Rollup merge of #111722 - mrcnski:stack-protector-doc, r=rcvalle

Document stack-protector option

Only updated `exploit-mitigations.md` to reflect that the option exists. Removed the alternatives mentioned as they are not actually implemented yet.

As this is an unstable feature, should it be added to `unstable-book` also? [Example](https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html). I didn't do that because I couldn't find the tracking issue for stack-protector. (There should be one to track stabilization of the feature, I think?)

cc `@rcvalle`

src/doc/rustc/src/exploit-mitigations.md

@@ -115,9 +115,9 @@ equivalent.
   <tr>
    <td>Stack smashing protection
    </td>
-   <td>No
+   <td>Yes
    </td>
-   <td>
+   <td>Nightly
    </td>
   </tr>
   <tr>
@@ -432,18 +432,16 @@ saved return instruction pointer, and checking if this value has changed
 when returning from a function. This is also known as “Stack Protector” or
 “Stack Smashing Protector (SSP)”.
 
-The Rust compiler does not support stack smashing protection. However, more
-comprehensive alternatives to stack smashing protection exist, such as
-shadow and safe stack (see backward-edge control flow protection).
+The Rust compiler supports stack smashing protection on nightly builds[42].
 
 ![Screenshot of IDA Pro listing cross references to __stack_chk_fail in hello-rust.](images/image3.png "Cross references to __stack_chk_fail in hello-rust.")
 Fig. 14. IDA Pro listing cross references to `__stack_chk_fail` in
 hello-rust.
 
 To check if stack smashing protection is enabled for a given binary, search
-for cross references to `__stack_chk_fail`. The only cross references to
-`__stack_chk_fail` in hello-rust are from the statically-linked libbacktrace
-library (see Fig. 14).
+for cross references to `__stack_chk_fail`. The presence of these
+cross-references in Rust-compiled code (e.g., `hello_rust::main`) indicates
+that the stack smashing protection is enabled (see Fig. 14).
 
 
 ### Forward-edge control flow protection
@@ -697,3 +695,6 @@ defaults (unrelated to `READ_IMPLIES_EXEC`).
 
 41. “ControlFlowIntegrity.” The Rust Unstable Book.
     [https://doc.rust-lang.org/unstable-book/compiler-flags/sanitizer.html#controlflowintegrity](../unstable-book/compiler-flags/sanitizer.html#controlflowintegrity).
+
+42. bbjornse. “add codegen option for using LLVM stack smash protection #84197.”
+    GitHub. <https://github.com/rust-lang/rust/pull/84197>