os x: re-enable PIE (ASLR)

[bnoordhuis]

Commit a5012a0 disables PIE (and therefore ASLR) on OS X because it breaks profiling of C++ code. Ideally, we'd figure out a way to keep it turned on except when -prof is specified on the command line.

I believe the only way to do that (except for having two separate binaries, which I don't think we want) is to re-exec the process with the _POSIX_SPAWN_DISABLE_ASLR (256) flag set. The flag is ignored for setuid/setgid binaries so in that respect -Wl,-no_pie is superior.

[bnoordhuis]

#6468

[indutny]

@bnoordhuis I wonder how lldb starts the process in such way that it is still able to resolve symbols.

[bnoordhuis]

@indutny In exactly the same way as the PR. :-)

[indutny]

While this is simple enough, lldb also works when attaching to existing process. I wonder if the image ranges reported by _dyld_get_image_header() may be used to determine ASLR shift in profiler.

[bnoordhuis]

I've been investigating that but it's not exactly trivial: we'd first need to teach the profiler about the VM slide for each shared object and every third-party profiling tool would have to duplicate the effort.

[indutny]

@bnoordhuis the reason why I ask this, is that with your patch one won't be able to start the profiler at runtime, and this is certainly desirable in some environments.

If the tooling needs to break, then we probably need to keep it -no_pie for v6, and break the things in v7. Sliding all symbols by fixed value in tools/tickprocessor.js doesn't seem to be complicated at all.

Will open an alternative PR in a bit.