os: ModeSetgid has no effect while using with Mkdir() on Linux

[Al2Klimov]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/root/go"
GORACE=""
GOROOT="/usr/lib/golang"
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build203196509=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

package main

import (
	"os"
)

func main() {
	os.Mkdir("test", 0770 | os.ModeSetgid)
}

What did you expect to see?

# ls -la test
insgesamt 4
drwxr-s---.  2 root root    6 24. Mai 04:09 .
dr-xr-x---. 13 root root 4096 24. Mai 04:09 ..
#

What did you see instead?

# ls -la test
insgesamt 4
drwxr-x---.  2 root root    6 24. Mai 04:09 .
dr-xr-x---. 13 root root 4096 24. Mai 04:09 ..
#

Why did this happen?

According to strace -f ./mkdir the Go stdlib behaves as expected...

[pid  6782] mkdirat(AT_FDCWD, "test", 02770 <unfinished ...>
[pid  6782] <... mkdirat resumed> )     = 0

... but on Linux this is not enough, see mkdirat(2):

The mkdirat() system call operates in exactly the same way as mkdir(2), (...)

... and mkdir(2):

The  argument  mode  specifies the permissions to use.
It is modified (...): the permissions of the created directory are (mode & ~umask & 0777).
Other mode bits of the created directory depend on the operating system.
For Linux, see below.
(...)
That is, under Linux the created directory actually gets mode (mode & ~umask & 01777)

How could this be fixed?

Similar to #8383, via Chmod.

[tklauser]

/cc @bradfitz @ianlancetaylor @rsc

[ianlancetaylor]

Seems that we should fix that along the lines that you point out. Want to send a patch?

[Al2Klimov]

I'm afraid I have to correct myself: It's not just Linux, it seems to be all *nix.

Discovered this by running this on my Mac workstation:

#include <sys/stat.h>

int main() {
	mkdir("test", 02770);
}

vs.

#include <sys/stat.h>

int main() {
	mkdir("test", 02770);
	chmod("test", 02770);
}

[odeke-em]

/cc @jessfraz @lizrice too

[jessfraz]

Dope, if you don't want to send a patch I can make one, just let me know

[dnsmichi]

@jessfraz Thanks. Haven't tested the linked PR yet, I'll talk to @Al2Klimov in terms of signing the CLA on Monday :)

[Al2Klimov]

As discussed w/ @dnsmichi (offline): I've just overseen an elephant:

I forgot to do this one.