Update ChromeOS logging doc

Give more actionable commands to increase Chrome's log level on
ChromeOS.

Bug: None
Change-Id: I1efc1a01133258e4abec8ce27f2e79e2395d2e11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3853113
Reviewed-by: Marcello Salomao <msalomao@google.com>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1038577}

docs/chrome_os_logging.md

@@ -5,38 +5,90 @@
 Messages written via the logging macros in [base/logging.h] end up in different
 locations depending on Chrome's state:
 
-* `/var/log/ui/ui.LATEST` contains data written to stdout and stderr by Chrome
-  (and technically also [session_manager]). This generally comprises messages
-  that are written very early in Chrome's startup process, before logging has
-  been initialized.
-* `/var/log/chrome/chrome` contains messages that are written before a user has
-  logged in. It also contains messages written after login on test images, where
-  chrome runs with `--disable-logging-redirect`.
-* `/home/chronos/user/log/chrome` contains messages that are written while a
-  user is logged in on non-test images. Note that this path is within the user's
-  encrypted home directory and is only accessible while the user is logged in.
+`/var/log/ui/ui.LATEST`
+:   contains data written to stdout and stderr by Chrome (and technically also
+    [session_manager]). This generally comprises messages that are written very
+    early in Chrome's startup process, before logging has been initialized.
 
-All of the above files are actually symlinks. Older log files can be found
+`/var/log/chrome/chrome`
+:   contains messages that are written before a user has logged in. It also
+    contains messages written after login on test images, where Chrome runs with
+    `--disable-logging-redirect`.
+
+`/home/chronos/user/log/chrome`
+:   contains messages that are written while a user is logged in on non-test
+    images. Note that this path is within the user's encrypted home directory
+    and is only accessible while the user is logged in.
+
+`/var/log/audit/audit.log`:
+:   contains SECCOMP violation messages.
+
+`/var/log/messages`
+:   contains messages written by services such as `session_manager`,
+    `cryptohomed` and `cros-disks` that may be useful in determining when or why
+    Chrome started or stopped.
+
+Some of the above files are actually symlinks. Older log files can be found
 alongside them in the same directories.
 
-Also notable is `/var/log/messages`. This file contains general syslog messages,
-but it also includes messages written by `session_manager` that may be useful in
-determining when or why Chrome started or stopped.
+## How to increase Chrome's log level to `INFO` on a test device
 
-## Severity
+By default, only `WARNING`, `ERROR` and `FATAL` messages are written to disk,
+whereas `INFO` and `VERBOSE` messages are discarded.
 
-By default, only messages logged at severity `WARNING` or higher are written to
-disk. More concretely, `LOG(INFO)` messages are discarded.
-
-To enable `LOG(INFO)` message, pass `--log-level=0` to Chrome. See the
+To log `INFO` messages, pass `--log-level=0` to Chrome. See the
 [Passing Chrome flags from session_manager] document for more details, and
-specifically the `/etc/chrome_dev.conf` mechanism that can be used to
-change flags on development devices.
+specifically the `/etc/chrome_dev.conf` configuration file that can be used to
+change flags on test devices.
+
+Remount the root filesystem in read-write mode (to be able to modify
+`chrome_dev.conf`):
+
+```sh
+(dut)$ sudo mount -o remount,rw /
+```
+
+Add `--log-level=0` to `chrome_dev.conf`:
+
+```sh
+(dut)$ echo "--log-level=0" | sudo tee -a /etc/chrome_dev.conf > /dev/null
+```
+
+Restart Chrome:
+
+```sh
+(dut)$ sudo restart ui
+```
+
+Follow Chrome's logs:
+
+```sh
+(dut)$ tail -F /var/log/chrome/chrome
+```
 
 ## Verbose Logging
 
-When actively debugging issues, Chrome's `--vmodule` flag is sometimes used to
-temporarily log messages at lower severities for particular modules.
+When actively debugging issues, Chrome's `--vmodule` flag can be used to log
+verbose messages for particular modules.
+
+For example, to log `VERBOSE1` messages produced by `VLOG(1)` in
+`volume_manager.cc` or `volume_manager.h`:
+
+```sh
+(dut)$ echo "--vmodule=volume_manager=1" | sudo tee -a /etc/chrome_dev.conf > /dev/null
+```
+
+Restart Chrome:
+
+```sh
+(dut)$ sudo restart ui
+```
+
+Follow `volume_manager`'s logs:
+
+```sh
+(dut)$ tail -F /var/log/chrome/chrome | grep volume_manager
+```
 
 [base/logging.h]: ../base/logging.h
 [session_manager]: https://chromium.googlesource.com/chromiumos/platform2/+/main/login_manager/