[PATCH 06/16] kbuild: doc: document the new syntax 'userprogs'

Masahiro Yamada
2020-04-23T07:41:46Z

Kbuild now supports the syntax 'userprogs' to compile userspace
programs for the same architecture as the kernel.

Insert the section '5 Userspace Program support' to explain it.

I copy-pasted '4 Host Program support' and fixed it up.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

Documentation/kbuild/makefiles.rst | 184 +++++++++++++++++++++--------
1 file changed, 136 insertions(+), 48 deletions(-)

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index b80257a03830..251e5431276e 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -29,31 +29,37 @@ This document describes the Linux kernel Makefiles.
--- 4.4 Controlling compiler options for host programs
--- 4.5 When host programs are actually built

- === 5 Kbuild clean infrastructure
-
- === 6 Architecture Makefiles
- --- 6.1 Set variables to tweak the build to the architecture
- --- 6.2 Add prerequisites to archheaders:
- --- 6.3 Add prerequisites to archprepare:
- --- 6.4 List directories to visit when descending
- --- 6.5 Architecture-specific boot images
- --- 6.6 Building non-kbuild targets
- --- 6.7 Commands useful for building a boot image
- --- 6.8 Custom kbuild commands
- --- 6.9 Preprocessing linker scripts
- --- 6.10 Generic header files
- --- 6.11 Post-link pass
-
- === 7 Kbuild syntax for exported headers
- --- 7.1 no-export-headers
- --- 7.2 generic-y
- --- 7.3 generated-y
- --- 7.4 mandatory-y
-
- === 8 Kbuild Variables
- === 9 Makefile language
- === 10 Credits
- === 11 TODO
+ === 5 Userspace Program support
+ --- 5.1 Simple Userspace Program
+ --- 5.2 Composite Userspace Programs
+ --- 5.3 Controlling compiler options for userspace programs
+ --- 5.4 When userspace programs are actually built
+
+ === 6 Kbuild clean infrastructure
+
+ === 7 Architecture Makefiles
+ --- 7.1 Set variables to tweak the build to the architecture
+ --- 7.2 Add prerequisites to archheaders:
+ --- 7.3 Add prerequisites to archprepare:
+ --- 7.4 List directories to visit when descending
+ --- 7.5 Architecture-specific boot images
+ --- 7.6 Building non-kbuild targets
+ --- 7.7 Commands useful for building a boot image
+ --- 7.8 Custom kbuild commands
+ --- 7.9 Preprocessing linker scripts
+ --- 7.10 Generic header files
+ --- 7.11 Post-link pass
+
+ === 8 Kbuild syntax for exported headers
+ --- 8.1 no-export-headers
+ --- 8.2 generic-y
+ --- 8.3 generated-y
+ --- 8.4 mandatory-y
+
+ === 9 Kbuild Variables
+ === 10 Makefile language
+ === 11 Credits
+ === 12 TODO

1 Overview
==========
@@ -732,7 +738,89 @@ Both possibilities are described in the following.
This will tell kbuild to build lxdialog even if not referenced in
any rule.

-5 Kbuild clean infrastructure
+5 Userspace Program support
+===========================
+
+Just like host programs, Kbuild also supports building userspace executables
+for the target architecture (i.e. the same architecture as you are building
+the kernel for).
+
+The syntax is quite similar. The difference is to use "userprogs" instead of
+"hostprogs".
+
+5.1 Simple Userspace Program
+----------------------------
+
+ The following line tells kbuild that the program bpf-direct shall be
+ built for the target architecture.
+
+ Example::
+
+ userprogs := bpf-direct
+
+ Kbuild assumes in the above example that bpf-direct is made from a
+ single C source file named bpf-direct.c located in the same directory
+ as the Makefile.
+
+5.2 Composite Userspace Programs
+--------------------------------
+
+ Userspace programs can be made up based on composite objects.
+ The syntax used to define composite objects for userspace programs is
+ similar to the syntax used for kernel objects.
+ $(<executable>-objs) lists all objects used to link the final
+ executable.
+
+ Example::
+
+ #samples/seccomp/Makefile
+ userprogs := bpf-fancy
+ bpf-fancy-objs := bpf-fancy.o bpf-helper.o
+
+ Objects with extension .o are compiled from the corresponding .c
+ files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
+ and bpf-helper.c is compiled to bpf-helper.o.
+
+ Finally, the two .o files are linked to the executable, bpf-fancy.
+ Note: The syntax <executable>-y is not permitted for userspace programs.
+
+5.3 Controlling compiler options for userspace programs
+-------------------------------------------------------
+
+ When compiling userspace programs, it is possible to set specific flags.
+ The programs will always be compiled utilising $(CC) passed
+ the options specified in $(KBUILD_USERCFLAGS).
+ To set flags that will take effect for all userspace programs created
+ in that Makefile, use the variable user-ccflags.
+
+ Example::
+
+ # samples/seccomp/Makefile
+ user-ccflags += -I usr/include
+
+ To set specific flags for a single file the following construction
+ is used:
+
+ Example::
+
+ bpf-helper-ccflags += -I user/include
+
+ It is also possible to specify additional options to the linker.
+
+ Example::
+
+ # net/bpfilter/Makefile
+ bpfilter_umh-ldflags += -static
+
+ When linking bpfilter_umh-ldflags, it will be passed the extra option
+ -static.
+
+5.4 When userspace programs are actually built
+----------------------------------------------
+
+ Same as "When host programs are actually built".
+
+6 Kbuild clean infrastructure
=============================

"make clean" deletes most generated files in the obj tree where the kernel
@@ -790,7 +878,7 @@ is not operational at that point.
Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
be visited during "make clean".

