TODO
Getting the sources
Shallow copy of most recent sources in git repository:
$ git clone --depth 1 -b master https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git ~/linux-src
KCONFIG variables
The KCONFIG variables found in the source tree can be found with
$ grep -r -h -o -P '\bKCONFIG_\w+' | sort | uniq
KCONFIG_ALLCONFIG | Specifies the location of a file that contains preset config symbols. If set to "" or 1, make *config tries to find such config symbols in a file named all{yes/mod/no/def/random}.config |
KCONFIG_CONFIG | Specifies the path to the kernel config file (Default is .config) |
KCONFIG_DEFCONFIG_LIST | A (white space separated) list of base config files, the first existing of which is used in case a .config is not present. |
KCONFIG_OVERWRITECONFIG | If set, Kconfig will not break symlinks when .config is symlinked to another file. |
KCONFIG_PATH | Only used as script variable in tools/testing/kunit/kunit_kernel.py |
KCONFIG_REL_PATHS | Only used as script variable in tools/testing/selftests/bpf/vmtest.sh |
KCONFIG_WARN_UNKNOWN_SYMBOLS | If set, Kconfig will warn about all unrecognized symbols in the config input |
KCONFIG_WERROR | If set, Kconfig will treat warnings as errors. |
Variables used in conjunction with randconfig:
KCONFIG_SEED | Sets the seed for the random number generator (default: current time) |
KCONFIG_PROBABILITY | Used to skew probabilities. |
Variables used in conjunction with syncconfig:
KCONFIG_AUTOCONFIG | Specifies the path of the auto.conf file (default: include/config/auto.conf) |
KCONFIG_AUTOHEADER | Specifies the path of the autoconf.h header file (default: include/generated/autoconf.h) |
KCONFIG_NOSILENTUPDATE | If value is non-blank, silent kernel config updates are prevented (i. e. explicit updates are required) |
Others
KCONFIG_ARG | Only used as shell script variable in tools/testing/selftests/rcutorture/bin/kvm.sh |
KCONFIG_ARGS | Only used as shell script variable in scripts/tags.sh |
KCONFIG_RUSTCCFG | Found only in scripts/kconfig/confdata.c |
KCONFIG_SEC | Found only in tools/lib/bpf/libbpf.c where it is #defined to .kconfig |
Memory
Page tables
A page table page can be thought of an array like
pXd_t levelY_kernel_pgt[PTRS_PER_PxD]
pXd_t levelY_ident_pgt [PTRS_PER_PxD]
X is one of 4, u, m, t; Y is 4, 3, 2, 1
There is also a top level tree: init_top_pgt.
pXx_index() returns the index of the entry in the page table page which would control the given virtual address.
On x86,
CONFIG_PGTABLE_LEVELS is an integer that is set to
-
5 if X86_5LEVEL is enabled,
-
4 if x86_64 is enabled,
-
3 if X86_PAE is enable,
-
2 otherwise
include/linux/pgtable.h
__PAGETABLE_P4D_FOLDED, __PAGETABLE_PUD_FOLDED, __PAGETABLE_PMD_FOLDED
5-level page tables
5-level page tables allow to access up to 128 PiB of virtual and 4 PiB of physical address space.
On x86_X64, the config option X86_5LEVEL must be enabled to use 5-level page tables.
An x86 CPU supports 5-level pages tables if the
la57 flag is present which can be queried from the
flags lines in
/proc/cpuinfo:
$ grep '^flags.*la57' /proc/cpuinfo && echo 'la57 is enabled'
__KERNEL__ and __MODULE__
The macro __KERNEL__ is defined (gcc … -D__KERNEL__ …`) when compiling a source file for the kernel.
This allows for example to include the same header file for both the kernel and a user space program.
__MODULE__ is defined if a driver is selected to be compiled as a module (rather than be built-in the kernel).
EFI
CONFIG_EFI_MIXED
The .compat section in header.S is only present if CONFIG_EFI_MIXED is enabled.
Handover Protocol
CONFIG_EFI_HANDOVER_PROTOCOL
Sections
objdump -h vmlinux prints all sections found in the kernel.
