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kexec-kdump-howto: Add note on setting correct value of kptr_restrict 

kdumpctl fails to load a crash kernel if KASLR is enabled.

The incorrect configuration of the kptr_restrict parameter causes that
the kdumpctl fails to load a crash kernel. The problem occurs when the
Kernel Address Space Layout Randomization (KASLR) mechanism is enabled.
The value of kptr_restrict decides how to expose the content of /proc
files. Inappropriate setting will cause that the /proc/kallsyms file is
being printed as all zeros. This configuration will break the kdump
loading process since it needs access to the conntent of /proc/kallsyms
if KASLR is enabled.

Set kptr_restrict to 1 to avoid the problem described above.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>
This commit is contained in:
Baoquan He 2019-03-26 16:33:18 +08:00 committed by Kairui Song
parent a53be67929
commit 50aa6dea24
1 changed files with 6 additions and 0 deletions
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kexec-kdump-howto.txt
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@ -91,6 +91,12 @@ This line tells kexec to reserve 64M of ram if the system contains between
512M and 2G of physical memory. If the system contains 2G or more of physical
memory, 128M should be reserved.
Besides, since kdump needs to access /proc/kallsyms during a kernel
loading if KASLR is enabled, check /proc/sys/kernel/kptr_restrict to
make sure that the content of /proc/kallsyms is exposed correctly.
We recommend to set the value of kptr_restrict to '1'. Otherwise
capture kernel loading could fail.
After making said changes, reboot your system, so that the X MB of memory is
left untouched by the normal system, reserved for the capture kernel. Take note
that the output of 'free -m' will show X MB less memory than without this