unix: generate all Linux go files from source

Right now the process for adding in new constants, errors, or syscalls
for Linux is a pain and unreliable. The scripts are designed to be run
on the target architecture and use the header files installed on the
user's system. This makes it hard to generate files for all the
architectures or to have consistency between users. See golang/go#15282.

This CL fixes this issue by making all of the files for the 11 supported
architectures directly from source checkouts of Linux, glibc, and bluez.
This is done using Docker, the gcc cross-compilers, and qemu emulation.
Previously discussed here:
    https://go-review.googlesource.com/c/37589/

A README.md file is also added to explain how all the parts of the build
system work.

In order to get the build working for all the architectures, I made
some changes to the other scripts called from mkall_linux.go:
  - Files only used for generating linux code, moved to linux/
  - linux/mksysnum.pl supports a specified CC compiler.
  - The generated C code in mkerrors.sh changed to avoid a warning
  - mkerrors.sh headers changed to fix powerpc64 bug in sys/ioctl.h
  - linux/types.go no longer needs to export Ptrace structs in lowercase

Build instructions:
  - Host system needs to be x86-64 Linux
  - Install Docker (https://docs.docker.com/engine/installation/)
  - ./mkall.sh (That's it!!!)

Change-Id: I87067c14442ba12f8d51991349a43a9d73f38ae0
Reviewed-on: https://go-review.googlesource.com/37943
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

unix/README.md

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+# Building `sys/unix`
+
+The sys/unix package provides access to the raw system call interface of the
+underlying operating system. See: https://godoc.org/golang.org/x/sys/unix
+
+Porting Go to a new architecture/OS combination or adding syscalls, types, or
+constants to an existing architecture/OS pair requires some manual effort;
+however, there are tools that automate much of the process.
+
+## Build Systems
+
+There are currently two ways we generate the necessary files. We are currently
+migrating the build system to use containers so the builds are reproducible.
+This is being done on an OS-by-OS basis. Please update this documentation as
+components of the build system change.
+
+### Old Build System (currently for `GOOS != "Linux" || GOARCH == "sparc64"`)
+
+The old build system generates the Go files based on the C header files
+present on your system. This means that files
+for a given GOOS/GOARCH pair must be generated on a system with that OS and
+architecture. This also means that the generated code can differ from system
+to system, based on differences in the header files.
+
+To avoid this, if you are using the old build system, only generate the Go
+files on an installation with unmodified header files. It is also important to
+keep track of which version of the OS the files were generated from (ex.
+Darwin 14 vs Darwin 15). This makes it easier to track the progress of changes
+and have each OS upgrade correspond to a single change.
+
+To build the files for your current OS and architecture, make sure GOOS and
+GOARCH are set correctly and run `mkall.sh`. This will generate the files for
+your specific system. Running `mkall.sh -n` shows the commands that will be run.
+
+Requirements: bash, perl, go
+
+### New Build System (currently for `GOOS == "Linux" && GOARCH != "sparc64"`)
+
+The new build system uses a Docker container to generate the go files directly
+from source checkouts of the kernel and various system libraries. This means
+that on any platform that supports Docker, all the files using the new build
+system can be generated at once, and generated files will not change based on
+what the person running the scripts has installed on their computer.
+
+The OS specific files for the new build system are located in the `${GOOS}`
+directory, and the build is coordinated by the `${GOOS}/mkall.go` program. When
+the kernel or system library updates, modify the Dockerfile at
+`${GOOS}/Dockerfile` to checkout the new release of the source.
+
+To build all the files under the new build system, you must be on an amd64/Linux
+system and have your GOOS and GOARCH set accordingly. Running `mkall.sh` will
+then generate all of the files for all of the GOOS/GOARCH pairs in the new build
+system. Running `mkall.sh -n` shows the commands that will be run.
+
+Requirements: bash, perl, go, docker
+
+## Component files
+
+This section describes the various files used in the code generation process.
+It also contains instructions on how to modify these files to add a new
+architecture/OS or to add additional syscalls, types, or constants. Note that
+if you are using the new build system, the scripts cannot be called normally.
+They must be called from within the docker container.
+
+### asm files
+
+The hand-written assembly file at `asm_${GOOS}_${GOARCH}.s` implements system
+call dispatch. There are three entry points:
+```
+  func Syscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
+  func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
+  func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
+```
+The first and second are the standard ones; they differ only in how many
+arguments can be passed to the kernel. The third is for low-level use by the
+ForkExec wrapper. Unlike the first two, it does not call into the scheduler to
+let it know that a system call is running.
+
+When porting Go to an new architecture/OS, this file must be implemented for
+each GOOS/GOARCH pair.
+
+### mksysnum
+
+Mksysnum is a script located at `${GOOS}/mksysnum.pl` (or `mksysnum_${GOOS}.pl`
+for the old system). This script takes in a list of header files containing the
+syscall number declarations and parses them to produce the corresponding list of
+Go numeric constants. See `zsysnum_${GOOS}_${GOARCH}.go` for the generated
+constants.
+
+Adding new syscall numbers is mostly done by running the build on a sufficiently
+new installation of the target OS (or updating the source checkouts for the
+new build system). However, depending on the OS, you make need to update the
+parsing in mksysnum.
+
+### mksyscall.pl
+
+The `syscall.go`, `syscall_${GOOS}.go`, `syscall_${GOOS}_${GOARCH}.go` are
+hand-written Go files which implement system calls (for unix, the specific OS,
+or the specific OS/Architecture pair respectively) that need special handling
+and list `//sys` comments giving prototypes for ones that can be generated.
+
+The mksyscall.pl script takes the `//sys` and `//sysnb` comments and converts
+them into syscalls. This requires the name of the prototype in the comment to
+match a syscall number in the `zsysnum_${GOOS}_${GOARCH}.go` file. The function
+prototype can be exported (capitalized) or not.
+
+Adding a new syscall often just requires adding a new `//sys` function prototype
+with the desired arguments and a capitalized name so it is exported. However, if
+you want the interface to the syscall to be different, often one will make an
+unexported `//sys` prototype, an then write a custom wrapper in
+`syscall_${GOOS}.go`.
+
+### types files
+
+For each OS, there is a hand-written Go file at `${GOOS}/types.go` (or
+`types_${GOOS}.go` on the old system). This file includes standard C headers and
+creates Go type aliases to the corresponding C types. The file is then fed
+through godef to get the Go compatible definitions. Finally, the generated code
+is fed though mkpost.go to format the code correctly and remove any hidden or
+private identifiers. This cleaned-up code is written to
+`ztypes_${GOOS}_${GOARCH}.go`.
+
+The hardest part about preparing this file is figuring out which headers to
+include and which symbols need to be `#define`d to get the actual data
+structures that pass through to the kernel system calls. Some C libraries
+preset alternate versions for binary compatibility and translate them on the
+way in and out of system calls, but there is almost always a `#define` that can
+get the real ones.
+See `types_darwin.go` and `linux/types.go` for examples.
+
+To add a new type, add in the necessary include statement at the top of the
+file (if it is not already there) and add in a type alias line. Note that if
+your type is significantly different on different architectures, you may need
+some `#if/#elif` macros in your include statements.
+
+### mkerrors.sh
+
+This script is used to generate the system's various constants. This doesn't
+just include the error numbers and error strings, but also the signal numbers
+an a wide variety of miscellaneous constants. The constants come from the list
+of include files in the `includes_${uname}` variable. A regex then picks out
+the desired `#define` statements, and generates the corresponding Go constants.
+The error numbers and strings are generated from `#include <errno.h>`, and the
+signal numbers and strings are generated from `#include <signal.h>`. All of
+these constants are written to `zerrors_${GOOS}_${GOARCH}.go` via a C program,
+`_errors.c`, which prints out all the constants.
+
+To add a constant, add the header that includes it to the appropriate variable.
+Then, edit the regex (if necessary) to match the desired constant. Avoid making
+the regex too broad to avoid matching unintended constants.
+
+
+## Generated files
+
+### `zerror_${GOOS}_${GOARCH}.go`
+
+A file containing all of the system's generated error numbers, error strings,
+signal numbers, and constants. Generated by `mkerrors.sh` (see above).
+
+### `zsyscall_${GOOS}_${GOARCH}.go`
+
+A file containing all the generated syscalls for a specific GOOS and GOARCH.
+Generated by `mksyscall.pl` (see above).
+
+### `zsysnum_${GOOS}_${GOARCH}.go`
+
+A list of numeric constants for all the syscall number of the specific GOOS
+and GOARCH. Generated by mksysnum (see above).
+
+### `ztypes_${GOOS}_${GOARCH}.go`
+
+A file containing Go types for passing into (or returning from) syscalls.
+Generated by godefs and the types file (see above).

