[lldb][Docs] Add Guarded Control Stack to AArch64 Linux page (#117860)

The meat of this is how we execute expressions and deal with the
aftermath. For most users this will never be a concern, so it functions
more as a design doc than anything else.

Author: DavidSpickett

lldb/docs/use/aarch64-linux.md

@@ -229,3 +229,64 @@ bytes.
 
 `zt0`'s value and whether it is active or not will be saved prior to
 expression evaluation and restored afterwards.
+
+## Guarded Control Stack Extension (GCS)
+
+GCS support includes the following new registers:
+
+* `gcs_features_enabled`
+* `gcs_features_locked`
+* `gcspr_el0`
+
+These map to the registers ptrace provides. The first two have a `gcs_`
+prefix added as their names are too generic without it.
+
+When the GCS is enabled the kernel allocates a memory region for it. This region
+has a special attribute that LLDB will detect and presents like this:
+```
+  (lldb) memory region --all
+  <...>
+  [0x0000fffff7a00000-0x0000fffff7e00000) rw-
+  shadow stack: yes
+  [0x0000fffff7e00000-0x0000fffff7e10000) ---
+```
+
+`shadow stack` is a generic term used in the kernel for secure stack
+extensions like GCS.
+
+### Expression Evaluation
+
+To execute an expression when GCS is enabled, LLDB must push the return
+address of the expression wrapper (usually the entry point of the program)
+to the Guarded Control Stack. It does this by decrementing `gcspr_el0` and
+writing to the location now pointed to by `gcspr_el0` (instead of using the
+GCS push instructions).
+
+After an expression finishes, LLDB will restore the contents of all 3
+GCS registers, apart from the enable bit of `gcs_features_enabled`. This is
+because there are limits on how often and from where you can set this
+bit.
+
+GCS cannot be enabled from ptrace and it is expected that a process which
+has enabled and then disabled GCS, will not enable it again. The simplest
+choice was to not restore the enable bit at all. It is up to the user or
+program to manage that bit.
+
+The return address that LLDB pushed onto the Guarded Control Stack will be left
+in place. As will any values that were pushed to the stack by functions run
+during the expression.
+
+When the process resumes, `gcspr_el0` will be pointing to the original entry
+on the guarded control stack. So the other values will have no effect and
+likely be overwritten by future function calls.
+
+LLDB does not track and restore changes to general memory during expressions,
+so not restoring the GCS contents fits with the current behaviour.
+
+Note that if GCS is disabled and an expression enables it, LLDB will not
+be able to setup the return address and it is up to that expression to do that
+if it wants to return to LLDB correctly.
+
+If it does not do this, the expression will fail and although most process
+state will be restored, GCS will be left enabled. Which means that the program
+is very unlikely to be able to progress.