Adding unwinding support for x86_64_fortanix_unknown_sgx target.

[VardhanThigle]

Unwinding support is provided by our port of LLVM's libunwind which is available from https://github.com/fortanix/libunwind/tree/release_50.

libunwind requires support for rwlock and printing to stderr, which is only provided by std for this target. This poses two problems: 1) how to expose the std functionality to C and 2) dependency inversion.

Exposing std

For exposing the functionality we chose to expose the following symbols:

• __rust_rwlock_rdlock
• __rust_rwlock_wrlock
• __rust_rwlock_unlock
• __rust_print_err
• __rust_abort

Also, the following are needed from alloc:

• __rust_alloc
• __rust_dealloc

Rust RWLock in C

In libunwind, RWLock is initialized as a templated static variable:

pthread_rwlock_t DwarfFDECache<A>::_lock = PTHREAD_RWLOCK_INITIALIZER;

I don't know of a good way to use the Rust sys::rwlock::RWLock type and initializer there. We could have a static global variable in Rust, but that doesn't work with the templating. The variable needs to be initialized statically, since this target doesn't support the .init section. Currently, I just used a byte array and standard C array initialization. The mapping between this C type and the Rust type needs to be manually maintained. There is a compile-time check and a unit test to make sure the Rust versions of these C definitions match the actual Rust type. If any reviewer knows of a better solution, please do tell.

Dependency inversion issue

std depends on panic_unwind which depends on libunwind, and libunwind depends on std. This is not normally supported by Rust's linking system. Therefore we use raw C exports from std and libunwind.a is linked last in the target post_link_objects instead of being built as part of the Rust libunwind. Currently, all C exports are defined in src/libstd/sys/sgx/rwlock.rs to overcome LTO issues. Only the __rust_rwlock_* definitions need to live there for privacy reasons. Once again, if any reviewer knows of a better solution, please do tell.

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:04ec8e44:start=1545222668182686871,finish=1545222768251413858,duration=100068726987
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---

[00:03:01] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:01] tidy error: /checkout/src/libstd/sys/sgx/rwlock.rs:228: trailing whitespace
[00:03:02] some tidy checks failed
[00:03:02] 
[00:03:02] 
[00:03:02] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:03:02] 
[00:03:02] 
[00:03:02] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:02] Build completed unsuccessfully in 0:00:43
[00:03:02] Build completed unsuccessfully in 0:00:43
[00:03:02] Makefile:79: recipe for target 'tidy' failed
[00:03:02] make: *** [tidy] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1bc56118
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Dec 19 12:35:59 UTC 2018
---
travis_time:end:17350ff4:start=1545222959795432139,finish=1545222959799395270,duration=3963131
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:06f78d38
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:193df331
travis_time:start:193df331
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:06a4dce4
$ dmesg | grep -i kill

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[alexcrichton]

@bors: r+

[bors]

📌 Commit 885cf2a has been approved by alexcrichton

[alexcrichton]

@bors: rollup