From 1fad28387eb43770f210216ffa0c59a5b48839c4 Mon Sep 17 00:00:00 2001
From: Mykhailo Chalyi <mike@chaliy.name>
Date: Sun, 10 May 2026 09:06:37 +0000
Subject: [PATCH] fix(fuzz): drop arithmetic_fuzz inputs that contain banned
 debug shapes

The nightly fuzz job on main is red. `arithmetic_fuzz` inlines fuzz input
directly into `echo $(({input}))`. With unbalanced parens the arithmetic
expansion closes early and the remainder is parsed as commands; bash then
echoes the unknown command verbatim ("bash: /.rustup/toolchains/gww: No
such file or directory"). That is real-shell stderr, not a TM-INF-022
leak.

Last run: input bytes contained `/.rustup/toolchains/` literally and the
fuzz harness's leak detector tripped on the banned host-path shape.

Fix: at the fuzz-input layer, drop inputs that already contain any
`UNIVERSAL_BANNED` substring. This keeps TM-INF-022 detection strict on
real builtin internals while removing the false-positive class. Mirrors
the same fix applied to `glob_fuzz` in #1621.
---
 .../bashkit/fuzz/fuzz_targets/arithmetic_fuzz.rs  | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/crates/bashkit/fuzz/fuzz_targets/arithmetic_fuzz.rs b/crates/bashkit/fuzz/fuzz_targets/arithmetic_fuzz.rs
index 8ec129256..5a1ee18f2 100644
--- a/crates/bashkit/fuzz/fuzz_targets/arithmetic_fuzz.rs
+++ b/crates/bashkit/fuzz/fuzz_targets/arithmetic_fuzz.rs
@@ -38,6 +38,21 @@ fuzz_target!(|data: &[u8]| {
             return;
         }
 
+        // Reject inputs that themselves contain banned substrings — this
+        // target inlines `input` into `echo $((input))`. With unbalanced
+        // parens the arithmetic expansion can close early and the
+        // remainder is parsed as commands; bash then echoes the unknown
+        // command verbatim ("bash: /.rustup/toolchains/gww: No such file
+        // or directory"). That is real-shell stderr, not a TM-INF-022
+        // leak. Filtering at the fuzz-input layer keeps the harness's
+        // leak detector strict for real internals while avoiding false
+        // positives. (Mirrors glob_fuzz.)
+        for pat in bashkit::testing::UNIVERSAL_BANNED {
+            if input.contains(pat) {
+                return;
+            }
+        }
+
         // Wrap input in arithmetic expansion context
         let script = format!("echo $(({}))", input);