syscall.Ptrace* always start in new thread, causing the tracee untraceable

[jm33-m0]

What version of Go are you using (go version)?

$ go version
go version go1.13.8 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""  
GOARCH="amd64"
GOBIN=""    
GOCACHE="/home/jm33/.cache/go-build"                                   
GOENV="/home/jm33/.config/go/env"                                      
GOEXE=""    
GOFLAGS=""                                             
GOHOSTARCH="amd64"       
GOHOSTOS="linux"        
GONOPROXY=""
GONOSUMDB=""                                           
GOOS="linux" 
GOPATH="/home/jm33/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"                              
GOROOT="/usr/lib/go-1.13"
GOSUMDB="sum.golang.org"
GOTMPDIR=""        
GOTOOLDIR="/usr/lib/go-1.13/pkg/tool/linux_amd64"                      
GCCGO="gccgo"        
AR="ar"            
CC="gcc"            
CXX="g++"              
CGO_ENABLED="1"                                                                                                
GOMOD=""             
CGO_CFLAGS="-g -O2"                                                    
CGO_CPPFLAGS=""                                                                                                
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"                                                    
CGO_LDFLAGS="-g -O2"                                                   
PKG_CONFIG="pkg-config"                                                
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build432196136=/tmp/go-build -gno-record-gcc-switches"

What did you do?

// attach to a running process
err = syscall.PtraceAttach(pid)
if err != nil {
	return fmt.Errorf("ptrace attach: %v", err)		
}

// read registers
regs := &syscall.PtraceRegs{}
err = syscall.PtraceGetRegs(pid, regs)
if err != nil {
	return fmt.Errorf("my pid is %d, reading regs from %d: %v", os.Getpid(), pid, err)
}

What did you expect to see?

Registers are successfully returned

What did you see instead?

my pid is 4018, reading regs from 4028: no such process from syscall.PtraceGetRegs(pid, regs)

The issue

• The process I was trying to attach to is attachable, as syscall.PtraceAttach(pid) returns no error
• Checking the /proc/$(pidof tracee)/status, found a TracerPid matching a non-existent process, I think it's a thread (which has already exited) of my go program
• Putting runtime.LockOSThread() at the beginning of my function doesn't solve this issue, TracerPid still doesn't match my current go process, which it should

[jm33-m0]

Adding:

runtime.LockOSThread()
defer runtime.UnlockOSThread()

right before syscall.PtraceAttach(pid) (instead of at the top of calling function) seems to fix this issue