[LLVMdev] Error handling in LLVMObject library

[Alexey Samsonov]

Hi everyone,

Having proper error handling in LLVM's Object parsing library is a nice
thing by itself, but it would additionally allow us to find bugs by fuzzing
(see r238451 that adds llvm-dwarfdump-fuzzer tool), for which the clean
input validation is essential.

This is a generic discussion of state of affairs. I want to do some
progress in fuzzing before we finish it (especially if we decide to make a
significant intrusive changes), you may scroll down for my plan.

The code in lib/Object calls report_fatal_error() far too often, both when
we're (a) just constructing the specific implementation of ObjectFile, and
(b) when we access its contents and find out the file is broken and can't
be parsed properly.

We should just go and fix (a): ObjectFile factory methods return
ErrorOr<std::unique_ptr<ObjectFile>>, and we should propagate the error
appropriately.

(b) is harder. The current approach is to use std::error_code as a return
type, and store the result in by-reference argument, for instance:
  std::error_code getSymbolAddress(DataRefImpl Symbol, uint64_t &Res);

I wanted to follow this approach in a proposed large MachO API change
(http://reviews.llvm.org/D10111), but it raised discussion on whether this
approach is right.
Moving this discussion here. I see the following options:

1. Use the current approach:
  std::error_code getSymbolAddress(DataRefImpl Symbol, uint64_t &Res);

Pros:
  * don't need to change a large number of (often virtual) API functions
  * has a nice error handling pattern used in LLVM tools:
  uint64_t Addr;
  if (error(getSymbolAddress(Symbol, Addr)))
    return;  // or continue, or do anything else.

Cons:
  * return value can just be silently ignored. Adding warn_unused_result
attribute on per-function basis is ugly
  * adds extra overhead for cases when we're certain the result would be
valid.

2. Switch to ErrorOr wrapper:
  ErrorOr<uint64_t> getSymbolAddress(DataRefImpl Symbol);

Pros:
  * handling the error is now mandatory and explicit.
  * callers can explicitly skip error handling if they know the result
would be valid:
    uint64_t Addr = getSymbolAddress(Symbol).get();
  and it would fail the assert if they are wrong.

Cons:
  * need to change lots of old code, or live with two versions of functions
  * error handling boilerplate in regular code on call site is ugly:
  auto AddrOrErr = getSymbolAddress(Symbol);
  if (AddrOrErr.hasError())
    return;  // or continue, or whatever
  uint64_t Addr = AddrOrErr.get();
  (can probably be improved with a macro)
  * adds extra overhead for cases when we're certain the result would be
valid.

On IRC discussion Lang suggested
3. Check the whole object file contents in constructor or validate()
method, and get rid
of all error codes in regular accessors.

Pros:
  * the interface is much cleaner
  * no extra overhead for trusted (e.g. JIT) object files.

Cons:
  * significant change, fundamentally shifting the way object files are
handled
  * verifier function should now absolutely everything about the object
file, and anticipate all possible use cases. Especially hard, assuming that
ObjectFile interface allows user to pass any garbage as input arguments
(e.g. as DataRefImpl in the example above).
  * verifier can be slow, and might be an overkill if we strongly desire to
parse some bits of object file lazily.

================

Instead of http://reviews.llvm.org/D10111, I'm going to proceed with
minimal incremental changes, that would allow fuzzer to move forward.
Namely, I want to keep the changes to headers as small as possible,
changing functions one by one, and preferring to use ErrorOr<> construct
(option 2 listed above). An additional benefit of this is that each small
incremental change would be accompanied by the test case generated by
fuzzer, that exposed this problem.

Let me know if you think it's a good or terrible idea.

-- 
Alexey Samsonov
vonosmas at gmail.com
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