cmd/link: zlib skip produces invalid binaries

[klauspost]

Go version

go tip

Output of go env in your module/workspace:

λ ..\bin\go env
set AR=ar
set CC=gcc
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_ENABLED=1
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set CXX=g++
set GCCGO=gccgo
set GO111MODULE=
set GOAMD64=v1
set GOARCH=amd64
set GOAUTH=netrc
set GOBIN=
set GOCACHE=e:\temp\gocache
set GOCACHEPROG=
set GODEBUG=
set GOENV=C:\Users\klaus\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFIPS140=off
set GOFLAGS=
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=d:\temp\wintemp\go-build235563609=/tmp/go-build -gno-record-gcc-switches
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMOD=e:\godev\src\go.mod
set GOMODCACHE=e:\gopath\pkg\mod
set GOOS=windows
set GOPATH=e:\gopath\
set GOPRIVATE=github.com/miniohq/,github.com/klauspost/minio-ec
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=e:\godev
set GOSUMDB=sum.golang.org
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\klaus\AppData\Roaming\go\telemetry
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=e:\godev\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.26-devel_0c8bc965d9 Thu Oct 23 12:01:03 2025 +0200
set GOWORK=
set PKG_CONFIG=pkg-config

What did you do?

Observed working on https://go.dev/issue/75532

If the datasection cannot be compressed and this check is hit and an invalid binary will be generated.

To reproduce, apply:

diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index dfb1d7bafb..ddbd7c5762 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -3210,7 +3210,7 @@ func compressSyms(ctxt *Link, syms []loader.Sym) []byte {
 	// compression levels of zlib.DefaultCompression, but takes
 	// substantially less time. This is important because DWARF
 	// compression can be a significant fraction of link time.
-	z, err := zlib.NewWriterLevel(&buf, zlib.BestSpeed)
+	z, err := zlib.NewWriterLevel(&buf, zlib.NoCompression)
 	if err != nil {
 		log.Fatalf("NewWriterLevel failed: %s", err)
 	}

Always taking the branch has same effect. It appears the caller isn't handling nil response correctly.

What did you see happen?

Compilation fails:

λ make.bat
Building Go cmd/dist using C:\go. (go1.25.1 windows/amd64)
Building Go toolchain1 using C:\go.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
e:\godev\pkg\tool\windows_amd64\link: .debug_frame.PointerToRawData = 0xb05200, want 0xf28000

go tool dist: FAILED: e:\godev\pkg\tool\windows_amd64\link -extld= -L=e:\godev\pkg\obj\go-bootstrap\windows_amd64 -o e:\godev\pkg\tool\windows_amd64\go_bootstrap.exe d:\temp\wintemp\go-tool-dist-2188564858\cmd\go\_go_.a: exit status 2

What did you expect to see?

Successful compilation.

[gabyhelp]

Related Issues

• cmd/link: unexpected fault in bootstrap toolchain #58530 (closed)
• debug/pe: TestInternalLinkerDWARF fail in windows #64200 (closed)
• Windows: Go 1.25 internal linker produces invalid PE (misaligned SizeOfHeaders and sections, EXE won’t run) #75121 (closed)
• debug/elf: Incorrectly double-decompressing ELF section #59208 (closed)
• cmd/link: build fails with: combining dwarf failed #75068 (closed)
• cmd/compile: windows/amd64 invalid binary when allocating array near symbol size limit #54564
• cmd/link: dwarf sections cause program not to run on Windows #20183 (closed)
• cmd/link: index out of range at cmd/link/internal/ld.decodetypeKind #62156 (closed)
• cmd/go: If strip the windows binary with binutils it won't run. #59193 (closed)

Related Code Changes

• cmd/link: compress DWARF sections in ELF binaries

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[thepudds]

Hi @cherrymui, for #75532 and https://go.dev/cl/707355, @klauspost is heads down working on the speeding up the stdlib flate compression, and this was causing trybot failures.

Do you think someone from the core Go team could take a brief look at this issue?

[klauspost]

It seems like there are also some readers that assume that decompressed is always same or smaller than input:

https://github.com/klauspost/go/blob/1340662ab4c56f946337518bda94f861478d2661/src/debug/pe/file.go#L240

https://github.com/klauspost/go/blob/253dd08f5df3a45eafc97eec388636fcabfe0174/src/debug/macho/file.go#L649

So if the check is omitted it can also break things.

[dr2chase]

@golang/compilers

[ianlancetaylor]

@klauspost The code in debug/pe and debug/macho looks correct to me. It reads the decompressed length from the section header, and it decompresses into a buffer of that size. It should work fine if the decompressed length is larger.

[klauspost]

@ianlancetaylor Yes. You are correct, but there is an issue. Must be the checks above. Either way, to reproduce, allowing bigger zlib output breaks pe/DWARF parsing...

diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index dfb1d7bafb..c42d681c73 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -3210,7 +3210,7 @@ func compressSyms(ctxt *Link, syms []loader.Sym) []byte {
 	// compression levels of zlib.DefaultCompression, but takes
 	// substantially less time. This is important because DWARF
 	// compression can be a significant fraction of link time.
-	z, err := zlib.NewWriterLevel(&buf, zlib.BestSpeed)
+	z, err := zlib.NewWriterLevel(&buf, zlib.NoCompression)
 	if err != nil {
 		log.Fatalf("NewWriterLevel failed: %s", err)
 	}
@@ -3243,7 +3243,7 @@ func compressSyms(ctxt *Link, syms []loader.Sym) []byte {
 	if err := z.Close(); err != nil {
 		log.Fatalf("compression failed: %s", err)
 	}
-	if int64(buf.Len()) >= total {
+	if false && int64(buf.Len()) >= total {
 		// Compression didn't save any space.
 		return nil
 	}

Produces...

e:\godev\src\debug\pe (master -> origin)
λ e:\godev\bin\go test -run=DWARF
--- FAIL: TestInternalLinkerDWARF (1.02s)
    file_test.go:395: Testprog output:
        base=0x7ff7ba4c0000
        main=0x7ff7ba572ee0
        offset=0xb2ee0
    file_test.go:421: .debug_gdb_scripts section is not found
    file_test.go:442: Found main.main
--- FAIL: TestDWARF (0.51s)
    file_test.go:395: Testprog output:
        base=0x7ff7bd8c0000
        main=0x7ff7bd972660
        offset=0xb2660
    file_test.go:421: .debug_gdb_scripts section is not found
    file_test.go:442: Found main.main
FAIL
exit status 1
FAIL    debug/pe        4.414s

[cherrymui]

CL 714461 should fix the failure in debug/pe in #76022 (comment) . I'll look into the original failure (i.e. without suppressing the length check).

[gopherbot]

Change https://go.dev/cl/714461 mentions this issue: debug/pe: check for zdebug_gdb_scripts section in testDWARF