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bug#61386: [PATCH] cp,mv,install: Disable sparse copy on macOS

From: Pádraig Brady
Subject: bug#61386: [PATCH] cp,mv,install: Disable sparse copy on macOS
Date: Sun, 12 Feb 2023 20:36:24 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:109.0) Gecko/20100101 Thunderbird/109.0

On 12/02/2023 14:15, Pádraig Brady wrote:
On 11/02/2023 21:53, Paul Eggert wrote:
On 2023-02-10 17:21, George Valkov wrote:

remember to trace all code paths errors.

Yes, I've been doing that. However, I don't use macOS and the two of us
are debugging remotely where I write the code and you debug it but
neither of us know how macOS works. Of course it would be much more
efficient if we weren't operating with these obstacles, but here we are.

Because the macOS behavior is poorly documented and we lack the source,
if we can't figure this out fairly quickly coreutils should simply stop
using fclonefileat because it's unreliable for users and a time sink for
us. I'm hoping, though, we can get something working with another round
or two.

With CLONE_ACL, I get 22 Invalid argument.

OK, this means that although Apple has documented CLONE_ACL and it's in
your include files, it doesn't work in your version of macOS, because
either it's not supported by the kernel, or by the filesystem code, or
by the particular filesystem instance, or for some other reason. EINVAL
hints that it's the kernel but that's by no means certain.

One possible way to defend against this macOS misbehavior is for cp to
try to use CLONE_ACL, and if that fails with errno == EINVAL try again
without CLONE_ACL. I attempted to implement this in the attached patch;
please give it a try.

I tested this on macOS 13.2 and the first fclonefileat(..., CLONE_ACL) succeeds.
I tested with various umask, --no-preserve=mode, -p, ... combinations
and didn't find any inconsistencies in permissions with --reflink=never copies.
Actually scratch that. There was an older xcode installed so CLONE_ACL was 0.
I've not got access at the moment, so will need to retest later with newer 

Good news is that with newer xcode supporting macOS 13.1
CLONE_ACL is set and that all works as expected, with the
first fclonefileat() succeeding.
I checked with actual ACLs (set with `chmod +a "user:$USER allow readattr" 
and they were copied or dropped as expected. Note ls -l on macOS will list
ACLs with a "+" rather than a "@", as was mentioned elsewhere in this thread.

One divergence with fclonefileat() is that it writes through a dangling symlink,
where a standard --reflink=never copy will fail.
I.e. `make TESTS=tests/cp/thru-dangling.sh SUBDIRS=. check` fails.
Now there is nothing wrong with writing through the dangling symlink,
and in fact standard cp will also with POSIXLY_CORRECT set in the environment.
So I may just need need to adjust the test to work / skip in this case.

Another divergence I noticed is wrt umask and setuid handling.
The following transcript shows that with fclonefileat() we do _not_
preserve setuid when it is preserved without.
Also with fclonefileat() the umask is not honored,
while it is honored without.

# id -u

# echo create setuid file
# rm src/cp.s; src/cp src/cp src/cp.s; chmod u+s src/cp.s

# umask; for reflink in always never; do
>  for preserve in mode timestamps; do
>   rm -f src/cp.s.$reflink.$preserve
>   src/cp --reflink=$reflink --preserve=$preserve src/cp.s 
>  done
> done
> ls -le src/cp.s*
-rwsrwxr-x  1 root  staff  234032 12 Feb 20:21 src/cp.s
-rwxrwxr-x  1 root  staff  234032 12 Feb 20:21 src/cp.s.always.mode
-rwxrwxr-x  1 root  staff  234032 12 Feb 20:21 src/cp.s.always.timestamps
-rwsrwxr-x  1 root  staff  234032 12 Feb 20:21 src/cp.s.never.mode
-rwxr-xr-x  1 root  staff  234032 12 Feb 20:21 src/cp.s.never.timestamps


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