fix(fuzz): strip real-shell error lines from stderr before banned-shape check

[chaliy]

Summary

When a fuzz/proptest target inlines arbitrary input bytes into a shell script, bash and ls produce error messages that quote the input verbatim:

bash: <cmd>: command not found
bash: <path>: No such file or directory
ls:   cannot access '<path>': No such file or directory

These echoes can accidentally form a banned substring even when no internal Debug formatter ran. Two real cases:

• arithmetic_fuzz / glob_fuzz: input contained /.rustup/toolchains/ literally; bash echoed it back. Filtered at the input layer in fix(fuzz): drop glob_fuzz inputs that contain banned debug shapes #1621 and fix(fuzz): drop arithmetic_fuzz inputs that contain banned debug shapes #1622.
• glob_fuzz (new, dispatched run on this branch): input ended with Tok". Bash treated Tok: as a command and rendered bash: Tok:: command not found — the trailing : chrome from bash's error format glued onto the input's final : to form the banned parser-token shape Tok::. Pre-filtering the input for Tok:: doesn't catch this — the substring only forms after bash's formatter runs.

Fix

Structural fix in assert_fuzz_invariants: strip lines that match a recognized real-shell error template before running the banned-shape check.

• Byte-length cap (MAX_STDERR_BYTES) still runs on the unfiltered stderr — flood regressions still caught.
• Host-canary check (TM-INF-013) still runs on the unfiltered stderr — env-leak regressions still caught.
• The strict assert_no_leak path (used by per-builtin tests) is unchanged — non-fuzz tests must not produce shell echoes in the first place.

Recognized templates are conservative: prefix bash: or ls: plus a known suffix. Lines that look similar but don't match the exact template stay in stderr, so real Debug leaks that happen to coexist with shell errors still trip the assertion.

Test plan

• 8 unit tests in testing::tests cover both directions — strip known shell echoes, keep internal panic/Debug lines, keep partial matches and lines from other tools.
• cargo test -p bashkit --lib testing:: green
• cargo test -p bashkit --test proptest_security --all-features green (18 cases)
• cargo clippy -p bashkit --lib --tests -- -D warnings clean
• cargo fmt --check clean
• Threat-model TM-INF-022 section updated to document the carve-out
• Manually dispatch Fuzz Testing on this branch and confirm glob_fuzz job is green

[cloudflare-workers-and-pages]

Deploying with    Cloudflare Workers

The latest updates on your project. Learn more about integrating Git with Workers.

Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	ca39280	Commit Preview URL Branch Preview URL	May 10 2026, 09:51 AM