Switch exploit mitigations to mdbook footnotes

This updates the exploit mitigations chapter in the rustc book to use
the footnote feature of mdbook instead of manually implementing
footnotes with HTML.

Author: ehuss

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

@@ -42,8 +42,7 @@ understood within a given context.
 
 This section documents the exploit mitigations applicable to the Rust compiler
 when building programs for the Linux operating system on the AMD64 architecture
-and equivalent.<sup id="fnref:1" role="doc-noteref"><a href="#fn:1"
-class="footnote">1</a></sup> All examples in this section were built using
+and equivalent.[^all-targets] All examples in this section were built using
 nightly builds of the Rust compiler on Debian testing.
 
 The Rust Programming Language currently has no specification. The Rust compiler
@@ -67,11 +66,8 @@ equivalent.
 | Forward-edge control flow protection | Yes | Nightly |
 | Backward-edge control flow protection (e.g., shadow and safe stack) | Yes | Nightly |
 
-<small id="fn:1">1\. See
-<https://github.com/rust-lang/rust/tree/master/compiler/rustc_target/src/spec>
-for a list of targets and their default options. <a href="#fnref:1"
-class="reversefootnote" role="doc-backlink">↩</a></small>
-
+[^all-targets]: See <https://github.com/rust-lang/rust/tree/master/compiler/rustc_target/src/spec>
+    for a list of targets and their default options.
 
 ### Position-independent executable
 
@@ -141,18 +137,15 @@ Integer overflow checks are enabled when debug assertions are enabled (see Fig.
 3), and disabled when debug assertions are disabled (see Fig. 4). To enable
 integer overflow checks independently, use the option to control integer
 overflow checks, scoped attributes, or explicit checking methods such as
-`checked_add`<sup id="fnref:2" role="doc-noteref"><a href="#fn:2"
-class="footnote">2</a></sup>.
+`checked_add`[^checked-methods].
 
 It is recommended that explicit wrapping methods such as `wrapping_add` be used
 when wrapping semantics are intended, and that explicit checking and wrapping
 methods always be used when using Unsafe Rust.
 
-<small id="fn:2">2\. See [the `u32` docs](../std/primitive.u32.html) for more
-information on the checked, overflowing, saturating, and wrapping methods
-(using u32 as an example). <a href="#fnref:2" class="reversefootnote"
-role="doc-backlink">↩</a></small>
-
+[^checked-methods]: See [the `u32` docs](../std/primitive.u32.html) for more
+    information on the checked, overflowing, saturating, and wrapping methods
+    (using u32 as an example).
 
 ### Non-executable memory regions
 
@@ -180,17 +173,14 @@ binary.
 
 The presence of an element of type `PT_GNU_STACK` in the program header table
 with the `PF_X` (i.e., executable) flag unset indicates non-executable memory
-regions<sup id="fnref:3" role="doc-noteref"><a href="#fn:3"
-class="footnote">3</a></sup> are enabled for a given binary (see Fig. 5).
+regions[^other-regions] are enabled for a given binary (see Fig. 5).
 Conversely, the presence of an element of type `PT_GNU_STACK` in the program
 header table with the `PF_X` flag set or the absence of an element of type
 `PT_GNU_STACK` in the program header table indicates non-executable memory
 regions are not enabled for a given binary.
 
-<small id="fn:3">3\. See the Appendix section for more information on why it
-affects other memory regions besides the stack. <a href="#fnref:3"
-class="reversefootnote" role="doc-backlink">↩</a></small>
-
+[^other-regions]: See the [Appendix section](#appendix) for more information
+    on why it affects other memory regions besides the stack.
 
 ### Stack clashing protection
 
@@ -270,8 +260,7 @@ $ readelf -d target/release/hello-rust | grep BIND_NOW
 Fig. 10. Checking if immediate binding is enabled for a given binary.
 
 The presence of an element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW`
-flag<sup id="fnref:4" role="doc-noteref"><a href="#fn:4"
-class="footnote">4</a></sup> in the dynamic section indicates immediate binding
+flag[^bind-now] in the dynamic section indicates immediate binding
 is enabled for a given binary (see Fig. 10). Conversely, the absence of an
 element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW` flag in the dynamic
 section indicates immediate binding is not enabled for a given binary.
@@ -281,9 +270,7 @@ table and of an element with the `DT_BIND_NOW` tag and the `DF_BIND_NOW` flag
 in the dynamic section indicates full RELRO is enabled for a given binary (see
 Figs. 9–10).
 
