Thanks for your replies. However, I am now thinking of another solution.
Let's say I can capture write() in the userspace by either instrumenting the LibC or LD_PRELOAD wrapper and store the string buffer passed to write().
Can I call/generate *some other non-instrusive* syscall which can take that string buffer that I stored earlier and that syscall with the buffer will be visible in the audit.log? I am not worried about performance hit right now.

I'm not 100% sure, but you might be able to do this with LandLock.

On Thu, Apr 26, 2018 at 7:46 PM Casey Schaufler <casey@schaufler-ca.com> wrote:

On 4/26/2018 5:08 PM, Sargun Dhillon wrote:
> On Thu, Apr 26, 2018 at 4:40 PM, Casey Schaufler <casey@schaufler-ca.com> wrote:
>> On 4/26/2018 3:57 PM, Steve Grubb wrote:
>>> On Thu, 26 Apr 2018 20:34:57 +0000
>>> Wajih Ul Hassan <wajih.lums@gmail.com> wrote:
>>>
>>>> Hi all,
>>>>  .....
>> You could write a Linux Security Module (LSM) to monitor the
>> content of writes. The performance impact would be rather
>> amazing.
>>
> I would recommend using BPF + kprobes + perf_event buffers for this
> purpose. There are enough places you can probe to grab these strings
> in the kernel, and if you do your filtering in BPF, you can then push
> it into kernel space based on filtering. Although, AFAIK, the BPF JITs
> don't do vectorization of instructions, but it's still not too bad. If
> you put your kprobe on the syscall itself, and probe the userspace
> addr, remember you're going to be open to a time-of-use, time-of-check
> style attack.

That looks like a whole lot of mechanism to perform a simple task.