From d8e9cb70b3652a1f3626d85cb7cb4e302c2edcfb Mon Sep 17 00:00:00 2001
From: Brian Anderson <andersrb@gmail.com>
Date: Mon, 31 Dec 2018 21:46:36 +0000
Subject: [PATCH] fuzz: make fuzzers work with nightly

Recent Rust compilers have bugs that appear when fuzzing
optimized binaries:

https://github.com/rust-lang/rust/issues/53945

This patch works around the issue by adding the "-C codegen-units=1 -C
incremental=fuzz-incremental" arguments to `RUSTFLAGS`.

Why this works I don't actually know. This workaround isn't mentioned
in the linked issue, and afaik the "incremental" flag is simply
changing the directory of the incremental cache, not turning it on or
off.

Signed-off-by: Brian Anderson <andersrb@gmail.com>
---
 .gitignore  | 3 +++
 fuzz/cli.rs | 7 +++++++
 2 files changed, 10 insertions(+)

diff --git a/.gitignore b/.gitignore
index a425915d911d..1aef5d4b5c9d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,3 +24,6 @@ out/
 target
 tmp
 /bin
+
+# fuzzing hack, see fuzz/cli.rs
+fuzz-incremental/
diff --git a/fuzz/cli.rs b/fuzz/cli.rs
index 7e3c1614942c..78853d83e116 100644
--- a/fuzz/cli.rs
+++ b/fuzz/cli.rs
@@ -211,9 +211,16 @@ fn run_libfuzzer(target: &str) -> Result<(), Error> {
     #[cfg(not(any(target_os = "linux", target_os = "macos")))]
     panic!("libfuzzer-sys only supports Linux and macOS");
 
+    // FIXME: The -C codegen-units=1 and -C incremental=..
+    // below seem to workaround some difficult issues in Rust nightly
+    // https://github.com/rust-lang/rust/issues/53945.
+    // If this is ever fixed remember to remove the fuzz-incremental
+    // entry from .gitignore.
     let mut rust_flags = env::var("RUSTFLAGS").unwrap_or_default();
     rust_flags.push_str(
         "--cfg fuzzing \
+         -C codegen-units=1 \
+         -C incremental=fuzz-incremental \
          -C passes=sancov \
          -C llvm-args=-sanitizer-coverage-level=4 \
          -C llvm-args=-sanitizer-coverage-trace-pc-guard \