runtime/cgo: improve error messages after pointer panic #75894

ariel-anieli · 2025-10-14T16:37:48Z

This commit improves the error messages after panics due to the
sharing of an unpinned Go pointer (or a pointer to an unpinned Go
pointer) between Go and C.

This occurs when it is:

returned from Go to C (through cgoCheckResult)
passed as argument to a C function (through cgoCheckPointer).

An unpinned Go pointer refers to a memory location that might be moved or
freed by the garbage collector.

Therefore:

change the signature of cgoCheckArg (it does the real work behind
cgoCheckResult and cgoCheckPointer)
change the signature of cgoCheckUnknownPointer (called by cgoCheckArg
for checking unexpected pointers)
introduce cgoFormatErr (it formats an error message with the caller name)
update the cgo pointer tests (add new tests, and a field errTextRegexp
to the struct ptrTest)
remove an unused assignment in TestPointerChecks.

cgoCheckResult

When an unpinned Go pointer (or a pointer to an unpinned Go pointer) is
returned from Go to C,

package main

import (
"C"
)

//export foo
func foo() map[int]int {
return map[int]int{0: 1,}
}

This error shows up at runtime:

panic: runtime error: cgo result is unpinned Go pointer or points to unpinned Go pointer

goroutine 17 [running, locked to thread]:
panic({0x78ac36001d40?, 0xc000194000?})
/usr/lib/go/src/runtime/panic.go:802 +0x168
runtime.cgoCheckArg(0x78ac36002f40, 0xc000190000, 0xd8?, 0x0, {0x78ac35fdda62, 0x42})
/usr/lib/go/src/runtime/cgocall.go:628 +0x4c5
runtime.cgoCheckResult({0x78ac36002f40, 0xc000190000})
/usr/lib/go/src/runtime/cgocall.go:795 +0x4b
_cgoexp_1c29bd2c0b96_foo(0x7fffc16d87b0)
_cgo_gotypes.go:46 +0x6f
runtime.cgocallbackg1(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0)
/usr/lib/go/src/runtime/cgocall.go:446 +0x289
runtime.cgocallbackg(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0)
/usr/lib/go/src/runtime/cgocall.go:350 +0x132
runtime.cgocallbackg(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0)
:1 +0x2b
runtime.cgocallback(0x0, 0x0, 0x0)
/usr/lib/go/src/runtime/asm_amd64.s:1082 +0xcd
runtime.goexit({})
/usr/lib/go/src/runtime/asm_amd64.s:1693 +0x1

_cgoexp_1c29bd2c0b96_foo is the faulty caller; it is not obvious to
find it in the stack trace. Moreover the error does say which kind of
pointer caused the panic; for instance, a Go map.

Retrieve caller name and kind of pointer; use them in the error message:

panic: runtime error: result of cgo function foo is unpinned Go map or points to unpinned Go map

goroutine 17 [running, locked to thread]:
panic({0x76f2fd69b3c0?, 0x1fba8c79c000?})
/mnt/go/src/runtime/panic.go:877 +0x16f
runtime.cgoCheckArg(0x76f2fd69c5c0, 0x1fba8c790000, 0x0?, 0x0, 0x1)
/mnt/go/src/runtime/cgocall.go:631 +0x499
runtime.cgoCheckResult({0x76f2fd69c5c0, 0x1fba8c790000})
/mnt/go/src/runtime/cgocall.go:798 +0x45
_cgoexp_1c29bd2c0b96_foo(0x7ffd0803b8c0)
_cgo_gotypes.go:46 +0x6f
runtime.cgocallbackg1(0x76f2fd667680, 0x7ffd0803b8c0, 0x0)
/mnt/go/src/runtime/cgocall.go:446 +0x289
runtime.cgocallbackg(0x76f2fd667680, 0x7ffd0803b8c0, 0x0)
/mnt/go/src/runtime/cgocall.go:350 +0x132
runtime.cgocallbackg(0x76f2fd667680, 0x7ffd0803b8c0, 0x0)
:1 +0x2b
runtime.cgocallback(0x0, 0x0, 0x0)
/mnt/go/src/runtime/asm_amd64.s:1101 +0xcd
runtime.goexit({})
/mnt/go/src/runtime/asm_amd64.s:1712 +0x1

cgoCheckPointer

When a pointer to an unpinned Go pointer is passed to a C function,

package main

/*
#include <stdio.h>
void foo(void *bar) {}
*/
import "C"
import "unsafe"

func main() {
m := map[int]int{0: 1,}
C.foo(unsafe.Pointer(&m))
}

This error shows up at runtime:

panic: runtime error: cgo argument has Go pointer to unpinned Go pointer

goroutine 1 [running]:
main.main.func1(...)
/mnt/go/cgopointer.go:12
main.main()
/mnt/go/cgopointer.go:12 +0x91
exit status 2

