x86/rust: support RETPOLINE

Support the `RETPOLINE` speculation mitigation by enabling the target
features that Clang does.

The existing target feature being enabled was a leftover from
our old `rust` branch, and it is not enough: the target feature
`retpoline-external-thunk` only implies `retpoline-indirect-calls`, but
not `retpoline-indirect-branches` (see LLVM's `X86.td`), unlike Clang's
flag of the same name `-mretpoline-external-thunk` which does imply both
(see Clang's `lib/Driver/ToolChains/Arch/X86.cpp`).

Without this, `objtool` would complain if enabled for individual object
files (like it is planned in the future), e.g.

    rust/core.o: warning: objtool:
    _R...escape_default+0x13: indirect jump found in RETPOLINE build

In addition, change the comment to note that LLVM is the one disabling
jump tables when retpoline is enabled, thus we do not need to use
`-Zno-jump-tables` for Rust here -- see commit c58f2166ab39 ("Introduce
the "retpoline" x86 mitigation technique ...") [1]:

    The goal is simple: avoid generating code which contains an indirect
    branch that could have its prediction poisoned by an attacker. In
    many cases, the compiler can simply use directed conditional
    branches and a small search tree. LLVM already has support for
    lowering switches in this way and the first step of this patch is
    to disable jump-table lowering of switches and introduce a pass to
    rewrite explicit indirectbr sequences into a switch over integers.

As well as a live example at [2].

Cc: Daniel Borkmann <[email protected]>
Link: llvm/llvm-project@c58f216 [1]
Link: https://godbolt.org/z/esT8xnaxj [2]
Signed-off-by: Miguel Ojeda <[email protected]>

arch/x86/Makefile

@@ -197,7 +197,7 @@ ifdef CONFIG_RETPOLINE
   KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
   # Additionally, avoid generating expensive indirect jumps which
   # are subject to retpolines for small number of switch cases.
-  # clang turns off jump table generation by default when under
+  # LLVM turns off jump table generation by default when under
   # retpoline builds, however, gcc does not for x86. This has
   # only been fixed starting from gcc stable version 8.4.0 and
   # onwards, but not for older ones. See gcc bug #86952.

scripts/generate_rust_target.rs

@@ -158,7 +158,14 @@ fn main() {
         );
         let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
         if cfg.has("RETPOLINE") {
+            // The kernel uses `-mretpoline-external-thunk` (for Clang), which Clang maps to the
+            // target feature of the same name plus the other two target features in
+            // `clang/lib/Driver/ToolChains/Arch/X86.cpp`. These should be eventually enabled via
+            // `-Ctarget-feature` when `rustc` starts recognizing them (or via a new dedicated
+            // flag); see https://github.com/rust-lang/rust/issues/116852.
             features += ",+retpoline-external-thunk";
+            features += ",+retpoline-indirect-branches";
+            features += ",+retpoline-indirect-calls";
         }
         ts.push("features", features);
         ts.push("llvm-target", "x86_64-linux-gnu");