|
|
stored with userdata. `false` means it is stored in the `Shells` section. If
|
|
the shell is stored "here", it is the first element in `stringdata`, and it's
|
|
length is `shell_len_or_idx`. If it is stored externally, the latter variable
|
|
points to it's index in the ShellIndex area.
|
|
|
|
Shells in the external storage are sorted by their weight, which is
|
|
`length*frequency`.
|
|
|
|
Variable-length integers (varints)
|
|
----------------------------------
|
|
|
|
Varint is an efficiently encoded integer (packed for small values). Same as
|
|
[protocol buffer varints][varint], except the largest possible value is `u64`.
|
|
They compress integers well.
|
|
|
|
`groupmembers`, `additional_gids`
|
|
---------------------------------
|
|
|
|
`groupmembers` and `additional_gids` store group and user memberships
|
|
respectively: for each group, a list of pointers (offsets) to User records, and
|
|
for each user — a list of pointers to Group records. These fields are always
|
|
used in their entirety — not necessitating random access, thus suitable for
|
|
tight packing.
|
|
|
|
An entry of `groupmembers` and `additional_gids` looks like this piece of
|
|
pseudo-code:
|
|
|
|
```
|
|
const PackedList = struct {
|
|
length: varint,
|
|
members: []varint
|
|
}
|
|
const Groupmembers = PackedList;
|
|
const AdditionalGids = PackedList;
|
|
```
|
|
|
|
An entry in `members` field points to the offset into a respective `User` or
|
|
`Group` entry (number of bytes relative to the first entry of the type).
|
|
`members` in `PackedList` are sorted the same way as in the input.
|
|
|
|
A packed list is a list of varints.
|
|
|
|
Complete file structure
|
|
-----------------------
|
|
|
|
`idx_*` sections are of type `[]PackedIntArray(u29)` and are pointing to the
|
|
respective `Groups` and `Users` entries (from the beginning of the respective
|
|
section). Since User and Group records are 8-byte aligned, 3 bits are saved
|
|
from every element.
|
|
|
|
Each section is padded to 64 bytes.
|
|
|
|
```
|
|
SECTION SIZE DESCRIPTION
|
|
Header 48 see "Turbonss header" section
|
|
bdz_gid2group ? gid->group bdz
|
|
bdz_uid2user ? uid->user bdz
|
|
bdz_groupname2group ? groupname->group bdz
|
|
bdz_name2user ? username->user bdz
|
|
idx_gid2group len(group)*4*29/32 bdz->offset gid2group
|
|
idx_groupname2group len(group)*4*29/32 bdz->offset groupname2group
|
|
idx_uid2user len(user)*4*29/32 bdz->offset uid2user
|
|
idx_name2user len(user)*4*29/32 bdz->offset name2user
|
|
ShellIndex len(shells)*2 Shell index array
|
|
ShellBlob <= 4032 Shell data blob (max 63*64 bytes)
|
|
Groups ? packed Group entries (8b padding)
|
|
Users ? packed User entries (8b padding)
|
|
groupmembers ? per-group memberlist (32b padding)
|
|
additional_gids ? per-user grouplist (8b padding)
|
|
```
|
|
|
|
[git-subtrac]: https://apenwarr.ca/log/20191109
|
|
[cmph]: http://cmph.sourceforge.net/
|
|
[id]: https://linux.die.net/man/1/id
|
|
[nsswitch]: https://linux.die.net/man/5/nsswitch.conf
|
|
[data-oriented-design]: https://media.handmade-seattle.com/practical-data-oriented-design/
|
|
[getpwnam_r]: https://linux.die.net/man/3/getpwnam_r
|
|
[varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
|