fix: printf %b support and no-progress guard against format-recycle hangs#50
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…angs printf recycles its format while arguments remain, but a directive the format regex doesn't recognize consumes nothing — so printf "%b" "x" (or any unknown directive with an argument) looped forever, hanging the host process. %b is now a real directive sharing echo -e's escape dialect, and a recycle pass that consumes no arguments stops after one iteration instead of recursing. Unknown directives pass through literally where bash errors; the divergence is noted in the tests. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FpxHS5iGm9ktgpnL9QZu9x
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Found while running agents against the sandbox:
printf "%b" "A\xffB\n"— an ordinary command any agent will eventually type — hungJustBash.exec/2forever. An untrusted script could take down the host with one line.printfrecycles its format string while arguments remain, but%bwas not in the format regex, so each recycle pass consumed zero arguments and recursed with the same list. The same hang fired for any unrecognized directive with at least one argument (%q,%v,%i, …).Two changes:
%bis now a real directive, sharingecho -e's escape dialect (\n,\t,\xHH,\0NNN— exposed asEcho.interpret_escapes/1), soprintf "%b" "A\xffB\n"emits the raw bytes bash does.invalid format character; the divergence is noted in the tests.🤖 Generated with Claude Code
https://claude.ai/code/session_01FpxHS5iGm9ktgpnL9QZu9x