BoehmGCStackAllocator: ignore stack protection page

This fixes a crash that looks like:

```
Thread 1 "nix-build" received signal SIGSEGV, Segmentation fault.
0x00007ffff7ad22a0 in GC_push_all_eager () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
(gdb) bt
0  0x00007ffff7ad22a0 in GC_push_all_eager () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
1  0x00007ffff7adeefb in GC_push_all_stacks () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
2  0x00007ffff7ad5ac7 in GC_mark_some () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
3  0x00007ffff7ad77bd in GC_stopped_mark () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
4  0x00007ffff7adbe3a in GC_try_to_collect_inner.part.0 () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
5  0x00007ffff7adc2a2 in GC_collect_or_expand () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
6  0x00007ffff7adc4f8 in GC_allocobj () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
7  0x00007ffff7adc88f in GC_generic_malloc_inner () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
8  0x00007ffff7ae1a04 in GC_generic_malloc_many () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
9  0x00007ffff7ae1c72 in GC_malloc_kind () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
10 0x00007ffff7e003d6 in nix::EvalState::allocValue() () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
11 0x00007ffff7e04b9c in nix::EvalState::callPrimOp(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
12 0x00007ffff7e0a773 in nix::EvalState::callFunction(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
13 0x00007ffff7e0a91d in nix::ExprApp::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
14 0x00007ffff7e0a8f8 in nix::ExprApp::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
15 0x00007ffff7e0e0e8 in nix::ExprOpNEq::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
16 0x00007ffff7e0d708 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
17 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
18 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
19 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
20 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
21 0x00007ffff7e09e19 in nix::ExprOpNot::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
22 0x00007ffff7e0a792 in nix::EvalState::callFunction(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
23 0x00007ffff7e8cba0 in nix::addPath(nix::EvalState&, nix::Pos const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Value*, nix::FileIngestionMethod, std::optional<nix::Hash>, nix::Value&)::{lambda(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)#1}::operator()(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
24 0x00007ffff752e6f9 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
25 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
26 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
27 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
28 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
29 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
30 0x00007ffff757f8c0 in void boost::context::detail::fiber_entry<boost::context::detail::fiber_record<boost::context::fiber, nix::VirtualStackAllocator, boost::coroutines2::detail::pull_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::control_block::control_block<nix::VirtualStackAllocator, nix::sinkToSource(std::function<void (nix::Sink&)>, std::function<void ()>)::SinkToSource::read(char*, unsigned long)::{lambda(boost::coroutines2::detail::push_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&)#1}>(boost::context::preallocated, nix::VirtualStackAllocator&&, nix::sinkToSource(std::function<void (nix::Sink&)>, std::function<void ()>)::SinkToSource::read(char*, unsigned long)::{lambda(boost::coroutines2::detail::push_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&)#1}&&)::{lambda(boost::context::fiber&&)#1}> >(boost::context::detail::transfer_t) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
31 0x00007ffff6f331ef in make_fcontext () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libboost_context.so.1.69.0
32 0x0000000000000000 in ?? ()
```

1 files changed, 19 insertions, 7 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index ef9f8efca..c078bf4a1 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -233,22 +233,34 @@ static void * oomHandler(size_t requested)
 }
 
 class BoehmGCStackAllocator : public StackAllocator {
-  boost::coroutines2::protected_fixedsize_stack stack {
-    // We allocate 8 MB, the default max stack size on NixOS.
-    // A smaller stack might be quicker to allocate but reduces the stack
-    // depth available for source filter expressions etc.
-    std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))
+    boost::coroutines2::protected_fixedsize_stack stack {
+        // We allocate 8 MB, the default max stack size on NixOS.
+        // A smaller stack might be quicker to allocate but reduces the stack
+        // depth available for source filter expressions etc.
+        std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))
     };
 
+    // This is specific to boost::coroutines2::protected_fixedsize_stack.
+    // The stack protection page is included in sctx.size, so we have to
+    // subtract one page size from the stack size.
+    std::size_t pfss_usable_stack_size(boost::context::stack_context &sctx) {
+        return sctx.size - boost::context::stack_traits::page_size();
+    }
+
   public:
     boost::context::stack_context allocate() override {
         auto sctx = stack.allocate();
-        GC_add_roots(static_cast<char *>(sctx.sp) - sctx.size, sctx.sp);
+
+        // Stacks generally start at a high address and grow to lower addresses.
+        // Architectures that do the opposite are rare; in fact so rare that
+        // boost_routine does not implement it.
+        // So we subtract the stack size.
+        GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
         return sctx;
     }
 
     void deallocate(boost::context::stack_context sctx) override {
-        GC_remove_roots(static_cast<char *>(sctx.sp) - sctx.size, sctx.sp);
+        GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
         stack.deallocate(sctx);
     }