Regarding getrandom syscall

[Avantika Rawat]

Hi,

Thanks for the suggestion but i am getting following error

 error: 'SYS_getrandom' undeclared (first use in this function)

then i have replaced the SYS_getrandom by 275 in syscall(275, -- args--);
as 275 is the  __NR_getrandom defines in unistd.h file,  but it is
returning -1.
Can somebody tell me what i am missing here??

 . Also in /proc/kallsyms it is showing two syscalls added for getrandom

ffffffff80417a38 T SyS_getrandom
ffffffff80417a38 T sys_getrandom

On Sat, Jun 18, 2016 at 7:12 PM, Sayutin Dmitry <cdkrot at yandex.ru> wrote:
> Well, in fact it is not hard.
>
> Just use syscall(2) provided by libc.
>
> You need to provide to this function syscall id and syscall args.
> Syscall id can be retrieved from macro constant
> Should look something like:
>
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <sys/syscall.h>
>
> syscall(SYS_getrandom, -- your - args - here --);
>
> It returns long value - the result of syscall.
> If it is between [-4095; -1] then it is negated errno, otherways it is return value.
>
>
> 18.06.2016, 16:32, "Anoop" <anoop.chargotra at gmail.com>:
>> Hi Avantika,
>>
>> On Sat, Jun 18, 2016 at 5:32 PM, Avantika Rawat <avani.rawat at gmail.com> wrote:
>>>  Hi ALL,
>>>
>>>  I am trying to use getrandom syscall in kernel 3.10.20 by following this
>>>  link
>>>
>>>  https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c6e9d6f38894798696f23c8084ca7edbf16ee895
>>>
>>>  i have compiled the kernel and now want to call the getrandom syscall from
>>>  userspace to generate random numbers. But i am getting following error while
>>>  calling the getrandom () from userspace.
>>>
>>>  (.text.startup+0x18): undefined reference to `getrandom'
>>>  (.text.startup+0x1c): undefined reference to `getrandom'
>>
>> Your user space program will not know where 'getrandom' is defined
>> unless it's in the C library. You need to research more on how to call
>> custom system calls.
>>
>> -Anoop
>>
>>>  --
>>>  Regards,
>>>  Avantika Rawat
>>>
>>>  _______________________________________________
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>>
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>
> -----
> Sayutin Dmitry <cdkrot at yandex.com>



-- 
Regards,
Avantika Rawat