Does not happen here (openSUSE Tumbleweed, ext4 filesystem), which filesystem do you use?

The fiemap feature was introduces lately in genimage (see [1]), and there is a fallback in case the fiemap ioctl fails (see [2]):

	/* If failure is due to no filesystem support, return a single extent */
	if (ret == -EOPNOTSUPP)
		return whole_file_exent(size, extents, extent_count);

You can try to extend this check for ENOTTY (25) like the following:

	if (ret == -EOPNOTSUPP || ret == -ENOTTY)

or more correct (as 'ret = -errno;' some lines above):

	if (ret == EOPNOTSUPP || ret == ENOTTY)

and/or fallback to an earlier version of genimage in buildroot
by reverting the latest version bump commit [3]...

[1] https://github.com/pengutronix/genimage/commit/1b98d6fadfca76cc0202a2df3debc67a8fe0ee1a
[2] https://github.com/pengutronix/genimage/blob/master/util.c#L390
[3] https://git.buildroot.net/buildroot/commit/?id=677b0d9f574e2860cd22cb36022e5c059b4e6af5

Some additional information:

I compiled in WSL (Ubuntu 20.04 LTS) under Windows 10:

marie@2020L009:~$ mount
rootfs on / type lxfs (rw,noatime)
none on /dev type tmpfs (rw,noatime,mode=755)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
devpts on /dev/pts type devpts (rw,nosuid,noexec,noatime,gid=5,mode=620)
none on /run type tmpfs (rw,nosuid,noexec,noatime,mode=755)
none on /run/lock type tmpfs (rw,nosuid,nodev,noexec,noatime)
none on /run/shm type tmpfs (rw,nosuid,nodev,noatime)
none on /run/user type tmpfs (rw,nosuid,nodev,noexec,noatime,mode=755)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,relatime)
tmpfs on /sys/fs/cgroup type tmpfs (rw,nosuid,nodev,noexec,relatime,mode=755)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
C:\ on /mnt/c type drvfs (rw,noatime,uid=1000,gid=1000,case=off)
marie@2020L009:~$ pwd
/home/marie

I added second suggested line and called "make" again.
Now I get the following output:

>>>   Generating filesystem image rootfs.ext2
mkdir -p /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images
rm -rf /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2
mkdir -p /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2
rsync -auH --exclude=/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/target/ /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target
echo '#!/bin/sh' > /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
echo "set -e" >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
echo "chown -h -R 0:0 /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target" >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
PATH="/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/bin:/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/mnt/c/Program Files/WindowsApps/CanonicalGroupLimited.UbuntuonWindows_2004.2020.424.0_x64__79rhkp1fndgsc:/mnt/c/Program Files (x86)/Common Files/Oracle/Java/javapath:/mnt/c/WINDOWS/system32:/mnt/c/WINDOWS:/mnt/c/WINDOWS/System32/Wbem:/mnt/c/WINDOWS/System32/WindowsPowerShell/v1.0/:/mnt/c/WINDOWS/System32/OpenSSH/:/mnt/c/Program Files/TortoiseGit/bin:/mnt/c/Program Files/Git/cmd:/mnt/c/Program Files/Nordic Semiconductor/nrf-command-line-tools/bin/:/mnt/c/Program Files (x86)/STMicroelectronics/STM32 ST-LINK Utility/ST-LINK Utility:/mnt/c/Users/marie.maurer/AppData/Local/Microsoft/WindowsApps:/mnt/c/util:/mnt/c/users/marie.maurer/appdata/local/programs/python/python38/Scripts:/snap/bin" /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/support/scripts/mkusers /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/full_users_table.txt /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
echo "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/bin/makedevs -d /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/full_devices_table.txt /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target" >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
printf '   \n' >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
printf '        rm -f /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rootfs.ext2\n       /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/sbin/mkfs.ext4 -d /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target -r 1 -N 0 -m 5 -L "rootfs" -O ^64bit /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rootfs.ext2 "120M" || { ret=$?; echo "*** Maybe you need to increase the filesystem size (BR2_TARGET_ROOTFS_EXT2_SIZE)" 1>&2; exit $ret; }\n' >> /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
chmod a+x /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
PATH="/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/bin:/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/mnt/c/Program Files/WindowsApps/CanonicalGroupLimited.UbuntuonWindows_2004.2020.424.0_x64__79rhkp1fndgsc:/mnt/c/Program Files (x86)/Common Files/Oracle/Java/javapath:/mnt/c/WINDOWS/system32:/mnt/c/WINDOWS:/mnt/c/WINDOWS/System32/Wbem:/mnt/c/WINDOWS/System32/WindowsPowerShell/v1.0/:/mnt/c/WINDOWS/System32/OpenSSH/:/mnt/c/Program Files/TortoiseGit/bin:/mnt/c/Program Files/Git/cmd:/mnt/c/Program Files/Nordic Semiconductor/nrf-command-line-tools/bin/:/mnt/c/Program Files (x86)/STMicroelectronics/STM32 ST-LINK Utility/ST-LINK Utility:/mnt/c/Users/marie.maurer/AppData/Local/Microsoft/WindowsApps:/mnt/c/util:/mnt/c/users/marie.maurer/appdata/local/programs/python/python38/Scripts:/snap/bin" FAKEROOTDONTTRYCHOWN=1 /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/host/bin/fakeroot -- /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/fakeroot
rootdir=/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/ext2/target
table='/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/buildroot-fs/full_devices_table.txt'
mke2fs 1.45.6 (20-Mar-2020)
Creating regular file /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rootfs.ext2
64-bit filesystem support is not enabled.  The larger fields afforded by this feature enable full-strength checksumming.  Pass -O 64bit to rectify.
Creating filesystem with 122880 1k blocks and 30720 inodes
Filesystem UUID: c81809b4-af64-4851-83b9-838060850343
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729

