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

[Chandler Carruth]

On Tue, Jun 19, 2012 at 10:46 PM, Kostya Serebryany <kcc at google.com> wrote:

>
>
> On Wed, Jun 20, 2012 at 9:39 AM, Chandler Carruth <chandlerc at google.com>wrote:
>
>> On Tue, Jun 19, 2012 at 9:07 PM, Kostya Serebryany <kcc at google.com>wrote:
>>
>>> +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.
>>>
>>
>> Well, I can't be sure, but this description certainly agrees with my
>> understanding -- you need *every* part of the runtime to be completely
>> separate from *every* part of the instrumented binary. I'm with you there.
>>
>> In particular, I think the current strategy for libc & system calls makes
>> perfect sense, and I'm not trying to suggest changing it.
>>
>> I think the most similar situation is is this one:
>>
>> In the previous version of ThreadSanitizer we used a private copy of
>>> STLport in a separate namespace and a custom libc (small subset).
>>>
>>
>> My proposal is very similar except without the need to modify the C++
>> standard library in use. Instead, I'm suggesting post-processing the
>> library to ensure that the standard C++ library code in the runtime is kept
>> complete distinct from that in the instrumented binary -- everything would
>> in fact be *mangled* differently.
>>
>> The goal would be to avoid the maintenance overhead of a custom C++
>> standard library, and instead use a normal one. My understanding is that
>> both GCC's libstdc++ and LLVM's libc++ are significantly higher quality
>> than STLport, and if we're doing static linking, the code bloat should be
>> greatly reduced. We could reduce it still further by doing LTO of the
>> runtime library, which should be very straight forward given the rest of my
>> proposal.
>>
>> It would still require a very small subset of libc, likely not much more
>> than you already have.
>>
>>  This worked, but had problems too (Dmitry was very angry at STLport for
>>> code bloat, stack size increase and some direct libc calls).
>>>
>>
>> I would be interested to know if the above addresses most of the problems
>> or not.
>>
>>
>>>  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)
>>>
>>
>> I think my proposal is essentially in between these two:
>>
>> - Avoid the need for a low quality STL by using a normal C++ standard
>> library implementation, and avoid maintenance burden by doing a link-time
>> mangling of the symbols.
>>
>
> re-linking might be too platform specific.
> How about compiling the library into LLVM bitcode and adding
> namespaces/prefixes to that bitcode?
>

Re-linking is a bit platform specific...

It would definitely work on ELF platforms, and likely on Darwin, but
Windows is tricky.

On windows we would at least need a custom tool, but such a tool would be
quite easy to write I suspect. We could even use the very LLVM libraries in
question to write it! ;] Amusingly, I think with the LLVM libraries we
could very easily write a custom tool just to mangle the symbol names in a
collection of object files very easily and have it work on *most* platforms!

Still, the bitcode idea is interesting. Doing this entirely in bitcode has
some advantages as these types of runtimes are among the best uses for
things like LTO: they're small, performance sensitive, can enumerate the
entry points easily, and are likely to have a particular need for dead code
elimination.

One nice thing is that I suspect we could do any of these three options,
and get equivalent output for them. It may not matter what strategy is used
long term, we can use the easiest to implement short term.


>
> --kcc
>
>
>
>> - Provide the minimal custom libc, and do the same to it
>> - Link the LLVM libraries against these, and munge their symbols as well
>> - LTO the whole thing if needed to get the code bloat down
>>
>> I think this is actually easier than changing the LLVM libraries to not
>> use the C++ standard libraries. I also think it is easier than
>> re-implementing the LLVM libraries in question. But that doesn't mean I
>> think it is easy. ;] I think it is quite hard, but it is the best solution
>> I can come up with.
>>
>>
>>>
>>> --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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