-6 Architecture Makefiles
+7 Architecture Makefiles
========================

The top level Makefile sets up the environment and does the preparation,
@@ -820,7 +908,7 @@ When kbuild executes, the following steps are followed (roughly):
- Preparing initrd images and the like

-6.1 Set variables to tweak the build to the architecture
+7.1 Set variables to tweak the build to the architecture
--------------------------------------------------------

LDFLAGS
@@ -967,7 +1055,7 @@ When kbuild executes, the following steps are followed (roughly):
KBUILD_VMLINUX_LIBS together specify all the object files used to
link vmlinux.

-6.2 Add prerequisites to archheaders
+7.2 Add prerequisites to archheaders
------------------------------------

The archheaders: rule is used to generate header files that
@@ -977,7 +1065,7 @@ When kbuild executes, the following steps are followed (roughly):
architecture itself.

-6.3 Add prerequisites to archprepare
+7.3 Add prerequisites to archprepare
------------------------------------

The archprepare: rule is used to list prerequisites that need to be
@@ -995,7 +1083,7 @@ When kbuild executes, the following steps are followed (roughly):
generating offset header files.

-6.4 List directories to visit when descending
+7.4 List directories to visit when descending
---------------------------------------------

An arch Makefile cooperates with the top Makefile to define variables
@@ -1030,7 +1118,7 @@ When kbuild executes, the following steps are followed (roughly):
drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/

-6.5 Architecture-specific boot images
+7.5 Architecture-specific boot images
-------------------------------------

An arch Makefile specifies goals that take the vmlinux file, compress
@@ -1085,7 +1173,7 @@ When kbuild executes, the following steps are followed (roughly):

When "make" is executed without arguments, bzImage will be built.

-6.6 Building non-kbuild targets
+7.6 Building non-kbuild targets
-------------------------------

extra-y
@@ -1108,7 +1196,7 @@ When kbuild executes, the following steps are followed (roughly):
In this example, extra-y is used to list object files that
shall be built, but shall not be linked as part of built-in.a.

-6.7 Commands useful for building a boot image
+7.7 Commands useful for building a boot image
---------------------------------------------

Kbuild provides a few macros that are useful when building a
@@ -1211,7 +1299,7 @@ When kbuild executes, the following steps are followed (roughly):
targets += $(dtb-y)
DTC_FLAGS ?= -p 1024

-6.8 Custom kbuild commands
+7.8 Custom kbuild commands
--------------------------

When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
@@ -1241,7 +1329,7 @@ When kbuild executes, the following steps are followed (roughly):
will be displayed with "make KBUILD_VERBOSE=0".

-6.9 Preprocessing linker scripts
+7.9 Preprocessing linker scripts
--------------------------------

When the vmlinux image is built, the linker script
@@ -1274,7 +1362,7 @@ When kbuild executes, the following steps are followed (roughly):
The kbuild infrastructure for `*lds` files is used in several
architecture-specific files.

-6.10 Generic header files
+7.10 Generic header files
-------------------------

The directory include/asm-generic contains the header files
@@ -1283,7 +1371,7 @@ When kbuild executes, the following steps are followed (roughly):
to list the file in the Kbuild file.
See "7.2 generic-y" for further info on syntax etc.

-6.11 Post-link pass
+7.11 Post-link pass
-------------------

If the file arch/xxx/Makefile.postlink exists, this makefile
@@ -1299,7 +1387,7 @@ When kbuild executes, the following steps are followed (roughly):
For example, powerpc uses this to check relocation sanity of
the linked vmlinux file.

-7 Kbuild syntax for exported headers
+8 Kbuild syntax for exported headers
------------------------------------

The kernel includes a set of headers that is exported to userspace.
@@ -1319,14 +1407,14 @@ A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
arch/<arch>/include/asm/ to list asm files coming from asm-generic.
See subsequent chapter for the syntax of the Kbuild file.

-7.1 no-export-headers
+8.1 no-export-headers
---------------------

no-export-headers is essentially used by include/uapi/linux/Kbuild to
avoid exporting specific headers (e.g. kvm.h) on architectures that do
not support it. It should be avoided as much as possible.

-7.2 generic-y
+8.2 generic-y
-------------

If an architecture uses a verbatim copy of a header from
@@ -1356,7 +1444,7 @@ See subsequent chapter for the syntax of the Kbuild file.

#include <asm-generic/termios.h>

-7.3 generated-y
+8.3 generated-y
---------------

If an architecture generates other header files alongside generic-y
@@ -1370,7 +1458,7 @@ See subsequent chapter for the syntax of the Kbuild file.
#arch/x86/include/asm/Kbuild
generated-y += syscalls_32.h

-7.4 mandatory-y
+8.4 mandatory-y
---------------

mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
@@ -1380,7 +1468,7 @@ See subsequent chapter for the syntax of the Kbuild file.
in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
a wrapper of the asm-generic one.

-8 Kbuild Variables
+9 Kbuild Variables
==================

The top Makefile exports the following variables:
@@ -1438,8 +1526,8 @@ The top Makefile exports the following variables:
command.

-9 Makefile language
-===================
+10 Makefile language
+====================

The kernel Makefiles are designed to be run with GNU Make. The Makefiles
use only the documented features of GNU Make, but they do use many
@@ -1458,7 +1546,7 @@ time the left-hand side is used.
There are some cases where "=" is appropriate. Usually, though, ":="
is the right choice.

-10 Credits
+11 Credits
==========

- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
@@ -1466,7 +1554,7 @@ is the right choice.
- Updates by Sam Ravnborg <sam@ravnborg.org>
- Language QA by Jan Engelhardt <jengelh@gmx.de>

-11 TODO
+12 TODO
=======

- Describe how kbuild supports shipped files with _shipped.
--
2.25.1