Allocating group tables: done
Writing inode tables: done
Creating journal (4096 blocks): done
Copying files into the device: done
Writing superblocks and filesystem accounting information: done

ln -sf rootfs.ext2 /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rootfs.ext4
>>>   Executing post-image script board/raspberrypi3/post-image.sh
board/raspberrypi3/genimage-raspberrypi3.cfg:34: no sub-section title/index for 'config'
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"" (stderr):
INFO: cmd: "rm -rf "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"/*" (stderr):
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"" (stderr):
INFO: cmd: "cp -a "/tmp/tmp.gpxMoEOrt8" "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root"" (stderr):
INFO: cmd: "find '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root' -depth -type d -printf '%P\0' | xargs -0 -I {} touch -r '/tmp/tmp.gpxMoEOrt8/{}' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root/{}'" (stderr):
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images"" (stderr):
INFO: vfat(boot.vfat): cmd: "dd if=/dev/zero of="/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat" seek=33554432 count=0 bs=1 2>/dev/null" (stderr):
INFO: vfat(boot.vfat): cmd: "mkdosfs   '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-3-b.dtb' as 'bcm2710-rpi-3-b.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-3-b.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-3-b-plus.dtb' as 'bcm2710-rpi-3-b-plus.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-3-b-plus.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-cm3.dtb' as 'bcm2710-rpi-cm3.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-cm3.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/bootcode.bin' as 'rpi-firmware/bootcode.bin' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/bootcode.bin' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/cmdline.txt' as 'rpi-firmware/cmdline.txt' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/cmdline.txt' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/config.txt' as 'rpi-firmware/config.txt' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/config.txt' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/fixup.dat' as 'rpi-firmware/fixup.dat' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/fixup.dat' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/start.elf' as 'rpi-firmware/start.elf' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/start.elf' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/overlays' as 'rpi-firmware/overlays' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/overlays' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'zImage' as 'zImage' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/zImage' '::'" (stderr):
INFO: hdimage(sdcard.img): adding partition 'boot' (in MBR) from 'boot.vfat' ...
ERROR: hdimage(sdcard.img): fiemap /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat: 25 Inappropriate ioctl for device
ERROR: hdimage(sdcard.img): failed to write image partition 'boot'
INFO: hdimage(sdcard.img): cmd: "rm -f "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/sdcard.img"" (stderr):
ERROR: hdimage(sdcard.img): failed to generate sdcard.img
make: *** [Makefile:835: target-post-image] Error 1

(In reply to Marie Maurer from comment #3)

Wrong log output in previous comment, sorry!

Here is the right one:

