Re: [PATCH 0/7] Uprobes: Support SDT markers having reference count (semaphore)

From: Ravi Bangoria
Date: Mon Jun 11 2018 - 00:32:26 EST

Hi Masami,

>>> Hmm, it sounds simple... maybe we can increment refctr in install_breakpoint/
>>> remove_breakpoint?
>> Not really, it would be simpler if I can put it inside install_breakpoint().
>> Consider an mmap() case. Probed instruction resides in the text section whereas
>> reference counter resides in the data section. These sections gets mapped using
>> separate mmap() calls. So, when process mmaps the text section we will change the
>> instruction, but section holding the reference counter may not have been mapped
>> yet in the virtual memory. If so, we will fail to update the reference counter.
> Got it.
> In such case, maybe we can hook the target page mmapped and do install_breakpoint()
> at that point. Since the instruction is protected by a refctr, unless mmap the
> page on where the refctr is, the program doesn't reach the tracepoint. Is that right?

You mean, when mmap(text) happens, save the target page somewhere and when
mmap(data) happens, update both instruction and ref_ctr?

This sounds feasible. Let me think on it.

Thanks for suggestion,