Retrieve file name and kind of pointer; use them in the error message.

panic: runtime error: file cgopointer.go, line 12: argument of cgo function has Go pointer to unpinned Go map

goroutine 1 [running]:
main.main.func1(...)
/mnt/go/cgopointer.go:12
main.main()
/mnt/go/cgopointer.go:12 +0x9b
exit status 2

Link: https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers
Suggested-by: Ian Lance Taylor iant@golang.org
Fixes #75856

gopherbot · 2025-10-14T16:44:48Z

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gopherbot · 2025-10-14T16:52:54Z

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gopherbot · 2025-10-14T16:52:54Z

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gopherbot · 2025-10-14T19:56:46Z

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gopherbot · 2025-10-14T20:12:43Z

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gopherbot · 2025-10-15T20:34:05Z

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gopherbot · 2025-10-17T10:07:59Z

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gopherbot · 2025-10-22T18:00:48Z

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gopherbot · 2025-10-28T09:09:02Z

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gopherbot · 2025-10-28T09:26:07Z

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gopherbot · 2025-10-28T10:16:42Z

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gopherbot · 2025-10-28T17:35:31Z

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gopherbot · 2025-10-28T17:44:10Z

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gopherbot · 2025-10-28T18:01:10Z

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This commit improves the error messages after panics due to the sharing of an unpinned Go pointer (or a pointer to an unpinned Go pointer) between Go and C. This occurs when it is: 1. returned from Go to C (through cgoCheckResult) 2. passed as argument to a C function (through cgoCheckPointer). An unpinned Go pointer refers to a memory location that might be moved or freed by the garbage collector. Therefore: - change the signature of cgoCheckArg (it does the real work behind cgoCheckResult and cgoCheckPointer) - change the signature of cgoCheckUnknownPointer (called by cgoCheckArg for checking unexpected pointers) - introduce cgoFormatErr (it formats an error message with the caller name) - update the cgo pointer tests (add new tests, and a field errTextRegexp to the struct ptrTest) - remove an unused assignment in TestPointerChecks. 1. cgoCheckResult When an unpinned Go pointer (or a pointer to an unpinned Go pointer) is returned from Go to C, > package main > > import ( > "C" > ) > > //export foo > func foo() map[int]int { > return map[int]int{0: 1,} > } This error shows up at runtime: > panic: runtime error: cgo result is unpinned Go pointer or points to unpinned Go pointer > > goroutine 17 [running, locked to thread]: > panic({0x78ac36001d40?, 0xc000194000?}) > /usr/lib/go/src/runtime/panic.go:802 +0x168 > runtime.cgoCheckArg(0x78ac36002f40, 0xc000190000, 0xd8?, 0x0, {0x78ac35fdda62, 0x42}) > /usr/lib/go/src/runtime/cgocall.go:628 +0x4c5 > runtime.cgoCheckResult({0x78ac36002f40, 0xc000190000}) > /usr/lib/go/src/runtime/cgocall.go:795 +0x4b > _cgoexp_1c29bd2c0b96_foo(0x7fffc16d87b0) > _cgo_gotypes.go:46 +0x6f > runtime.cgocallbackg1(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0) > /usr/lib/go/src/runtime/cgocall.go:446 +0x289 > runtime.cgocallbackg(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0) > /usr/lib/go/src/runtime/cgocall.go:350 +0x132 > runtime.cgocallbackg(0x78ac35fd4d20, 0x7fffc16d87b0, 0x0) > <autogenerated>:1 +0x2b > runtime.cgocallback(0x0, 0x0, 0x0) > /usr/lib/go/src/runtime/asm_amd64.s:1082 +0xcd > runtime.goexit({}) > /usr/lib/go/src/runtime/asm_amd64.s:1693 +0x1 _cgoexp_1c29bd2c0b96_foo is the faulty caller; it is not obvious to find it in the stack trace. Moreover the error does say which kind of pointer caused the panic; for instance, a Go map. Retrieve caller name and kind of pointer; use them in the error message: > panic: runtime error: result of cgo function foo is unpinned Go map or points to unpinned Go map > > goroutine 17 [running, locked to thread]: > panic({0x76f2fd69b3c0?, 0x1fba8c79c000?}) > /mnt/go/src/runtime/panic.go:877 +0x16f > runtime.cgoCheckArg(0x76f2fd69c5c0, 0x1fba8c790000, 0x0?, 0x0, 0x1) > /mnt/go/src/runtime/cgocall.go:631 +0x499 > runtime.cgoCheckResult({0x76f2fd69c5c0, 0x1fba8c790000}) > /mnt/go/src/runtime/cgocall.go:798 +0x45 > _cgoexp_1c29bd2c0b96_foo(0x7ffd0803b8c0) > _cgo_gotypes.go:46 +0x6f > runtime.cgocallbackg1(0x76f2fd667680, 0x7ffd0803b8c0, 0x0) > /mnt/go/src/runtime/cgocall.go:446 +0x289 > runtime.cgocallbackg(0x76f2fd667680, 0x7ffd0803b8c0, 0x0) > /mnt/go/src/runtime/cgocall.go:350 +0x132 > runtime.cgocallbackg(0x76f2fd667680, 0x7ffd0803b8c0, 0x0) > <autogenerated>:1 +0x2b > runtime.cgocallback(0x0, 0x0, 0x0) > /mnt/go/src/runtime/asm_amd64.s:1101 +0xcd > runtime.goexit({}) > /mnt/go/src/runtime/asm_amd64.s:1712 +0x1 2. cgoCheckPointer When a pointer to an unpinned Go pointer is passed to a C function, > package main > > /* > #include <stdio.h> > void foo(void *bar) {} > */ > import "C" > import "unsafe" > > func main() { > m := map[int]int{0: 1,} > C.foo(unsafe.Pointer(&m)) > } This error shows up at runtime: > panic: runtime error: cgo argument has Go pointer to unpinned Go pointer > > goroutine 1 [running]: > main.main.func1(...) > /mnt/go/cgopointer.go:12 > main.main() > /mnt/go/cgopointer.go:12 +0x91 > exit status 2 Retrieve file name and kind of pointer; use them in the error message. > panic: runtime error: file cgopointer.go, line 12: argument of cgo function has Go pointer to unpinned Go map > > goroutine 1 [running]: > main.main.func1(...) > /mnt/go/cgopointer.go:12 > main.main() > /mnt/go/cgopointer.go:12 +0x9b > exit status 2 Link: https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers Suggested-by: Ian Lance Taylor <iant@golang.org> Fixes golang#75856

