[compiler-rt] [tsan] Lazily call 'personality' to minimize sandbox violations (PR #79334)

[Thurston Dang via llvm-commits]

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@@ -267,6 +267,14 @@ static void ReExecIfNeeded() {
   if (reexec) {
     // Don't check the address space since we're going to re-exec anyway.
   } else if (!CheckAndProtect(false, false, false)) {
+    // ASLR personality check.
+    // N.B. 'personality' is sometimes forbidden by sandboxes, so we only call
+    // this as a last resort (when the memory mapping is incompatible and TSan
+    // would fail anyway).
+    int old_personality = personality(0xffffffff);
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thurstond wrote:

I've removed the test in the latest commit (https://github.com/llvm/llvm-project/pull/79334/commits/0b4c8464b5045e80eccd9acc11e7312d6caa3c88).

With the re-exec patch + this patch:
- low entropy ASLR, no sandbox: TSan works
- low entropy ASLR, with sandbox: TSan works
- high entropy ASLR, no sandbox: TSan works
- high entropy ASLR, with sandbox: TSan will trigger a sandbox violation, but TSan didn't work anyway with high entropy ASLR. Moreover, this is currently only a hypothetical case.

https://github.com/llvm/llvm-project/pull/79334