Bionic comes with a processed set of all of the uapi Linux kernel headers that can safely be included by userland applications and libraries.
These clean headers are automatically generated by several scripts located in the tools/ directory. The tools process the original unmodified kernel headers in order to get rid of many annoying declarations and constructs that usually result in compilation failure.
The ‘clean headers’ only contain type and macro definitions, with the exception of a couple static inline functions used for performance reason (e.g. optimized CPU-specific byte-swapping routines).
They can be included from C++, or when compiling code in strict ANSI mode. They can be also included before or after any Bionic C library header.
Description of the directories involved in generating the parsed kernel headers:
external/kernel-headers/original/uapi/ Contains the uapi kernel headers found in the Android kernel. Note this also includes the header files that are generated by building the kernel sources.
external/kernel-headers/original/scsi/ Contains copies of the kernel scsi header files. These where never made into uapi files, but some user space code expects that these headers are available.
external/kernel-headers/modified/scsi/ Contains hand-modified versions of a few files from original/scsi/ that removes the kernel specific code from these files so they can be used as uapi headers. The tools to process the kernel headers will warn if any scsi header files have changed and require new versions to be hand-modified.
bionic/libc/kernel/uapi/ Contains the cleaned kernel headers and mirrors the directory structure in external/kernel-headers/original/uapi/.
bionic/libc/kernel/tools/ Contains various Python and shell scripts used to get and re-generate the headers.
The tools to get/parse the headers:
tools/generate_uapi_headers.sh Checks out the Android kernel and generates all uapi header files. copies all the changed files into external/kernel-headers.
tools/clean_header.py Prints the clean version of a given kernel header. With the -u option, this will also update the corresponding clean header file if its content has changed. You can also process more than one file with -u.
tools/update_all.py Automatically update all clean headers from the content of external/kernel-headers/original/.
IMPORTANT IMPORTANT:
WHEN UPDATING THE HEADERS, ALWAYS CHECK THAT THE NEW CLEAN HEADERS DO NOT BREAK THE KERNEL <-> USER ABI, FOR EXAMPLE BY CHANGING THE SIZE OF A GIVEN TYPE. THIS TASK CANNOT BE EASILY AUTOMATED AT THE MOMENT.
Download the Android mainline kernel source code:
> mkdir kernel_src > cd kernel_src kernel_src> git clone https://android.googlesource.com/kernel/common/ -b android-mainline
The Android mainline kernel source has tags that indicate the kernel version to which they correspond. The format of a tag is android-mainline-XXX, where XXX is the kernel version. For example, android-mainline-5.10 corresponds to linux stable kernel 5.10. To check out a particular tag:
kernel_src> cd common kernel_src/common> git checkout tags/android-mainline-XXX
It is expected that a kernel update should only be performed on a valid tag. For testing purposes, it is possible that you can use the top of tree version, but never use that as the basis for importing new kernel headers.
Before running the command to import the headers, make sure that you have done a lunch TARGET. The script uses a variable set by the lunch command to determine which directory to use as the destination directory.
After running lunch, run this command to import the headers into the Android source tree if there is a kernel source tree already checked out:
bionic/libc/kernel/tools/generate_uapi_headers.sh --use-kernel-dir kernel_src
Run this command to automatically download the latest version of the headers and import them if there is no checked out kernel source tree:
# For testing only, not for use in production! bionic/libc/kernel/tools/generate_uapi_headers.sh --download-kernel
Next, run this command to copy the parsed files to bionic/libc/kernel/uapi:
bionic/libc/kernel/tools/update_all.py
After this, you will need to build/test the tree to make sure that these changes do not introduce any errors.