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cmd/compile: checkptr false positive from (*[Big]T)(ptr)[:n:n] pattern #46938

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mdempsky opened this issue Jun 26, 2021 · 1 comment
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cmd/compile: checkptr false positive from (*[Big]T)(ptr)[:n:n] pattern #46938

mdempsky opened this issue Jun 26, 2021 · 1 comment
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@mdempsky
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@mdempsky mdempsky commented Jun 26, 2021

checkptr has special case logic for recognizing the pattern (*[Big]T)(ptr)[:n:n]. In particular, instead of diagnosing whether (*[Big]T)(ptr) is an invalid conversion, it diagnoses (*[n]T)(ptr).

However, currently the checkptr instrumentation is inserted before the slice operation has validated n <= Big, so it's possible to have false positive throws in cases that should just be panics.

For example:

package main

import "unsafe"

func main() {
	s := make([]int64, 100)
	p := unsafe.Pointer(&s[0])
	n := 1000

	_ = (*[10]int64)(p)[:n:n] // throws; should just panic
}

This issue has existed since checkptr's inclusion in Go 1.14, and I'm not aware of any user reports about it. It's also easy to workaround.

Marking for Go 1.18.

@mdempsky mdempsky added the NeedsFix label Jun 26, 2021
@mdempsky mdempsky added this to the Go1.18 milestone Jun 26, 2021
@mdempsky mdempsky self-assigned this Jun 26, 2021
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@gopherbot gopherbot commented Aug 21, 2021

Change https://golang.org/cl/343972 mentions this issue: cmd/compile: fix checkptr false positive for (*[Big]T)(ptr)[:n:n] pattern

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