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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

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 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 mentions this issue: cmd/compile: fix checkptr false positive for (*[Big]T)(ptr)[:n:n] pattern

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