[LLVMdev] Intercepting dlinfo in memory sanitizer

[Evgeniy Stepanov]

On Thu, Apr 9, 2015 at 9:24 PM, Keno Fischer
<kfischer at college.harvard.edu> wrote:
> Hi everyone,
>
> I ran into a false positive with memory sanitizer due to it not intercepting
> dlinfo. I tried to get started on writing such an interceptor, but dlinfo
> seems like an extraordinarily difficult function to intercept. The three
> considerations that I would like somebody to look at are:
>
> 1) How do we get the enum values to decide what kind of pointer dlinfo is
> getting. Ideally we'd include dlfcn.h, but the interceptor file doesn't
> generally include system headers. What's the right thing to do here?

We generally redefine system constants and types in
sanitizer_platform_limits_posix.h and verify correctness with
assertions in .cc.

> 2) The structures pointed to contain linked lists, etc. of dynamic linker
> state. In order to avoid spurious failures, it seems like we should iterate
> the linked lists and unpoison all of them, but that seems expensive. Do you
> see a better solution?

Nope, sounds like that's want you need to do.

> 3) The return values include pointers into the object files. It seems
> perfectly reasonable for a user to want to read these object files based on
> information from dlinfo. That probably means we should unpoison them as
> well? Do we need to do something here or are these already unpoisoned by
> some other mechanism (e.g. because msan realizes they are part of a loaded
> object file or unposions them on load - does this happen?).

They are unpoisoned. All memory defaults to unpoisoned (shadow == 0),
and we take care of unpoisoning all memory we return to the OS.

>
> Thanks,
> Keno
>
>