pkg/system: Allow EnsureRemoveAll to delete trees with immutable files

[dfr]

Fixes #1354.

Signed-off-by: Doug Rabson [email protected]

[rhatdan]

LGTM
@nalind @vrothberg @giuseppe @flouthoc @saschagrunert PTAL

[nalind]

There's probably a corresponding function on Linux that uses the FS_IOC_SETFLAGS ioctl to clear immutable flags, but we don't tend to run into that in routine use, so we can leave that for another time.
LGTM.

[rhatdan]

int attr;
fd = open("pathname", ...);

ioctl(fd, FS_IOC_GETFLAGS, &attr);  /* Read current flags */
attr |= FS_NOATIME_FL;              /* Add desired flag */
ioctl(fd, FS_IOC_SETFLAGS, &attr);  /* Set updated flags */

The interesting part is the flags themselves. There are some that help optimize file-operations, others that manipulate the file operational mode and yes, there are always some exotic flags that even after reading their description twice I still don’t know how it would actually affect the system.

But, inside this haystack I found two shiny needles that really caught my eye and got my heart rate up, and yeah, I know this sounds nerdy :)

    FS_APPEND_FL: Mark a file, so that it can be opened only with the O_APPEND flag.
    FS_IMMUTABLE_FL: The file is immutable: no changes are permitted to the file contents or metadata (permissions, timestamps, ownership, link count and so on).

[rhatdan]

https://github.com/snapcore/snapd/blob/master/osutil/chattr.go

[rhatdan]

One issue with this is that it will be slow, since it needs to walk the trees twice.

Would it be better to RemoveAll then if it fails attempt to change the immutable and try to removeall again.

[dfr]

I think immutable and append flags are generally useful. BSDs have used immutable, combined with securelevel, to protect suid utilities like login and su for a very long time. The point about efficiency is a good one - I used WalkDir which should avoid excessive stat calls but still, it would be better to check for an EPERM error from RemoveAll first.

I was working on a separate PR which will preserve the BSD flags in pkg/archive but got slightly distracted by fixing tests on FreeBSD. I will add a commit to that to avoid the double tree walk in simple cases.