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/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.
INFO on a test deviceBy default, only WARNING, ERROR and FATAL messages are written to disk, whereas INFO and VERBOSE messages are discarded.
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 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):
(dut)$ sudo mount -o remount,rw /
Add --log-level=0 to chrome_dev.conf:
(dut)$ echo "--log-level=0" | sudo tee -a /etc/chrome_dev.conf > /dev/null
Restart Chrome:
(dut)$ sudo restart ui
Follow Chrome's logs:
(dut)$ tail -F /var/log/chrome/chrome
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:
(dut)$ echo "--vmodule=volume_manager=1" | sudo tee -a /etc/chrome_dev.conf > /dev/null
Restart Chrome:
(dut)$ sudo restart ui
Follow volume_manager's logs:
(dut)$ tail -F /var/log/chrome/chrome | grep volume_manager