[LLVMdev] RFC: How can AddressSanitizer, ThreadSanitizer, and similar runtime libraries leverage shared library code?

[Kostya Serebryany]

+dvyukov

On Wed, Jun 20, 2012 at 7:12 AM, Chandler Carruth <chandlerc at google.com>wrote:

> Hello folks (and sorry if I've forgotten to CC anyone with particular
> interest to this discussion...):
>
> I've been thinking a lot about how best to build advanced runtime
> libraries like ASan, and scale them up. Note that this does *not* try to
> address any licensing issues. For now, I'll consider those orthogonal /
> solvable w/o technical contortions. =]
>
> My primary motivation: we really, *really* need runtime libraries to be
> able to use common, shared libraries.
>

I am not sure you understand the problem as we do.

In short, asan/tsan/msan/etc can not use any function which is also called
from the instrumented binary.
E.g. it can not use malloc() for internal allocations because malloc is
intercepted/replaced. We use raw mmap.
It can not use functions like strlen, memset, etc, because those functions
generate memory access events. We use our own implementations or sometimes
steal them from libc using dlsym.
Ideally, asan/etc should not even use libc functions like read() -- on
linux we currently use raw system call for some of those.

In Valgrind, they struggled with the same problem and made 2 or 3 attempts
to reuse the system libc.
Every time it ended with a maintenance nightmare; so currently valgrind has
its own private subset of libc.
In PIN, they have a private copy of system libc/libstdc++ and, afaict, it
is constantly causing pain for the maintainers.
In the previous version of ThreadSanitizer we used a private copy of
STLport in a separate namespace and a custom libc (small subset). This
worked, but had problems too (Dmitry was very angry at STLport for code
bloat, stack size increase and some direct libc calls).

Until recently this was not causing too much pain in asan/tsan, but our
attempts to use the LLVM DWARF readers made it worse.
When tsan finds a race, we need to symbolize it online to be able to match
against a suppression and decide whether we want to emit the warning. Today
we do it in a separate addr2line process (ugly and slow).
But if we start calling the LLVM dwarf reader we end up with all possible
dependency problems (Dmitry and Alexey will know the exact ones) because
the LLVM code calls to malloc, memcpy, etc.

Frankly, I don't have any solution other than to change the code such that
it does not call libc/libc++.
Some of that may be solved by a private copy of STLport + a bit of custom
libc (but see above about STLport)

--kcc



> This starts with libraries such as the C++ standard library -- a runtime
> shouldn't need to re-implement std::vector. It includes other primitive
> libraries that have had significant effort put into them in LLVM such as
> the ADT and Support libraries. But, IMO, it has even more importance as we
> start looking at libraries such as ELF readers, DWARF readers, symbolizers,
> etc. This code should shared, and shared easily, with other LLVM projects.
>
> However, clearly the runtime must at some point be linked against a
> program, and indeed programs which may be using *the same set of
> libraries*. It is crucially important that the runtime uses a separate
> implementation of the libraries from the ones used by the program itself:
> we will often compile the program's libraries with instrumentation and
> other features which we explicitly wish to avoid in the runtime. Even
> simple name clashes can cause problems, leading to the current practice of
> putting all of these runtime libraries into a '__sanitizer' or other
> specially spelled namespace.
>
> A final unusual requirement is that at least *some* of the code for the
> runtime libraries must be statically linked to have reasonable efficiency.
> We also have several use cases where it would be very convenient to link
> *all* of the runtime statically, so I prefer a solution that preserves this
> option.
>
> So how can we effectively share code? Here is my proposal, and a few
> alternate strategies.
>
> I suggest that we build the runtime library as-if it were not a runtime
> library at all, and just a normal library. No strange namespaces, no
> restrictions on what other libraries it uses with one exception: they must
> *all* be statically linkable. We build this as a normal archive library,
> nothing special. One nice property is that testing the runtime library
> becomes the same as testing any other library.
>
> Then, we have a special build step to produce a final archive which is
> actually *used* as the runtime library. This step works not dissimilarly to
> the step to link an executable: we build the list of archive libraries
> depended on, but instead of linking an executable, we run a linker script
> over them. This script will re-link each '.o' file from the transitive
> closure of archives, prepending a '__asan__' (or other runtime library
> prefix) onto each symbol; effectively mangling each symbol. All of these
> processed '.o' files would go into a single, final archive that would be
> the installed runtime library. The only functions not processed in this
> manner are a white list of "exported" functions from the runtime (C-library
> routines provided by the runtime, and runtime entry points, et.).
>
> The result should be a runtime library that is essentially hermetic, and
> should have no clashes with binaries it links against. It would be free to
> use standard libraries, LLVM libraries, whatever it needs. That said, there
> are some clear disadvantages:
> - Bizarre name mangling, especially for C++
> - Potentially incompatible with C++ EH, libunwind, or other tools (I just
> don't know, haven't done enough research here)
> - Requires "relinking" the final runtime
> - Definitely implementable on Linux & ELF-based BSDs, I *think* do-able on
> Darwin, but I have no idea about Windows.
> - Other downsides? I'm probably missing some big problems here... ;]
>
> However, if we can make this (possibly with tweaks/modifications) work, I
> think the upside is quite large -- the runtime library stops having to be
> written in such a strange special sub-set of the language, etc.
>
>
> Note that this proposal is orthogonal to the issue of minimizing the
> binary size and cost of the runtime library -- that is clearly still an
> important concern, but that can be addressed both with or without using
> other libraries. LLVM has lots of libraries specifically engineered to be
> lightweight in situations like this.
>
>
> Other alternatives that have been discussed:
>
> - Require isolating all shared code into a shared library (.so) than is
> loaded as-needed. This helps some, but it doesn't seem to fully solve the
> issues (where does the shared code go? the .so? What happens when it is
> loaded into a program that already has copies of the same code? What
> happens when one is instrumented and the other isn't). It also requires us
> to ship the '.so' with the binary to get full functionality, something that
> would be at least somewhat undesirable. It also requires the runtime
> library developers to carefully partition the code into that which can go
> in the .a and that which can go in the .so.
>
> - The current strategy of re-implementing everything needed from
> (essentially) the ground up inside the runtime library. I think that this
> has serious long-term maintenance problems.... but who knows, maybe?
>
> - Other ideas?
>
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