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The source for the Linux kernel used in Windows Subsystem for Linux 2 (WSL2)
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Thomas Hebb e3232c2f39 tools build feature: Use CC and CXX from parent 
commit c8c188679c ("tools build: Use the same CC for feature detection
and actual build") changed these assignments from unconditional (:=) to
conditional (?=) so that they wouldn't clobber values from the
environment. However, conditional assignment does not work properly for
variables that Make implicitly sets, among which are CC and CXX. To
quote tools/scripts/Makefile.include, which handles this properly:

  # Makefiles suck: This macro sets a default value of $(2) for the
  # variable named by $(1), unless the variable has been set by
  # environment or command line. This is necessary for CC and AR
  # because make sets default values, so the simpler ?= approach
  # won't work as expected.

In other words, the conditional assignments will not run even if the
variables are not overridden in the environment; Make will set CC to
"cc" and CXX to "g++" when it starts[1], meaning the variables are not
empty by the time the conditional assignments are evaluated. This breaks
cross-compilation when CROSS_COMPILE is set but CC isn't, since "cc"
gets used for feature detection instead of the cross compiler (and
likewise for CXX).

To fix the issue, just pass down the values of CC and CXX computed by
the parent Makefile, which gets included by the Makefile that actually
builds whatever we're detecting features for and so is guaranteed to
have good values. This is a better solution anyway, since it means we
aren't trying to replicate the logic of the parent build system and so
don't risk it getting out of sync.

Leave PKG_CONFIG alone, since 1) there's no common logic to compute it
in Makefile.include, and 2) it's not an implicit variable, so
conditional assignment works properly.

[1] https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

Fixes: c8c188679c ("tools build: Use the same CC for feature detection and actual build")
Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Igor Lubashev <ilubashe@akamai.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Quentin Monnet <quentin@isovalent.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: thomas hebb <tommyhebb@gmail.com>
Link: http://lore.kernel.org/lkml/0a6e69d1736b0fa231a648f50b0cce5d8a6734ef.1595822871.git.tommyhebb@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
 		2020-07-31 09:27:58 -03:00
Documentation Kbuild fixes for v5.8 (2nd) 2020-07-05 12:14:24 -07:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch ARC updates for 5.8-rc5 2020-07-07 13:43:43 -07:00
block block-5.8-2020-07-05 2020-07-05 10:45:31 -07:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto crypto: af_alg - fix use-after-free in af_alg_accept() due to bh_lock_sock() 2020-06-18 17:09:54 +10:00
drivers MTD: 2020-07-07 14:54:02 -07:00
fs for-5.8-rc4-tag 2020-07-07 14:10:33 -07:00
include pci-v5.8-fixes-1 2020-07-03 12:14:51 -07:00
init kbuild: fix CONFIG_CC_CAN_LINK(_STATIC) for cross-compilation with Clang 2020-07-02 00:57:45 +09:00
ipc mmap locking API: use coccinelle to convert mmap_sem rwsem call sites 2020-06-09 09:39:14 -07:00
kernel A single fix for a printk format warning in RCU. 2020-07-05 12:21:28 -07:00
lib Peter Zijlstra says: 2020-06-28 09:42:47 -07:00
mm mm/page_alloc: fix documentation error 2020-07-03 16:15:25 -07:00
net Fixes for a umask bug on exported filesystems lacking ACL support, a 2020-07-02 20:35:33 -07:00
samples samples/vfs: avoid warning in statx override 2020-07-03 16:15:25 -07:00
scripts Kbuild fixes for v5.8 (2nd) 2020-07-05 12:14:24 -07:00
security Two simple fixes for v5.8: 2020-06-30 12:21:53 -07:00
sound sound fixes for 5.8-rc3 2020-06-25 09:15:24 -07:00
tools tools build feature: Use CC and CXX from parent 2020-07-31 09:27:58 -03:00
usr bpfilter: match bit size of bpfilter_umh to that of the kernel 2020-05-17 18:52:01 +09:00
virt kvm: use more precise cast and do not drop __user 2020-07-02 05:39:31 -04:00
.clang-format block: add bio_for_each_bvec_all() 2020-05-25 11:25:24 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: Do not track `defconfig` from `make savedefconfig` 2020-07-05 16:15:46 +09:00
.mailmap A fair amount of stuff this time around, dominated by yet another massive 2020-06-01 15:45:27 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS mailmap: change email for Ricardo Ribalda 2020-05-25 18:59:59 -06:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Devicetree fixes for v5.8, take 2: 2020-07-02 22:46:05 -07:00
Makefile Linux 5.8-rc4 2020-07-05 16:20:22 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.