xref: /bionic/README.md

# bionic maintainer overview

[bionic](https://en.wikipedia.org/wiki/Bionic_(software)) is Android's
C library, math library, and dynamic linker.

This document is a high-level overview of making changes to bionic itself.
If you're trying to _use_ bionic, or want more in-depth information about
some part of the implementation, see [all the bionic documentation](docs/).

## What are the big pieces of bionic?

#### libc/ --- libc.so, libc.a

The C library. Stuff like `fopen(3)` and `kill(2)`.

#### libm/ --- libm.so, libm.a

The math library. Traditionally Unix systems kept stuff like `sin(3)` and
`cos(3)` in a separate library to save space in the days before shared
libraries.

#### libdl/ --- libdl.so

The dynamic linker interface library. This is actually just a bunch of stubs
that the dynamic linker replaces with pointers to its own implementation at
runtime. This is where stuff like `dlopen(3)` lives.

#### libstdc++/ --- libstdc++.so

The C++ ABI support functions. The C++ compiler doesn't know how to implement
thread-safe static initialization and the like, so it just calls functions that
are supplied by the system. Stuff like `__cxa_guard_acquire` and
`__cxa_pure_virtual` live here.

#### linker/ --- /system/bin/linker and /system/bin/linker64

The dynamic linker. When you run a dynamically-linked executable, its ELF file
has a `DT_INTERP` entry that says "use the following program to start me".  On
Android, that's either `linker` or `linker64` (depending on whether it's a
32-bit or 64-bit executable). It's responsible for loading the ELF executable
into memory and resolving references to symbols (so that when your code tries to
jump to `fopen(3)`, say, it lands in the right place).

#### tests/ --- unit tests

The `tests/` directory contains unit tests. Roughly arranged as one file per
publicly-exported header file. `tests/headers/` contains compile-only tests
that just check that things are _in_ the headers, whereas the "real" tests
check actual _behavior_.

#### benchmarks/ --- benchmarks

The `benchmarks/` directory contains benchmarks, with its own [documentation](benchmarks/README.md).


## What's in libc/?

```
libc/
  arch-arm/
  arch-arm64/
  arch-common/
  arch-x86/
  arch-x86_64/
    # Each architecture has its own subdirectory for stuff that isn't shared
    # because it's architecture-specific. There will be a .mk file in here that
    # drags in all the architecture-specific files.
    bionic/
      # Every architecture needs a handful of machine-specific assembler files.
      # They live here.
    string/
      # Most architectures have a handful of optional assembler files
      # implementing optimized versions of various routines. The <string.h>
      # functions are particular favorites.
    syscalls/
      # The syscalls directories contain script-generated assembler files.
      # See 'Adding system calls' later.

  include/
    # The public header files on everyone's include path. These are a mixture of
    # files written by us and files taken from BSD.

  kernel/
    # The kernel uapi header files. The "libc" headers that developers actually
    # use are a mixture of headers provided by the C library itself (which,
    # for bionic, are in bionic/libc/include/) and headers provided by the
    # kernel. This is because ISO C and POSIX will say things like "there is
    # a constant called PROT_NONE" or "there is a type called struct stat,
    # and it contains a field called st_size", but they won't necessarily say
    # what _value_ that constant has, or what _order_ the fields in a type
    # are in. Those are left to individual kernels' ABIs. In an effort to --
    # amongst other things, see https://lwn.net/Articles/507794/ for more
    # background -- reduce copy & paste, the Linux kernel makes all the types
    # and constants that make up the "userspace API" (uapi) available as
    # headers separate from their internal-use headers (which contain all kinds
    # of extra stuff that isn't available to userspace). We import the latest
    # released kernel's uapi headers in external/kernel-headers/, but we don't
    # use those headers directly in bionic. The bionic/libc/kernel/ directory
    # contains scrubbed copies of the originals from external/kernel-headers/.
    # The generate_uapi_headers.sh script should be used to go from a kernel
    # tree to external/kernel-headers/ --- this takes care of the
    # architecture-specific details. The update_all.py script should then be
    # used to regenerate bionic's copy from external/kernel-headers/.
    # The files in bionic must not be edited directly because any local changes
    # will be overwritten by the next update. "Updating kernel header files"
    # below has more information on this process.

  private/
    # These are private header files meant for use within bionic itself.

  dns/
    # Contains the DNS resolver (originates from NetBSD code).

  upstream-freebsd/
  upstream-netbsd/
  upstream-openbsd/
    # These directories contain upstream source with no local changes.
    # Any time we can just use a BSD implementation of something unmodified,
    # we should. Ideally these should probably have been three separate git
    # projects in external/, but they're here instead mostly by historical
    # accident (because it wouldn't have been easy to import just the tiny
    # subset of these operating systems that -- unlike Android -- just have
    # one huge repository rather than lots of little ones and a mechanism
    # like our `repo` tool).
    # The structure under these directories mimics the relevant upstream tree,
    # but in order to actually be able to compile this code in our tree
    # _without_ making modifications to the source files directly, we also
    # have the following subdirectories in each one that aren't upstream:
    android/
      include/
        # This is where we keep the hacks necessary to build BSD source
        # in our world. The *-compat.h files are automatically included
        # using -include, but we also provide equivalents for missing
        # header/source files needed by the BSD implementation.

  bionic/
    # This is the biggest mess. The C++ files are files we own, typically
    # because the Linux kernel interface is sufficiently different that we
    # can't use any of the BSD implementations. The C files are usually
    # legacy mess that needs to be sorted out, either by replacing it with
    # current upstream source in one of the upstream directories or by
    # switching the file to C++ and cleaning it up.

  malloc_debug/
    # The code that implements the functionality to enable debugging of
    # native allocation problems.

  stdio/
    # These are legacy files of dubious provenance. We're working to clean
    # this mess up, and this directory should disappear.

  tools/
    # Various tools used to maintain bionic.

  tzcode/
    # A modified superset of the IANA tzcode. Most of the modifications relate
    # to Android's use of a single file (with corresponding index) to contain
    # timezone data.
  zoneinfo/
    # Android-format timezone data.
    # See 'Updating tzdata' later.
```