-<small id="fn:4">4\. And the `DF_1_NOW` flag for some link editors. <a
-href="#fnref:4" class="reversefootnote" role="doc-backlink">↩</a></small>
-
+[^bind-now]: And the `DF_1_NOW` flag for some link editors.
 
 ### Heap corruption protection
 
@@ -303,8 +290,7 @@ Rust’s default allocator has historically been
 [jemalloc](http://jemalloc.net/), and it has long been the cause of issues and
 the subject of much discussion[32]–[38]. Consequently, it has been removed as
 the default allocator in favor of the operating system’s standard C library
-default allocator<sup id="fnref:5" role="doc-noteref"><a href="#fn:5"
-class="footnote">5</a></sup> since version 1.32.0 (2019-01-17)[39].
+default allocator[^linx-allocator] since version 1.32.0 (2019-01-17)[39].
 
 ```rust,no_run
 fn main() {
@@ -343,11 +329,9 @@ Fig. 13. Build and execution of hello-rust-heap with debug assertions disabled
 Heap corruption checks are performed when using the default allocator (i.e.,
 the GNU Allocator) (see Figs. 12–13).
 
-<small id="fn:5">5\. Linux's standard C library default allocator is the GNU
-Allocator, which is derived from ptmalloc (pthreads malloc) by Wolfram Gloger,
-which in turn is derived from dlmalloc (Doug Lea malloc) by Doug Lea. <a
-href="#fnref:5" class="reversefootnote" role="doc-backlink">↩</a></small>
-
+[^linx-allocator]: Linux's standard C library default allocator is the GNU
+    Allocator, which is derived from ptmalloc (pthreads malloc) by Wolfram Gloger,
+    which in turn is derived from dlmalloc (Doug Lea malloc) by Doug Lea.
 
 ### Stack smashing protection
 
@@ -385,8 +369,7 @@ commercially available [grsecurity/PaX Reuse Attack Protector
 (RAP)](https://grsecurity.net/rap_faq).
 
 The Rust compiler supports forward-edge control flow protection on nightly
-builds[41]-[42] <sup id="fnref:6" role="doc-noteref"><a href="#fn:6"
-class="footnote">6</a></sup>.
+builds[41]-[42] [^win-cfg].
 
 ```text
 $ readelf -s -W target/release/hello-rust | grep "\.cfi"
@@ -401,10 +384,8 @@ of symbols suffixed with ".cfi" or the `__cfi_init` symbol (and references to
 `__cfi_check`) indicates that LLVM CFI is not enabled for a given binary (see
 Fig. 15).
 
-<small id="fn:6">6\. It also supports Control Flow Guard (CFG) on Windows (see
-<https://github.com/rust-lang/rust/issues/68793>). <a href="#fnref:6"
-class="reversefootnote" role="doc-backlink">↩</a></small>
-
+[^win-cfg]: It also supports Control Flow Guard (CFG) on Windows (see
+    <https://github.com/rust-lang/rust/issues/68793>).
 
 ### Backward-edge control flow protection
 
@@ -431,8 +412,7 @@ Newer processors provide hardware assistance for backward-edge control flow
 protection, such as ARM Pointer Authentication, and Intel Shadow Stack as part
 of Intel CET.
 
-The Rust compiler supports shadow stack for the AArch64 architecture<sup
-id="fnref:7" role="doc-noteref"><a href="#fn:7" class="footnote">7</a></sup>on
+The Rust compiler supports shadow stack for the AArch64 architecture[^amd64-shadow] on
 nightly builds[43]-[44], and also supports safe stack on nightly
 builds[45]-[46].
 
@@ -447,9 +427,8 @@ enabled for a given binary. Conversely, the absence of the `__safestack_init`
 symbol indicates that LLVM SafeStack is not enabled for a given binary (see
 Fig. 16).
 
-<small id="fn:7">7\. The shadow stack implementation for the AMD64 architecture
-and equivalent in LLVM was removed due to performance and security issues. <a
-href="#fnref:7" class="reversefootnote" role="doc-backlink">↩</a></small>
+[^amd64-shadow]: The shadow stack implementation for the AMD64 architecture
+    and equivalent in LLVM was removed due to performance and security issues.
 
 
 ## Appendix