Init data sections
.init.setup | |
.init.rodata | |
.meminit.rodata | |
.init.data | |
.meminit.data | |
Text sections
.text | |
.text.* | |
.sched.text | |
.kprobes.text | |
.cpuidle.text | |
.noinstr.text | |
Other text sections
.ref.text | |
.head.text | This section is placed at the head of vmlinux, before the normal .text section, and is typically used for entry point functions. The macro __HEAD is defined to be __section(".head.text","ax"). See also this comment in scripts/head-object-list.txt. |
.spinlock.text | |
.fixup | |
.entry.text | |
.exception.text | |
.coldtext | |
.softirqentry.text | |
.bstext | The bootsector? Found in arch/x86/boot/header.S |
PCI init sections
.pci_fixup_early | |
.pci_fixup_header | |
.pci_fixup_final | |
.pci_fixup_enable | |
.pci_fixup_resume | |
.pci_fixup_resume_early | |
.pci_fixup_suspend | |
White list
.comment* | echo $(objcopy --dump-section .comment=/dev/tty vmlinux) prints something like GCC: (Debian 10.2.1-6) 10.2.1 20210110 |
.debug* | |
.zdebug* | Compressed debug sections. |
.GCC.command.line | record-gcc-switches |
.mdebug* | alpha, score, mips etc. |
.pdr | alpha, score, mips etc. |
.stab* | |
.note* | |
.got* | |
.toc* | |
.xt.prop | xtensa |
.xt.lit | xtensa |
.arcextmap* | arc |
.gnu.linkonce.arcext* | arc : modules |
.cmem* | EZchip |
.fmt_slot* | EZchip |
.gnu.lto* | |
.discard.* | |
.llvm.call-graph-profile | call graph |
kallsyms
With CONFIG_KALLSYMS enabled, the kernel is built with the symbols of functions (text and inittext sections) in order to print out stack backtraces and crash information with these symbols
CONFIG_KALLSYMS_ALL includes all symbols (i. e. names of variables from the data section)
These features comes with the cost of an increased kernel size because it contains the symbols.
kallsyms_lookup_name is used by the in-kernel kprobe address resolution code.
Wih enabled kallsyms, the s and S format specifies in printk print the symbol names rather than their address.
Kprobes
Kprobes allows to dynamically break into (non-blacklisted) kernel function and collect debugging and performance information non-disruptively.
See also Documentation/trace/kprobes.rst
Determine the GCC options with which a source file is compiled
$ touch kernel/sys.c
$ make V=1 2>/dev/null | grep 'gcc.*sys.c' > /tmp/compile-command
Of course, the created
/tmp/compile-command file allows to produced the pre processed source file with a given configuration: the
-E option must be added and the
-c -o ….o options must be removed.
Minimal requirements to compile the Kernel
Documentation/process/changes.rst provides a list of the minimal requirements (software and version) to compile the Kernel.
BTF
With
CONFIG_DEBUG_INFO_BTF enabled, building the Kernel also generates BTF (BPF type format) from DWARF in
vmlinux and from kernel modules.
This feature requires pahole or llvm.
pahole acts as a dwarf2btf converter.
See also
-
include/uapi/linux/btf.h
-
Documentation/bpf/btf.rst
- The function
gen_btf() in scripts/link-vmlinux.sh.
- The
resolve_btfids tool (tools/bpf/resolve_btfids) scanes ELF objects for .BTF_ids sections and resolves its symbols with BTF ID values.
Most frequent first line
Count the number each different first line occurrs:
$ find . -type f \( -name '*.c' -o -name '*.h' -o -name '*.rst' \) -exec head -1 {} \+ | sort | uniq -c | sort -nr | head -20
60399
10189 // SPDX-License-Identifier: GPL-2.0
8898 // SPDX-License-Identifier: GPL-2.0-only
8526 /* SPDX-License-Identifier: GPL-2.0 */
6600 /*
5411 // SPDX-License-Identifier: GPL-2.0-or-later
3970 /* SPDX-License-Identifier: GPL-2.0-only */
2376 /* SPDX-License-Identifier: GPL-2.0-or-later */
2022 // SPDX-License-Identifier: GPL-2.0+
1203 .. SPDX-License-Identifier: GPL-2.0
1093 /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
736 /* SPDX-License-Identifier: MIT */
643 /* SPDX-License-Identifier: GPL-2.0+ */
546 /* SPDX-License-Identifier: GPL-2.0
363 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
337 // SPDX-License-Identifier: MIT
261 // SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0
258 // SPDX-License-Identifier: ISC
207 // SPDX-License-Identifier: GPL-2.0 OR Linux-OpenIB
207 // SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
See also the
scripts spdxcheck.py,
spdxcheck-test.sh and
spdxexclude.