## Adding libc wrappers for system calls

The first question you should ask is "should I add a libc wrapper for
this system call?". The answer is usually "no".

The answer is "yes" if the system call is part of the POSIX standard.

The answer is probably "yes" if the system call has a wrapper in at
least one other C library (typically glibc/musl or Apple's libc).

The answer may be "yes" if the system call has three/four distinct
users in different projects, and there isn't a more specific higher-level
library that would make more sense as the place to add the wrapper.

In all other cases, you should use
[syscall(3)](http://man7.org/linux/man-pages/man2/syscall.2.html) instead.

Adding a system call usually involves:

  1. Add an entry (or entries, in some cases) to SYSCALLS.TXT.
     See SYSCALLS.TXT itself for documentation on the format.
     See also the notes below for how to deal with tricky cases like `off_t`.
  2. Find the right header file to work in by looking up your system call
     on [man7.org](https://man7.org/linux/man-pages/dir_section_2.html).
     (If there's no header file given, see the points above about whether we
     should really be adding this or not!)
  3. Add constants (and perhaps types) to the appropriate header file.
     Note that you should check to see whether the constants are already in
     kernel uapi header files, in which case you just need to make sure that
     the appropriate header file in libc/include/ `#include`s the relevant
     `linux/` file or files.
  4. Add function declarations to the appropriate header file. Don't forget
     to include the appropriate `__INTRODUCED_IN()`, with the right API level
     for the first release your system call wrapper will be in. See
     libc/include/android/api_level.h for the API levels.
     If the header file doesn't exist, copy all of libc/include/sys/sysinfo.h
     into your new file --- it's a good short example to start from.

     Note also our style for naming arguments: always use two leading
     underscores (so developers are free to use any of the unadorned names as
     macros without breaking things), avoid abbreviations, and ideally try to
     use the same name as an existing system call (to reduce the amount of
     English vocabulary required by people who just want to use the function
     signatures). If there's a similar function already in the C library,
     check what names it's used. Finally, prefer the `void*` orthography we
     use over the `void *` you'll see on man7.org.)
  5. Add basic documentation to the header file. Again, the existing
     libc/include/sys/sysinfo.h is a good short example that shows the
     expected style.

