os: ModeSetgid has no effect while using with Mkdir() on Linux #25539
Description
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)