unix/linux/Dockerfile

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+FROM ubuntu:16.04
+
+# Dependencies to get the git sources and go binaries
+RUN apt-get update && apt-get install -y \
+        curl \
+        git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Get the git sources. If not cached, this takes O(5 minutes).
+WORKDIR /git
+RUN git config --global advice.detachedHead false
+# Linux Kernel: Released 19 Feb 2017
+RUN git clone --branch v4.10 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
+# GNU C library: Released 05 Feb 2017 (we should try to get a secure way to clone this)
+RUN git clone --branch glibc-2.25 --depth 1 git://sourceware.org/git/glibc.git
+
+# Get Go 1.8 (https://github.com/docker-library/golang/blob/master/1.8/Dockerfile)
+ENV GOLANG_VERSION 1.8
+ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
+ENV GOLANG_DOWNLOAD_SHA256 53ab94104ee3923e228a2cb2116e5e462ad3ebaeea06ff04463479d7f12d27ca
+
+RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
+    && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \
+    && tar -C /usr/local -xzf golang.tar.gz \
+    && rm golang.tar.gz
+
+ENV PATH /usr/local/go/bin:$PATH
+
+# Linux and Glibc build dependencies
+RUN apt-get update && apt-get install -y \
+        gawk make python \
+        gcc gcc-multilib \
+        gettext  texinfo \
+    && rm -rf /var/lib/apt/lists/*
+# Emulator and cross compilers
+RUN apt-get update && apt-get install -y \
+        qemu \
+        gcc-aarch64-linux-gnu       gcc-arm-linux-gnueabi     \
+        gcc-mips-linux-gnu          gcc-mips64-linux-gnuabi64 \
+        gcc-mips64el-linux-gnuabi64 gcc-mipsel-linux-gnu      \
+        gcc-powerpc64-linux-gnu     gcc-powerpc64le-linux-gnu \
+        gcc-s390x-linux-gnu         gcc-sparc64-linux-gnu     \
+    && rm -rf /var/lib/apt/lists/*
+
+# Let the scripts know they are in the docker environment
+ENV GOLANG_SYS_BUILD docker
+WORKDIR /build
+ENTRYPOINT ["go", "run", "linux/mkall.go", "/git/linux", "/git/glibc"]