     Most of the detail should actually be left to the man7.org page, with
     only a brief one-sentence explanation (usually based on the description
     in the NAME section of the man page) in our documentation. Always
     include the return value/error reporting details (you can find out
     what the system call returns from the RETURN VALUE of the man page),
     but try to match the wording and style wording from _our_ existing
     documentation; we're trying to minimize the amount of English readers
     need to understand by using the exact same wording where possible).
     Explicitly say which version of Android the function was added to in
     the documentation because the documentation generation tool doesn't yet
     understand `__INTRODUCED_IN()`.

     Explicitly call out any Android-specific changes/additions/limitations
     because they won't be on the man7.org page.
  6. Add the function name to the correct section in libc/libc.map.txt; it'll
     be near the end of the file. You may need to add a new section if you're
     the first to add a system call to this version of Android.
  7. Add a basic test. Don't try to test everything; concentrate on just testing
     the code that's actually in *bionic*, not all the functionality that's
     implemented in the kernel. For simple syscalls, that's just the
     auto-generated argument and return value marshalling.

     Add a test in the right file in tests/. We have one file per header, so if
     your system call is exposed in <unistd.h>, for example, your test would go
     in tests/unistd_test.cpp.

     A trivial test that deliberately supplies an invalid argument helps check
     that we're generating the right symbol and have the right declaration in
     the header file, and that the change to libc.map.txt from step 5 is
     correct. (You can use strace(1) manually to confirm that the correct
     system call is being made.)

     For testing the *kernel* side of things, we should prefer to rely on
     https://github.com/linux-test-project/ltp for kernel testing, but you'll
     want to check that external/ltp does contain tests for the syscall you're
     adding. Also check that external/ltp is using the libc wrapper for the
     syscall rather than calling it "directly" via syscall(3)!

Some system calls are harder than others. The most common problem is a 64-bit
argument such as `off64_t` (a *pointer* to a 64-bit argument is fine, since
pointers are always the "natural" size for the architecture regardless of the
size of the thing they point to). Whenever you have a function that takes
`off_t` or `off64_t`, you'll need to consider whether you actually need a foo()
and a foo64(), and whether they will use the same underlying system call or are
implemented as two different system calls. It's usually easiest to find a
similar system call and copy and paste from that. You'll definitely need to test
both on 32-bit and 64-bit. (These special cases warrant more testing than the
easy cases, even if only manual testing with strace. Sadly it isn't always
feasible to write a working test for the interesting cases -- offsets larger
than 2GiB, say -- so you may end up just writing a "meaningless" program whose
only purpose is to give you patterns to look for when run under strace(1).)

A general example of adding a system call:
https://android-review.googlesource.com/c/platform/bionic/+/2073827

### Debugging tips
1. Key error for a new codename in libc/libc.map.txt

e.g. what you add in libc/libc.map.txt is:

```
LIBC_V { # introduced=Vanilla
  global:
    xxx; // the new system call you add
} LIBC_U;
```

The error output is:

```
Traceback (most recent call last):
  File "/path/tp/out/soong/.temp/Soong.python_qucjwd7g/symbolfile/__init__.py", line 171,
  in decode_api_level_tag
    decoded = str(decode_api_level(value, api_map))
  File "/path/to/out/soong/.temp/Soong.python_qucjwd7g/symbolfile/__init__.py", line 157,
  in decode_api_level
    return api_map[api]
KeyError: 'Vanilla'
```

Solution: Ask in the team and wait for the update.

2. Use of undeclared identifier of the new system call in the test

Possible Solution: Check everything ready in the files mentioned above first.
Maybe glibc matters. Follow the example and try #if defined(__GLIBC__).

## Updating kernel header files

As mentioned above, this is currently a two-step process:

  1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
     contents for external/kernel-headers/.
  2. Run update_all.py to scrub those headers and import them into bionic.

Note that if you're actually just trying to expose device-specific headers to
build your device drivers, you shouldn't modify bionic. Instead use
`TARGET_DEVICE_KERNEL_HEADERS` and friends described in [config.mk](https://android.googlesource.com/platform/build/+/main/core/config.mk#186).


