another question - VM mappings

[Oliver Pinter]

On Mon, Apr 13, 2015 at 1:34 PM, Kimmo Paasiala <kpaasial at gmail.com> wrote:
> On Mon, Apr 13, 2015 at 2:25 PM, Oliver Pinter
> <oliver.pinter at hardenedbsd.org> wrote:
>> Under amd64 exists an so called shared-page - similar to linux's vdso
>> mechanism - https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_exec.c#L1045
>> .
>>
>> This page is double mapped with user-space and with kernel. From
>> security reason it's only RO, otherwise, when mapped with RW or RWX,
>> then you could write kernel memory from user-space.
>>
>> On Mon, Apr 13, 2015 at 9:31 AM, Wojciech Puchar <wojtek at puchar.net> wrote:
>>> below is mapping for very simple process (no libc etc)
>>>
>>> [wojtek at laptop ~]$ procstat -v 917
>>>   PID              START                END PRT  RES PRES REF SHD   FL TP
>>> PATH
>>>   917           0x400000           0x401000 r-x    1    0   1   0 CN-- vn
>>> /home/wojtek/test/1
>>>   917           0x600000           0x601000 rw-    1    0   1   0 ---- df
>>>   917     0x7ffffffdf000     0x7ffffffff000 rw-    1    0   1   0 ---D df
>>>   917     0x7ffffffff000     0x800000000000 r-x    0    0  39   0 ---- ph
>>>
>>>
>>> what is "ph" mapping and why read&executable?
>> _______________________________________________
>
> What is the function is this page, why is it there for every process?

The rationale behind is to speed up things. With the shared page, the
system able to eliminate some syscall overhead especially gettimeofday
family of syscalls.
The shared-page currently used only to hold time specific informations
( kernel part: https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_sharedpage.c
libc part: https://github.com/freebsd/freebsd/blob/master/lib/libc/sys/__vdso_gettimeofday.c
).
With these solution the system able to get the current time with a
"simple" memory read, rather the a syscall.


>
> -Kimmo