84 lines
3.8 KiB
Markdown
84 lines
3.8 KiB
Markdown
# ShellFlow — Script Reduction Playbook
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Most container entry points eventually execute a shell script. The ShellFlow analyser resolves these scripts without executing user code, providing deterministic, explainable reductions.
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## 1) Scope
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- POSIX `sh` subset with common Bash extensions (control flow, functions, parameter expansion).
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- Handles idioms from official Docker images (`if [ "$1" = "server" ]; then …`, `exec gosu "$USER" "$@"`, `set -- java -jar …`).
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- Tracks positional parameters (`$@`, `$1..$9`), environment variables, and `set --` mutations.
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- Produces one or more candidate commands with supporting evidence.
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## 2) Architecture
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```
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ShellFlow/
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Parser/ // POSIX sh lexer + recursive descent parser
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Ast/ // nodes for lists, pipelines, conditionals, functions
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Evaluator/ // partial evaluation & taint tracking
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Idioms/ // pattern library for common Docker entrypoints
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Planner/ // emits CommandPlan[]
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```
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### 2.1 CommandPlan
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```csharp
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public sealed record CommandPlan(
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string[] Argv,
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double HeuristicScore,
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IReadOnlyList<string> Evidence,
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IReadOnlyList<ReductionEdge> Chain,
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bool IsFallback = false
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);
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```
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Plans feed directly into the static reducer, which selects the highest-confidence plan but keeps alternates as evidence.
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## 3) Parsing & AST
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- Tokenise words, assignments, pipelines (`|`), lists (`;`, `&&`, `||`), conditionals (`if`, `case`), loops (`for`, `while`, `until`), functions, and redirections.
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- Preserve heredocs and subshells as opaque nodes (evaluated conservatively).
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- Record source spans to surface meaningful evidence (`"line 12: exec java -jar $APP_JAR"`).
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## 4) Partial evaluation
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- Initialise symbol table from image environment plus caller-supplied args.
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- Treat `$@`, `$*`, `$1..$9` as tainted; propagate taint through assignments.
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- Resolve `${VAR:-default}` and `${VAR:+alt}` when `VAR` known; otherwise branch.
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- Support `set -- …` (resets positional parameters) and `shift`.
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- `source`/`.` commands are parsed recursively when files are available; else fallback to low-confidence branch.
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## 5) Exec sink detection
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- `exec <cmd>` dominates the remainder of the script.
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- Chains such as `exec gosu "$USER" "$@"` feed into wrapper collapse.
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- When no `exec` is present, pick the last reachable simple command in the main path.
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- Multi-branch scripts yield multiple plans with adjusted scores; unresolved branches are marked `IsFallback`.
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## 6) Idiom library
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| Pattern | Action |
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| --- | --- |
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| `if [ "${1:0:1}" = '-' ]; then set -- server "$@"; fi` | Rewrite argv to prepend default command. |
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| `if [ "$1" = "bash" ]; then exec "$@"; fi` | Pass-through for manual shells. |
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| `exec "$@"` + non-empty CMD | Substitute CMD vector into plan. |
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| `exec java -jar "$APP_JAR" "$@"` | Resolve JAR via env or filesystem. |
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| `set -- gosu "$APP_USER" "$@"` | Collapse into wrapper plan. |
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Idioms are implemented as AST visitors; each adds evidence strings when triggered.
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## 7) Confidence scoring
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- Base score from plan heuristics (`HeuristicScore`).
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- Penalties for unresolved taint (`$@` unknown), missing files, nested subshells, or fallbacks.
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- Bonus when idioms match, artefacts exist, or env values resolve cleanly.
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- Final confidence is combined with the outer static scoring model.
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## 8) Failure modes
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- Missing script (`ENTRYPOINT` points to deleted file): emit fallback plan with low confidence.
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- Self-modifying scripts or heavy dynamic features (`eval`, backticks): mark plan as low-confidence and surface warning evidence.
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- Commands that spawn supervisors without `exec`: return both the supervisor and inferred children (if configuration files are present).
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ShellFlow keeps the static reducer explainable: every inferred command is accompanied by the script span and reasoning used to reach it.
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