Panic handler that passes info to crash reporter via memfds.

Crashes of rust executables have a generic signature since the user
crash collector is not able to compute a reasonable signature for them.
This custom panic hook allows passing a signature derived from PanicInfo
to the crash collector.

BUG=b:234093439
TEST=See crrev.com/c/3834930 and crrev.com/c/3836804

Change-Id: Ida7f655d3a2463e8a42602d8e32588195a0ce36b
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3842808
Commit-Queue: Paramjit Oberoi <psoberoi@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: Paramjit Oberoi <psoberoi@google.com>

base/src/sys/unix/mod.rs

@@ -30,6 +30,7 @@ mod mmap;
 pub mod net;
 mod netlink;
 mod notifiers;
+pub mod panic_handler;
 pub mod platform_timer_resolution;
 mod poll;
 mod priority;

base/src/sys/unix/panic_handler.rs

@@ -0,0 +1,34 @@
+// Copyright 2022 The ChromiumOS Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+//! A panic handler for better crash signatures for rust apps.
+
+use super::SharedMemory;
+use std::ffi::CString;
+use std::io;
+use std::mem;
+use std::panic;
+
+const PANIC_MEMFD_NAME: &str = "RUST_PANIC_SIG";
+
+/// Inserts a panic handler that writes the panic info to a memfd called
+/// "RUST_PANIC_SIG" before calling the original panic handler. This
+/// makes it possible for external crash handlers to recover the panic info.
+pub fn install_memfd_handler() {
+    let hook = panic::take_hook();
+    panic::set_hook(Box::new(move |p| {
+        let panic_info = format!("{}\n", &p);
+        let panic_bytes = panic_info.as_bytes();
+        // On failure, ignore the error and call the original handler.
+        if let Ok(mut panic_memfd) = SharedMemory::new(
+            &CString::new(PANIC_MEMFD_NAME).unwrap(),
+            panic_bytes.len() as u64,
+        ) {
+            io::Write::write_all(&mut panic_memfd, panic_bytes).ok();
+            // Intentionally leak panic_memfd so it is picked up by the crash handler.
+            mem::forget(panic_memfd);
+        }
+        hook(p)
+    }));
+}