I wrote:
> But what I observe is that the behaviour is the same on all major file
> systems on Linux.
>
> Tested on Linux 5.3.7 (fc31), with the trivial program below, on
> - ext4
> - btrfs
> - jfs
> - reiserfs
> - xfs
This are different when using the code which goes through the /proc
file system:
===============================================================================
#define _GNU_SOURCE 1
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
int main (int argc, char *argv[])
{
int mode = strtol (argv[1], NULL, 8);
char* path = argv[2];
int ret;
int fd = openat (AT_FDCWD, path, O_PATH | O_NOFOLLOW | O_CLOEXEC);
if (fd < 0)
ret = -1;
else {
char buf[100];
sprintf (buf, "/proc/self/fd/%d", fd);
ret = chmod (buf, mode);
}
if (ret < 0)
perror ("lchmod");
}
===============================================================================
In this case:
- On ext4, btrfs, jfs, xfs:
mode changed, but error "Operation not supported".
- On reiserfs:
mode changed, success.
