kernel: add section about /proc/version

Customize user, host and date

README.adoc

@@ -5283,6 +5283,26 @@ Sources:
 
 Bibliography: https://stackoverflow.com/questions/8516021/proc-create-example-for-kernel-module/18924359#18924359
 
+===== /proc/version
+
+Its data is shared with `uname()`, which is a POSIX C function and has a Linux syscall to back it up.
+
+Where the data comes from and how to modify it:
+
+* https://unix.stackexchange.com/questions/136959/where-does-uname-get-its-information-from/485962#485962
+* https://stackoverflow.com/questions/23424174/how-to-customize-or-remove-extra-linux-kernel-version-details-shown-at-boot
+
+In this repo, leaking host information, and to make builds more reproducible, we are setting:
+
+- user and date to dummy values
+- hostname to the kernel git commit
+
+So the file contains something like:
+
+....
+Linux version 4.19.0-dirty (lkmc@84df9525b0c27f3ebc2ebb1864fa62a97fdedb7d) (gcc version 6.4.0 (Buildroot 2018.05-00002-gbc60382b8f)) #1 SMP Thu Jan 1 00:00:00 UTC 1970
+....
+
 ==== sysfs
 
 Sysfs is more restricted than <<procfs>>, as it does not take an arbitrary `file_operations`:

build-linux

@@ -130,6 +130,12 @@ Configure the kernel, but don't build it.
                     common_make_args +
                     common.add_newlines(args.extra_make_args)
                 ),
+                extra_env={
+                    'KBUILD_BUILD_VERSION': '1',
+                    'KBUILD_BUILD_TIMESTAMP': 'Thu Jan  1 00:00:00 UTC 1970',
+                    'KBUILD_BUILD_USER': 'lkmc',
+                    'KBUILD_BUILD_HOST': common.git_sha(common.linux_source_dir),
+                },
                 **common_args
             )
             common.run_cmd(

common.py

@@ -66,7 +66,9 @@ for key in common.arch_short_to_long_dict:
     arch_choices.append(common.arch_short_to_long_dict[key])
 default_arch = 'x86_64'
 gem5_cpt_prefix = '^cpt\.'
-sha = subprocess.check_output(['git', '-C', root_dir, 'log', '-1', '--format=%H']).decode().rstrip()
+def git_sha(repo_path):
+    return subprocess.check_output(['git', '-C', repo_path, 'log', '-1', '--format=%H']).decode().rstrip()
+sha = common.git_sha(root_dir)
 release_dir = os.path.join(common.out_dir, 'release')
 release_zip_file = os.path.join(common.release_dir, 'lkmc-{}.zip'.format(common.sha))
 github_repo_id = 'cirosantilli/linux-kernel-module-cheat'