Issue #2953 has been reported by zhtw.

Bug #2953: open(2) returns "Permission denied" instead of "File exists"

* Author: zhtw
* Status: New
* Priority: Normal
* Assignee: 
* Category: Kernel
* Target version: 
I am porting ChezScheme to DragonFly. Turned out ChezScheme has very thorough 
set of tests. I found that open(2) returns wrong error code when a file is 
opened for writing with both O_EXCL and O_CREAT flags set, but it (the file) is 
not writable.
Not sure if this is actually considered "wrong" by Posix, but most (if not all) 
Unixes agree on that and return "File exists."

I only checked Darwin and FreeBSD (besides DragonFly), but all ChezScheme tests 
pass on Linux as well, so I'm guessing Linux behaves the same way.

a@kl:~/tmp$ uname -a
DragonFly 4.6-RELEASE DragonFly v4.6.0.10.g16fba-RELEASE #10: Wed Aug 
17 14:26:31 CEST 2016  
a@kl:~/tmp$ cat test-open.c
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>

int main()
  int fd = open("aoeu", O_WRONLY | O_EXCL | O_CREAT);
  if (fd == -1) {
    return 1;
a@kl:~/tmp$ ls -l aoeu
-r--r--r--  1 a  users  0 23-Sep-2016 07:51 aoeu
a@kl:~/tmp$ cc test-open.c
a@kl:~/tmp$ ./a.out
Permission denied

The same program returns "File exists." both on FreeBSD 10.3 & Darwin 15.6.0.
(Almost certainly the same on Linux.)

$ uname -a
FreeBSD  10.3-RELEASE FreeBSD 10.3-RELEASE #0 r297264: Fri Mar 25 02:10:02 UTC 
2016  amd64
$ ls -l aoeu
-r--r--r--  1 a  a  0 Sep 22 23:28 aoeu
$ cc test-open.c
$ ./a.out
File exists

a@zdev:~/tmp$ uname -a
Darwin zdev.local 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 
2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64
a@zdev:~/tmp$ ls -l aoeu
-r--r--r--  1 a  staff  0 Sep 22 22:54 aoeu
a@zdev:~/tmp$ cc test-open.c
a@zdev:~/tmp$ ./a.out
File exists

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