<s>
vDSO	B-Application
(	O
virtual	B-Application
dynamic	I-Application
shared	I-Application
object	I-Application
)	O
is	O
a	O
kernel	O
mechanism	O
for	O
exporting	O
a	O
carefully	O
selected	O
set	O
of	O
kernel	B-Operating_System
space	I-Operating_System
routines	O
to	O
user	B-Operating_System
space	I-Operating_System
applications	O
so	O
that	O
applications	O
can	O
call	O
these	O
kernel	B-Operating_System
space	I-Operating_System
routines	O
in-process	O
,	O
without	O
incurring	O
the	O
performance	O
penalty	O
of	O
a	O
mode	O
switch	O
from	O
user	B-Operating_System
mode	I-Operating_System
to	O
kernel	O
mode	O
that	O
is	O
inherent	O
when	O
calling	O
these	O
same	O
kernel	B-Operating_System
space	I-Operating_System
routines	O
by	O
means	O
of	O
the	O
system	B-Operating_System
call	I-Operating_System
interface	O
.	O
</s>
<s>
vDSO	B-Application
uses	O
standard	O
mechanisms	O
for	O
linking	B-Application
and	O
loading	B-Operating_System
i.e.	O
</s>
<s>
vDSO	B-Application
is	O
a	O
memory	O
area	O
allocated	O
in	O
user	B-Operating_System
space	I-Operating_System
which	O
exposes	O
some	O
kernel	O
functionalities	O
.	O
</s>
<s>
vDSO	B-Application
is	O
dynamically	B-Application
allocated	I-Application
,	O
offers	O
improved	O
safety	O
through	O
address	O
space	O
layout	O
randomization	O
,	O
and	O
supports	O
more	O
than	O
four	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Some	O
C	B-Language
standard	I-Language
libraries	I-Language
,	O
like	O
glibc	B-Language
,	O
may	O
provide	O
vDSO	B-Application
links	O
so	O
that	O
if	O
the	O
kernel	O
does	O
not	O
have	O
vDSO	B-Application
support	O
,	O
a	O
traditional	O
syscall	B-Operating_System
is	O
made	O
.	O
</s>
<s>
vDSO	B-Application
helps	O
to	O
reduce	O
the	O
calling	O
overhead	O
on	O
simple	O
kernel	O
routines	O
,	O
and	O
it	O
also	O
can	O
work	O
as	O
a	O
way	O
to	O
select	O
the	O
best	O
system-call	O
method	O
on	O
some	O
computer	B-General_Concept
architectures	I-General_Concept
such	O
as	O
IA-32	B-Device
.	O
</s>
<s>
Implementation	O
generally	O
implies	O
hooks	O
in	O
the	O
dynamic	O
linker	B-Application
to	O
find	O
the	O
vDSOs	O
.	O
</s>
<s>
vDSO	B-Application
was	O
developed	O
to	O
offer	O
the	O
vsyscall	O
features	O
while	O
overcoming	O
its	O
limitations	O
:	O
a	O
small	O
amount	O
of	O
statically	B-General_Concept
allocated	I-General_Concept
memory	O
,	O
which	O
allows	O
only	O
four	O
system	B-Operating_System
calls	I-Operating_System
,	O
and	O
the	O
same	O
addresses	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
(	O
ABI	O
)	O
in	O
each	O
process	O
,	O
which	O
compromises	O
security	O
.	O
</s>
<s>
This	O
security	O
issue	O
has	O
been	O
mitigated	O
by	O
emulating	O
a	O
virtual	O
system	B-Operating_System
call	I-Operating_System
,	O
but	O
the	O
emulation	O
introduces	O
additional	O
latency	O
.	O
</s>
