From 311780dbc75cbf3cd4ab7b23ffa39e94b942cf50 Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Wed, 28 Jan 2026 21:09:30 +0000
Subject: [PATCH] =?UTF-8?q?=F0=9F=9B=A1=EF=B8=8F=20Sentinel:=20[CRITICAL]?=
 =?UTF-8?q?=20Fix=20SIGBUS=20on=20input/output=20path=20collision?=
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-   🚨 Severity: CRITICAL
-   💡 Vulnerability: A SIGBUS crash occurs when the output file path is the same as the input file path.
-   🎯 Impact: Opening the output file for writing truncates the input file while it is still memory-mapped, causing the application to crash immediately with a Bus Error (SIGBUS).
-   🔧 Fix: Use \`std::fs::canonicalize\` to verify that input and output file paths are distinct before opening the output file.
-   ✅ Verification: Verified with a reproduction script that formerly crashed and now reports a proper error message. All existing tests pass.

Co-authored-by: kassoulet <1905+kassoulet@users.noreply.github.com>
---
 .jules/sentinel.md  |  4 ++++
 bz2zstd/src/main.rs | 42 +++++++++++++++++++++++++++---------------
 2 files changed, 31 insertions(+), 15 deletions(-)
 create mode 100644 .jules/sentinel.md

diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..c9ef1be
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2025-05-22 - SIGBUS on Memory-Mapped File Truncation
+**Vulnerability:** A SIGBUS crash occurs when an output file truncates the input file while it is currently memory-mapped.
+**Learning:** `File::create` truncates the file immediately. If that file is backed by a `mmap`, any subsequent access to the mapping results in a SIGBUS signal because the mapped pages no longer exist in the file.
+**Prevention:** Always use `std::fs::canonicalize` to verify that input and output file paths are distinct before opening any file for writing, especially when using memory mapping.
diff --git a/bz2zstd/src/main.rs b/bz2zstd/src/main.rs
index ced1e01..d777199 100644
--- a/bz2zstd/src/main.rs
+++ b/bz2zstd/src/main.rs
@@ -86,6 +86,33 @@ fn main() -> Result<()> {
             .context("Failed to build global thread pool")?;
     }
 
+    // Determine output file path early to check against input
+    let output_path = if let Some(path) = args.output.clone() {
+        path
+    } else {
+        // Auto-generate output filename by replacing .bz2 with .zst
+        let input_str = args.input.to_string_lossy();
+        if input_str.ends_with("bz2") {
+            PathBuf::from(input_str.replace("bz2", "zst"))
+        } else {
+            let mut path = args.input.clone();
+            path.set_extension("zst");
+            path
+        }
+    };
+
+    // Security Requirement: Verify that input and output file paths are distinct
+    // to prevent SIGBUS caused by truncating a memory-mapped input file.
+    if let Ok(input_canonical) = std::fs::canonicalize(&args.input) {
+        if output_path.exists() {
+            if let Ok(output_canonical) = std::fs::canonicalize(&output_path) {
+                if input_canonical == output_canonical {
+                    anyhow::bail!("Input and output files must be different");
+                }
+            }
+        }
+    }
+
     // Memory-map the input file for efficient random access
     // Benefits:
     // - No need to load entire file into memory
@@ -148,21 +175,6 @@ fn main() -> Result<()> {
     // Receives compressed blocks from workers and writes them in order.
     // Uses a HashMap to buffer out-of-order blocks.
     let writer_handle = thread::spawn(move || -> Result<()> {
-        // Determine output file path
-        let output_path = if let Some(path) = args.output {
-            path
-        } else {
-            // Auto-generate output filename by replacing .bz2 with .zst
-            let input_str = args.input.to_string_lossy();
-            if input_str.ends_with("bz2") {
-                PathBuf::from(input_str.replace("bz2", "zst"))
-            } else {
-                let mut path = args.input.clone();
-                path.set_extension("zst");
-                path
-            }
-        };
-
         let raw_out: Box<dyn Write + Send> =
             Box::new(File::create(output_path).context("Failed to create output file")?);