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

Co-authored-by: Tarun Prabhu <tarunprabhu@gmail.com>
Co-authored-by: David Spickett <david.spickett@linaro.org>

flang/docs/FAQ.md

@@ -0,0 +1,52 @@
+<!--===- 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
+
+-->
+
+# Frequently Asked Questions (FAQ)
+
+```{contents}
+---
+local:
+---
+```
+
+## Driver
+
+### Why do I get a warning or an error about an executable stack?
+
+This occurs because Flang's implementation of pointers to internal procedures requires an executable stack.
+
+An internal procedure has a "host scope", which is the scope in which it is contained.
+It can access variables defined in that host scope.
+When an internal procedure is referenced from outside its host scope (for example, via a procedure pointer), the implementation must ensure that it can still access variables from the host scope.
+To achieve this, the current implementation of Flang generates a small piece of code, called a "trampoline", on the stack.
+When the procedure is called, this trampoline is executed.
+The trampoline is on the stack, so the stack itself must be executable.
+For a more detailed explanation of trampolines, please refer to the [design document](InternalProcedureTrampolines.md).
+
+An executable stack increases the risk and impact of certain classes of security vulnerabilities, such as [stack buffer overflows](https://llsoftsec.github.io/llsoftsecbook/#stack-buffer-overflows).
+Therefore, modern linkers often issue a warning or an error if an executable stack is not explicitly requested by the developer.
+For instance, the GNU Linker (`ld`) issues a warning while the LLVM Linker (`lld`) emits an error.
+
+```{note}
+The trampoline code generated by Flang is not itself a security risk.
+The risk comes from the possibility of executing malicious code that an attacker has placed on the stack.
+You should determine whether such risks are appropriate for your software.
+```
+
+When you use the Flang driver (the `flang` command) to generate executables, you can instruct the linker to enable an executable stack with the `-Wl,-z,execstack` or `-Xlinker -zexecstack` flag.
+
+```console
+$ flang src.f90 -fuse-ld=ld
+/path/to/ld: warning: src.o: requires executable stack (because the .note.GNU-stack section is executable)
+$ flang src.f90 -fuse-ld=ld -Wl,-z,execstack
+
+$ flang src.f90 -fuse-ld=lld
+ld.lld: error: src.o: requires an executable stack, but -z execstack is not specified
+flang-22: error: linker command failed with exit code 1 (use -v to see invocation)
+$ flang src.f90 -fuse-ld=lld -Wl,-z,execstack
+```

flang/docs/FortranStandardsSupport.md

@@ -95,7 +95,7 @@ All features except those listed in the following table are supported.
 |------------------------------------------------------------|--------|---------------------------------------------------------|
 | Coarrays                                                   | N      | Lowering and runtime support is not implemented         |
 | do concurrent                                              | P      | Sequential execution works. Parallel support in progress|
-| Internal procedure as an actual argument or pointer target | Y      | Current implementation requires stack to be executable. See [Proposal](InternalProcedureTrampolines.md) |
+| Internal procedure as an actual argument or pointer target | Y      | Current implementation requires stack to be executable. See [FAQ](FAQ.md#why-do-i-get-a-warning-or-an-error-about-an-executable-stack) and [Proposal](InternalProcedureTrampolines.md) |
 
 ## Fortran 2003
 All features except those listed in the following table are supported.

flang/docs/index.md

@@ -30,6 +30,7 @@ on how to get in touch with us and to learn more about the current status.
    OpenMPSupport
    Real16MathSupport
    Unsigned
+   FAQ
 ```
 
 # Contributing to Flang