[NaCl SDK] nacl_io: Fix select() implementation so it doesn't assume entire fs_sets.

[NaCl SDK] nacl_io: Fix select() implementation so it doesn't assume entire fd_sets.

Turns out the application code (specifically openssh does
this) is allows to pass incomplete fd_set structs as long
as they can store at least nfds worth of information.
This means that we can't call FD_ZERO on the pointers
given to use since FD_ZERO assumes a full fd_set.

This fixed a long standing memory corruption issue with
openssh.

R=binji@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=262849

[binji, 2014-04-09 00:47:45]

lgtm, though you should add a test for this behavior.

[Sam Clegg, 2014-04-09 00:52:21]

On 2014/04/09 00:47:45, binji wrote:
> lgtm, though you should add a test for this behavior.

Really?  Writing tests for every function to test for memory corruption around
its inputs seems like a very high bar for unit tests.  Isn't that more the job
of asan/valgrind and friends?

[binji, 2014-04-09 00:57:52]

On 2014/04/09 00:52:21, Sam Clegg wrote:
> On 2014/04/09 00:47:45, binji wrote:
> > lgtm, though you should add a test for this behavior.
> 
> Really?  Writing tests for every function to test for memory corruption around
> its inputs seems like a very high bar for unit tests.  Isn't that more the job
> of asan/valgrind and friends?

Not for every function, just this one.

As you've described it, this is special, non-obvious behavior that we should
make sure doesn't regress. At the very least we should have a comment about it.

[Sam Clegg, 2014-04-09 21:03:11]

Now with test!

[binji, 2014-04-09 21:26:21]

slgtm

https://codereview.chromium.org/229863002/diff/20001/native_client_sdk/src/te...
File native_client_sdk/src/tests/nacl_io_test/event_test.cc (right):

https://codereview.chromium.org/229863002/diff/20001/native_client_sdk/src/te...
native_client_sdk/src/tests/nacl_io_test/event_test.cc:318: * We had an issue
when select() was calling FD_ZERO() on the incoming fs_sets
sp: fd_sets

https://codereview.chromium.org/229863002/diff/20001/native_client_sdk/src/te...
native_client_sdk/src/tests/nacl_io_test/event_test.cc:319: * which was causing
curruption in ssh which always allocates the fs_sets to be
sp: corruption

[Sam Clegg, 2014-04-09 23:09:54]

Committed patchset #3 manually as r262849 (presubmit successful).