fix(builtin): limit recursion depth #870
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Add builtin.MaxDepth (default 10k) to prevent stack overflows when processing deeply nested or cyclic structures in builtin functions. The functions flatten, min, max, mean, and median now return a "recursion depth exceeded" error instead of crashing the runtime. Signed-off-by: Ville Vesilehto <[email protected]>
antonmedv
approved these changes
Dec 4, 2025
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Motivation
The builtin functions
flatten,min,max,mean, andmedianrecursively traverse nested arrays. If the environment provides a deeply nested structure or one containing a cycle (e.g., a slice containing itself), these functions would recurse indefinitely until the Go runtime panics due to stack overflow. This panic is unrecoverable and crashes the host application, presenting a DoS risk.Changes
The
builtinpackage now has aMaxDepthinteger (defaults to 10k).These recursive helper functions now accept a
depthargument. This is incremented on recursive calls, and errors propagate up the stack if limit is exceeded. Initial function definitions call the helpers with an initial depth of 0 and handle returned errors.Tests
Added various tests for the affected builtin functions using self-referencing slices. Also a test about customising the MaxDepth for users who rely on expr-lang as a library, and need more or less depth.