Go: os: os.OpenFile() ignores FileMode when truncating a existing file

[WestleyK]

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

$ go version
go version go1.12.7 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

GOARCH="amd64"

GOBIN=""

GOCACHE="/home/westleyk/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="/home/westleyk/go"

GOPROXY=""

GORACE=""

GOROOT="/usr/local/go"

GOTMPDIR=""

GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

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-build074655655=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Here is a test code, that will use os.OpenFile() to open a test file with 0777:

package main

import (
    "log"
    "os"
)

func main() {
    f, err := os.OpenFile("foo.txt", os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0777)
    if err != nil {
        log.Fatalf("failed opening file: %s", err)
    }

    f.Close()
}

What did you expect to see?

When I run the code with go run test.go, and foo.txt does not exist, I get the expected results:

$ ls -l foo.txt
ls: cannot access 'foo.txt': No such file or directory

$ go run test.go 

$ ls -l foo.txt
-rwxr-xr-x 1 westleyk westleyk 0 Aug 12 12:16 foo.txt

But, when foo.txt already exist (with the "default" permission), the specified 777 file permission is ignored and not applied to foo.txt...

$ touch foo.txt
$ ls -l foo.txt 
-rw-r--r-- 1 westleyk westleyk 0 Aug 12 12:17 foo.txt

$ go run test.go 

$ ls -l foo.txt 
-rw-r--r-- 1 westleyk westleyk 0 Aug 12 12:17 foo.txt

I expected when truncating the already-existing file, os.OpenFile will truncated it, and apply the specified file permission.

What did you see instead?

os.OpenFile() ignored the 0777 permission, and continued without an error.

[dmitshur]

Thanks for reporting.

Do you know (or can you check) what was the behavior in Go 1.11 or older versions? In other words, is this a recent regression, or has it always behaved like that?

[WestleyK]

@dmitshur, I got the same behavior as above with go1.11, all the way down to go1.2.2. So _all_ the go versions are like this.

[networkimprov]

I believe you'll see the same behavior from a C program; i.e. that's POSIX.

[dmitshur]

Thank you for confirming this is not a recent regression, that's very helpful.

This issue is under the Go1.14 milestone, so we can investigate it during the upcoming development cycle.

[WestleyK]

Yeah, a similar c program does this same behavior, which seems a little weird... I guess this is not a issue with Go. Thanks for the help! Close when ready :+1:

[odeke-em]

Closing as per https://github.com/golang/go/issues/33605#issuecomment-520672726 and thanks @WestleyK for the report, @dmitshur for the triage and @networkimprov for catching that an equivalent C program produces the same result.

For the record here is the equivalent C program

#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/errno.h>
#include <stdio.h>

int main() {
    int fd = open("foo.txt", O_CREAT|O_TRUNC|O_WRONLY, 0777);
    if (fd < 0) {
        fprintf(stderr, "opening file: %s\n", strerror(errno));
        return -1;
    }
    close(fd);
    return 0;
}

and when run, here are the results:

$ gcc main.c -o main
$ ls -lrth
total 32
-rw-r--r--  1 emmanuelodeke  staff   315B 13 Aug 00:18 main.c
-rwxr-xr-x  1 emmanuelodeke  staff   8.5K 13 Aug 00:19 main
$ ./main 
$ ls -lrth
total 32
-rw-r--r--  1 emmanuelodeke  staff   315B 13 Aug 00:18 main.c
-rwxr-xr-x  1 emmanuelodeke  staff   8.5K 13 Aug 00:19 main
-rwxr-xr-x  1 emmanuelodeke  staff     0B 13 Aug 00:19 foo.txt
$ ./main 
$ ls -lrth
total 32
-rw-r--r--  1 emmanuelodeke  staff   315B 13 Aug 00:18 main.c
-rwxr-xr-x  1 emmanuelodeke  staff   8.5K 13 Aug 00:19 main
-rwxr-xr-x  1 emmanuelodeke  staff     0B 13 Aug 00:20 foo.txt
$ chmod 0777 foo.txt # Now trying with chmod and this should change the permissions
$ ls -lrth
total 32
-rw-r--r--  1 emmanuelodeke  staff   315B 13 Aug 00:18 main.c
-rwxr-xr-x  1 emmanuelodeke  staff   8.5K 13 Aug 00:19 main
-rwxrwxrwx  1 emmanuelodeke  staff     0B 13 Aug 00:20 foo.txt