## Updating tzdata

This is handled by the libcore team, because they own icu, and that needs to be
updated in sync with bionic). See
[system/timezone/README.android](https://android.googlesource.com/platform/system/timezone/+/main/README.android).


## Verifying changes

If you make a change that is likely to have a wide effect on the tree (such as a
libc header change), you should run `make checkbuild`. A regular `make` will
_not_ build the entire tree; just the minimum number of projects that are
required for the device. Tests, additional developer tools, and various other
modules will not be built. Note that `make checkbuild` will not be complete
either, as `make tests` covers a few additional modules, but generally speaking
`make checkbuild` is enough.


## Running the tests

The tests are all built from the tests/ directory. There is a separate
directory `benchmarks/` containing benchmarks, and that has its own
documentation on [running the benchmarks](benchmarks/README.md).

### Device tests

    $ mma # In $ANDROID_ROOT/bionic.
    $ adb root && adb remount && adb sync
    $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
    $ adb shell \
        /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static
    # Only for 64-bit targets
    $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests
    $ adb shell \
        /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static

Note that we use our own custom gtest runner that offers a superset of the
options documented at
<https://github.com/google/googletest/blob/main/docs/advanced.md#running-test-programs-advanced-options>,
in particular for test isolation and parallelism (both on by default).

### Device tests via CTS

Most of the unit tests are executed by CTS. By default, CTS runs as
a non-root user, so the unit tests must also pass when not run as root.
Some tests cannot do any useful work unless run as root. In this case,
the test should check `getuid() == 0` and do nothing otherwise (typically
we log in this case to prevent accidents!). Obviously, if the test can be
rewritten to not require root, that's an even better solution.

Currently, the list of bionic CTS tests is generated at build time by
running a host version of the test executable and dumping the list of
all tests. In order for this to continue to work, all architectures must
have the same number of tests, and the host version of the executable
must also have the same number of tests.

Running the gtests directly is orders of magnitude faster than using CTS,
but in cases where you really have to run CTS:

    $ make cts # In $ANDROID_ROOT.
    $ adb unroot # Because real CTS doesn't run as root.
    # This will sync any *test* changes, but not *code* changes:
    $ cts-tradefed \
        run singleCommand cts --skip-preconditions -m CtsBionicTestCases

### Host tests

The host tests require that you have `lunch`ed either an x86 or x86_64 target.
Note that due to ABI limitations (specifically, the size of pthread_mutex_t),
32-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max
to 65536.

    $ ./tests/run-on-host.sh 32
    $ ./tests/run-on-host.sh 64   # For x86_64-bit *targets* only.

You can supply gtest flags as extra arguments to this script.

### Against glibc

As a way to check that our tests do in fact test the correct behavior (and not
just the behavior we think is correct), it is possible to run the tests against
the host's glibc.

    $ ./tests/run-on-host.sh glibc

### Against musl

Another way to verify test behavior is to run against musl on the host. glibc
musl don't always match, so this can be a good way to find the more complicated
corners of the spec. If they *do* match, bionic probably should too!

    $ OUT_DIR=$(ANDROID_BUILD_TOP)/musl-out ./tests/run-on-host.sh musl

Note: the alternate OUT_DIR is used to avoid causing excessive rebuilding when
switching between glibc and musl. The first musl test run will be expensive
because it will not reuse any already built artifacts, but subsequent runs will
be cheaper than if you hadn't used it.

## Gathering test coverage

To get test coverage for bionic, use `//bionic/build/coverage.sh`. Before
running, follow the instructions at the top of the file to rebuild bionic with
coverage instrumentation.

## Attaching GDB to the tests

Bionic's test runner will run each test in its own process by default to prevent
tests failures from impacting other tests. This also has the added benefit of
running them in parallel, so they are much faster.

However, this also makes it difficult to run the tests under GDB. To prevent
each test from being forked, run the tests with the flag `--no-isolate`.


## 32-bit ABI bugs

See [32-bit ABI bugs](docs/32-bit-abi.md).