[flang-commits] [flang] [flang][docs] Add an FAQ about an executable stack (PR #171241)

[Tarun Prabhu via flang-commits]

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+<!--===- docs/FAQ.md
+
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+-->
+
+<!-- TODO: remove this after adding this page to ToC -->
+```{eval-rst}
+:orphan:
+```
+
+# Frequently Asked Questions (FAQ)
+
+```{contents}
+---
+local:
+---
+```
+
+## Driver
+
+### Why do I get a warning or error about an executable stack?
+
+This warning or error occurs because Flang's implementation of pointers to internal procedures requires an executable stack.
+
+When an internal procedure is referenced from outside its host scope (e.g., via a procedure pointer), the implementation must ensure it can still access its host-associated variables.
+To achieve this, the current implementation of Flang generates a small piece of code, called a "trampoline", on the stack dynamically.
+Since this trampoline code must be executed, the stack needs to be executable.
+For a more detailed explanation of trampolines, please refer to the [design document](InternalProcedureTrampolines.md).
+
+However, an executable stack can introduce security vulnerabilities (e.g., by enabling [buffer overflow attacks](https://en.wikipedia.org/wiki/Buffer_overflow#Stack-based_exploitation)).
+For this reason, modern linkers often issue a warning or error.
+In particular, the LLVM Linker (`lld`) never enables an executable stack unless you explicitly allow it.
----------------
tarunprabhu wrote:

I think one example of a linker that is known to issue a warning by default and one that is known to issue an error by default would be good. For instance, we could say that GNU's `ld` issues a warning while LLVM's `lld` emits an error. If we can enumerate the behavior of some of the more widely used linkers such as `ld64` (MacOSX) and the Windows linker, that would be good, but we don't have to.

https://github.com/llvm/llvm-project/pull/171241