gopherbot · 2025-10-28T22:20:22Z

Message from Ariel Otilibili Anieli:

Patch Set 19:

(2 comments)

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gopherbot · 2025-10-28T22:20:29Z

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gopherbot · 2025-10-28T23:29:26Z

Message from Keith Randall:

Patch Set 20:

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austinderek mentioned this pull request Oct 14, 2025

cmd/cgo: improve error messages PullRequestInc/go#108

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ariel-anieli force-pushed the cgo-error-messages branch from 74f2fb0 to e723643 Compare October 14, 2025 19:44

ariel-anieli force-pushed the cgo-error-messages branch from e723643 to b5f4ef2 Compare October 16, 2025 23:15

ariel-anieli force-pushed the cgo-error-messages branch from b5f4ef2 to 405155c Compare October 17, 2025 00:50

ariel-anieli changed the title ~~cmd/cgo: improve error messages~~ runtime/cgo: improve error messages after panic Oct 17, 2025

ariel-anieli changed the title ~~runtime/cgo: improve error messages after panic~~ runtime/cgo: improve error messages after pointer panic Oct 17, 2025

ariel-anieli force-pushed the cgo-error-messages branch from 405155c to aca5ab4 Compare October 17, 2025 09:47

ariel-anieli force-pushed the cgo-error-messages branch from aca5ab4 to fe7c70a Compare October 19, 2025 14:14

ariel-anieli force-pushed the cgo-error-messages branch from 78e1263 to a9be65a Compare October 28, 2025 09:08

ariel-anieli changed the title ~~runtime/cgo: improve error messages after pointer panic~~ runtime/cgo: improve error messages after ptr panic Oct 28, 2025

ariel-anieli changed the title ~~runtime/cgo: improve error messages after ptr panic~~ runtime/cgo: improve error messages after pointer panic Oct 28, 2025

ariel-anieli force-pushed the cgo-error-messages branch from a9be65a to fea390c Compare October 28, 2025 17:28

ariel-anieli force-pushed the cgo-error-messages branch from fea390c to 83742a3 Compare October 28, 2025 22:13

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