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v8: loosen artificial mmap constraint

Fixes #4010.
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1 parent 017009f commit 7bd84de5ed7c5e8c072bcc02fb50037b8f3aac3f @bcantrill bcantrill committed with bnoordhuis
Showing with 13 additions and 4 deletions.
  1. +13 −4 deps/v8/src/platform-posix.cc
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17 deps/v8/src/platform-posix.cc
@@ -109,11 +109,20 @@ void* OS::GetRandomMmapAddr() {
raw_addr &= V8_UINT64_C(0x3ffffffff000);
#else
uint32_t raw_addr = V8::RandomPrivate(isolate);
- // The range 0x20000000 - 0x60000000 is relatively unpopulated across a
- // variety of ASLR modes (PAE kernel, NX compat mode, etc) and on macos
- // 10.6 and 10.7.
+
+ // For our 32-bit mmap() hint, we pick a random address in the bottom
+ // half of the top half of the address space (that is, the third quarter).
+ // Because we do not MAP_FIXED, this will be treated only as a hint -- the
+ // system will not fail to mmap() because something else happens to already
+ // be mapped at our random address. We deliberately set the hint high enough
+ // to get well above the system's break (that is, the heap); systems will
+ // either try the hint and if that fails move higher (MacOS and other BSD
+ // derivatives) or try the hint and if that fails allocate as if there were
+ // no hint at all (Linux, Solaris, illumos and derivatives). The high hint
+ // prevents the break from getting hemmed in at low values, ceding half of
+ // the address space to the system heap.
raw_addr &= 0x3ffff000;
- raw_addr += 0x20000000;
+ raw_addr += 0x80000000;
#endif
return reinterpret_cast<void*>(raw_addr);
}

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