Subject: bug#79139: cp --reflink truncates sparse files on ZFS

From: Leah Neukirchen <[email-masked]>
Date: Fri, 01 Aug 2025 17:00:36 +0200

Hello,

I found the following issue with coreutils 9.7, Linux 6.12.40-1-lts,
zfs 2.3.3 on Arch x86_64, glibc 2.42:

Copying a file with sparse holes using "cp --reflink=auto" truncates
the file before the final segment.  The relevant strace is:

openat(AT_FDCWD, "celestis.img", O_RDONLY|O_PATH|O_DIRECTORY) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/.zfs/snapshot/pre-fixup/var/lib/libvirt/images/celestis.img", {st_mode=S_IFREG|0644,>
openat(AT_FDCWD, "/.zfs/snapshot/pre-fixup/var/lib/libvirt/images/celestis.img", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=137438953472, ...}) = 0
openat(AT_FDCWD, "celestis.img", O_WRONLY|O_CREAT|O_EXCL, 0644) = 4
ioctl(4, BTRFS_IOC_CLONE or FICLONE, 3) = -1 EXDEV (Invalid cross-device link)
fstat(4, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lseek(3, 0, SEEK_DATA)                  = 0
fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
lseek(3, 0, SEEK_HOLE)                  = 131072
lseek(3, 0, SEEK_SET)                   = 0
copy_file_range(3, NULL, 4, NULL, 131072, 0) = 131072
lseek(3, 131072, SEEK_DATA)             = 1048576
lseek(3, 1048576, SEEK_HOLE)            = 1179648
lseek(3, 1048576, SEEK_SET)             = 1048576
lseek(4, 917504, SEEK_CUR)              = 1048576
copy_file_range(3, NULL, 4, NULL, 131072, 0) = 131072
lseek(3, 1179648, SEEK_DATA)            = 4194304
lseek(3, 4194304, SEEK_HOLE)            = 16646144
lseek(3, 4194304, SEEK_SET)             = 4194304
lseek(4, 3014656, SEEK_CUR)             = 4194304
copy_file_range(3, NULL, 4, NULL, 12451840, 0) = 12451840
lseek(3, 16646144, SEEK_DATA)           = 134217728
lseek(3, 134217728, SEEK_HOLE)          = 137438953472
lseek(3, 134217728, SEEK_SET)           = 134217728
lseek(4, 117571584, SEEK_CUR)           = 134217728
copy_file_range(3, NULL, 4, NULL, 137304735744, 0) = 137304735744
mmap(NULL, 270336, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x76c5df0ce000
read(3, "", 262144)                     = 0
ftruncate(4, 134217728)                 = 0
close(4)                                = 0
close(3)                                = 0

As we can see, there's a hole from 16646144 to 134217728, then data up
to the end at 137438953472 (= the total file size).  Both fd are thus
moved to 134217728, and a copy_file_range for the rest of the file is
issued and successful.

However, in the end the file is truncated to the first 128MB... why?

For comparison, a plain cat simply does this:

fstat(1, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
openat(AT_FDCWD, "/.zfs/snapshot/pre-fixup/var/lib/libvirt/images/celestis.img", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=137438953472, ...}) = 0
fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 137438953472
mmap(NULL, 270336, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7e7af83a9000
read(3, "", 262144)                     = 0
munmap(0x7e7af83a9000, 270336)          = 0
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0

This works correctly and the source and destination agree in the end.
Likewise for xcp(1), which uses copy_file_range in 1MB blocks by
default and does not care for holes.

Thus I think this is a logic bug in cp and not a ZFS issue.

Do not hesitate to contact me if you inquire further details.

Thanks,
-- 
Leah Neukirchen  <[email-masked]>  https://leahneukirchen.org/