<s>
kexec	B-Application
,	O
abbreviated	O
from	O
kernel	B-Operating_System
execute	O
and	O
analogous	O
to	O
the	O
Unix/Linux	O
kernel	B-Operating_System
call	O
exec	B-Operating_System
,	O
is	O
a	O
mechanism	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
that	O
allows	O
booting	O
of	O
a	O
new	O
kernel	B-Operating_System
from	O
the	O
currently	O
running	O
one	O
.	O
</s>
<s>
Essentially	O
,	O
kexec	B-Application
skips	O
the	O
bootloader	B-Application
stage	O
and	O
hardware	B-Architecture
initialization	O
phase	O
performed	O
by	O
the	O
system	O
firmware	B-Application
(	O
BIOS	B-Operating_System
or	O
UEFI	B-Architecture
)	O
,	O
and	O
directly	O
loads	O
the	O
new	O
kernel	B-Operating_System
into	O
main	O
memory	O
and	O
starts	O
executing	O
it	O
immediately	O
.	O
</s>
<s>
While	O
feasible	O
,	O
implementing	O
a	O
mechanism	O
such	O
as	O
kexec	B-Application
raises	O
two	O
major	O
challenges	O
:	O
</s>
<s>
Memory	O
of	O
the	O
currently	O
running	O
kernel	B-Operating_System
is	O
overwritten	O
by	O
the	O
new	O
kernel	B-Operating_System
,	O
while	O
the	O
old	O
one	O
is	O
still	O
executing	O
.	O
</s>
<s>
The	O
new	O
kernel	B-Operating_System
will	O
usually	O
expect	O
all	O
hardware	B-Architecture
devices	O
to	O
be	O
in	O
a	O
well	O
defined	O
state	O
,	O
in	O
which	O
they	O
are	O
after	O
a	O
system	O
reboot	O
because	O
the	O
system	O
firmware	B-Application
resets	O
them	O
to	O
a	O
"	O
sane	O
"	O
state	O
.	O
</s>
<s>
Bypassing	O
a	O
real	O
reboot	O
may	O
leave	O
devices	O
in	O
an	O
unknown	O
state	O
,	O
and	O
the	O
new	O
kernel	B-Operating_System
will	O
have	O
to	O
recover	O
from	O
that	O
.	O
</s>
<s>
Support	O
for	O
allowing	O
only	O
signed	O
kernels	B-Operating_System
to	O
be	O
booted	O
through	O
kexec	B-Application
was	O
merged	O
into	O
version	O
3.17	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
mainline	O
,	O
which	O
was	O
released	O
on	O
October	O
5	O
,	O
2014	O
.	O
</s>
<s>
This	O
disallows	O
a	O
root	B-Application
user	I-Application
to	O
load	O
arbitrary	O
code	O
via	O
kexec	B-Application
and	O
execute	O
it	O
,	O
complementing	O
the	O
UEFI	B-Architecture
secure	O
boot	O
and	O
in-kernel	O
security	O
mechanisms	O
for	O
ensuring	O
that	O
only	O
signed	O
Linux	B-Application
kernel	I-Application
modules	I-Application
can	O
be	O
inserted	O
into	O
the	O
running	O
kernel	B-Operating_System
.	O
</s>
<s>
Kexec	B-Application
is	O
used	O
by	O
LinuxBoot	B-Device
to	O
boot	O
the	O
main	O
kernel	B-Operating_System
from	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
located	O
in	O
the	O
firmware	B-Application
.	O
</s>
