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When a header file is changed, kernel/gen_kheaders.sh may fail to update
kernel/kheaders_data.tar.xz.
[steps to reproduce]
[1] Build kernel/kheaders_data.tar.xz
$ make -j$(nproc) kernel/kheaders.o
DESCEND objtool
INSTALL libsubcmd_headers
CALL scripts/checksyscalls.sh
CHK kernel/kheaders_data.tar.xz
GEN kernel/kheaders_data.tar.xz
CC kernel/kheaders.o
[2] Modify a header without changing the file size
$ sed -i s/0xdeadbeef/0xfeedbeef/ include/linux/elfnote.h
[3] Rebuild kernel/kheaders_data.tar.xz
$ make -j$(nproc) kernel/kheaders.o
DESCEND objtool
INSTALL libsubcmd_headers
CALL scripts/checksyscalls.sh
CHK kernel/kheaders_data.tar.xz
kernel/kheaders_data.tar.xz is not updated if steps [1] - [3] are run
within the same minute.
The headers_md5 variable stores the MD5 hash of the 'ls -l' output
for all header files. This hash value is used to determine whether
kheaders_data.tar.xz needs to be rebuilt. However, 'ls -l' prints the
modification times with minute-level granularity. If a file is modified
within the same minute and its size remains the same, the MD5 hash does
not change.
To reliably detect file modifications, this commit rewrites
kernel/gen_kheaders.sh to output header dependencies to
kernel/.kheaders_data.tar.xz.cmd. Then, Make compares the timestamps
and reruns kernel/gen_kheaders.sh when necessary. This is the standard
mechanism used by Make and Kbuild.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
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 reStructuredText 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.