dualfroz
650669002f
Substr panicked with a slice-bounds error when a negative length trimmed more runes than remained after pos, because stopIdx (strLn + length) could fall before pos. Clamp stopIdx to pos so an over-long negative trim returns an empty string, matching the existing pos >= strLn guard.
65 lines
1.8 KiB
Markdown
65 lines
1.8 KiB
Markdown
# fix(strutil): prevent Substr panic on out-of-range negative length
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## Problem
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`strutil.Substr` panics with a slice-bounds error when a negative length trims
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more characters than remain after `pos`:
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```go
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strutil.Substr("hello", 4, -3) // panic: slice bounds out of range [4:2]
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strutil.Substr("hello", 3, -5) // panic
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strutil.Substr("abcDEF123", 8, -3) // panic
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```
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A negative `length` means "stop that many runes before the end of the string"
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(this is the documented, tested behaviour, e.g. `Substr("abcDEF123", 2, -3) ==
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"cDEF"`). When the resulting stop index falls before `pos`, the final slice
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`runes[pos:stopIdx]` has `low > high` and panics.
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## Root cause
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`strutil/split.go`, `Substr`:
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```go
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stopIdx := pos + length
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if length == 0 || stopIdx > strLn {
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stopIdx = strLn
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} else if length < 0 {
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stopIdx = strLn + length // can become < pos
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}
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return string(runes[pos:stopIdx])
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```
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For example `Substr("hello", 4, -3)`: `strLn == 5`, `pos == 4`, so
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`stopIdx = 5 + (-3) = 2`, and `runes[4:2]` panics.
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## Fix
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Clamp `stopIdx` to `pos` so an over-long negative trim yields an empty string
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instead of panicking:
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```go
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if stopIdx < pos {
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stopIdx = pos
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}
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```
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This is consistent with the existing out-of-range guard `if pos >= strLn` which
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also returns an empty string.
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## Test
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Extended `TestSubstr` in `strutil/split_test.go` with the three panicking cases,
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each asserting an empty-string result.
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- Gate: `go test ./strutil/ -run 'TestSubstr$'` -> ok (PASS)
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- `go vet ./strutil/` -> exit 0
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- `go build ./...` -> exit 0
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Counterfactual: reverting the clamp makes `TestSubstr` panic with
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`slice bounds out of range [4:2]`.
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Note: `TestDateSNv3/dateLen4_base62` in `strutil/gensn_test.go` is flaky and
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already fails on the unmodified base commit (it asserts uniqueness of randomly
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generated serial numbers); it is unrelated to this change.
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