>>>   Executing post-image script board/raspberrypi3/post-image.sh
board/raspberrypi3/genimage-raspberrypi3.cfg:34: no sub-section title/index for 'config'
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"" (stderr):
INFO: cmd: "rm -rf "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"/*" (stderr):
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp"" (stderr):
INFO: cmd: "cp -a "/tmp/tmp.gpxMoEOrt8" "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root"" (stderr):
INFO: cmd: "find '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root' -depth -type d -printf '%P\0' | xargs -0 -I {} touch -r '/tmp/tmp.gpxMoEOrt8/{}' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/build/genimage.tmp/root/{}'" (stderr):
INFO: cmd: "mkdir -p "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images"" (stderr):
INFO: vfat(boot.vfat): cmd: "dd if=/dev/zero of="/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat" seek=33554432 count=0 bs=1 2>/dev/null" (stderr):
INFO: vfat(boot.vfat): cmd: "mkdosfs   '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-3-b.dtb' as 'bcm2710-rpi-3-b.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-3-b.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-3-b-plus.dtb' as 'bcm2710-rpi-3-b-plus.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-3-b-plus.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'bcm2710-rpi-cm3.dtb' as 'bcm2710-rpi-cm3.dtb' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/bcm2710-rpi-cm3.dtb' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/bootcode.bin' as 'rpi-firmware/bootcode.bin' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/bootcode.bin' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/cmdline.txt' as 'rpi-firmware/cmdline.txt' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/cmdline.txt' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/config.txt' as 'rpi-firmware/config.txt' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/config.txt' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/fixup.dat' as 'rpi-firmware/fixup.dat' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/fixup.dat' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/start.elf' as 'rpi-firmware/start.elf' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/start.elf' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'rpi-firmware/overlays' as 'rpi-firmware/overlays' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/rpi-firmware/overlays' '::'" (stderr):
INFO: vfat(boot.vfat): adding file 'zImage' as 'zImage' ...
INFO: vfat(boot.vfat): cmd: "MTOOLS_SKIP_CHECK=1 mcopy -bsp -i '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat' '/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/zImage' '::'" (stderr):
INFO: hdimage(sdcard.img): adding partition 'boot' (in MBR) from 'boot.vfat' ...
ERROR: hdimage(sdcard.img): fiemap /home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/boot.vfat: 25 Inappropriate ioctl for device
ERROR: hdimage(sdcard.img): failed to write image partition 'boot'
INFO: hdimage(sdcard.img): cmd: "rm -f "/home/marie/buildroot-2020.08-rc1/buildroot-2020.08-rc1/output/images/sdcard.img"" (stderr):
ERROR: hdimage(sdcard.img): failed to generate sdcard.img
make: *** [Makefile:835: target-post-image] Error 1

I had to use this line to get it working:

if (ret == -EOPNOTSUPP || ret == -ENOTTY)

Now a sdcard.img was generated. No more error message.

I've just given a test-build in Ubuntu 20.04, on augfs over ext4 (in docker),
and the build succeeds without issue (as ;aready reported by Peter).

Can you provide more info on your running environment:
  - what distribution?
  - what filesystem?
  - are you running in WSL (windows Subsystem for Linux)? What version?

Note that running under WSL1 is not guaranteed to work. WSL2 is also very
very seldom tested.

I compiled in WSL (Ubuntu 20.04 LTS) under Windows 10.
I think since yesterday it updated to 20.04.1?
I don't know WSL version. How to get it?

Regarding Filesystem: See https://bugs.busybox.net/show_bug.cgi?id=13146#c2

(In reply to Marie Maurer from comment #5)

Thanks for feedback, good to hear it now works for you (the version without the minus sign was definitely wrong, did read the code wrong yesterday)...

(In reply to Yann E. MORIN from comment #6)

Is resolved/worksforme not a little harsh as there is definitely a existing problem (and a fix for it) and not yet determined if it is caused by the  filesystem and/or a WSL?

(In reply to Marie Maurer from comment #5)

Mind to suggest the patch/fix upstream (pengutronix/genimage)?

(In reply to Peter Seiderer from comment #10)

> Is resolved/worksforme not a little harsh as there is definitely a existing
> problem (and a fix for it) and not yet determined if it is caused by the
> filesystem and/or a WSL?

genimage does not work on WSL1 (from a previous IRC discussion some weeks
ago); WSL1 was too limited to be really workable anyway. It *might* work on
WSL2 (WSL2 is usually much better since it is in fact just a VM in disguise).

And yes, "works for me", because, yes, it works for me on a *real* Linux
machine, and that's what Buildroot is made to run on:

https://buildroot.org/manual.html#requirement

Quoting: "Buildroot is designed to run on Linux systems."

If it works on other systems, all the better. If it does not and you can
come with a non-invasive fix, good, please send it. Otherwise, meh.

(In reply to Marie Maurer from comment #7)

> I compiled in WSL (Ubuntu 20.04 LTS) under Windows 10.

Please my reply in #12.

> I don't know WSL version. How to get it?

I have absolutely no idea, I have not used Windows for the
past 18 years or so... But fortunately:

https://lmgtfy.com/?q=how+can+I+know+the+version+of+WSL+I+use+%3F&s=d

Regards.

This is the info I got under Linux:

marie@2020L009:~$ uname -a
Linux 2020L009 4.4.0-19041-Microsoft #1-Microsoft Fri Dec 06 14:06:00 PST 2019 x86_64 x86_64 x86_64 GNU/Linux

This is the info I got under Windows (partly Powershell):

PS C:\WINDOWS\system32> wsl -l -v
  NAME      STATE           VERSION
* Ubuntu    Stopped         1
PS C:\WINDOWS\system32>

C:\tmp>ver

Microsoft Windows [Version 10.0.19041.450]

C:\tmp>systeminfo.exe

Betriebssystemversion: 10.0.19041 Nicht zutreffend Build 19041

=> I assume it is WSL 1

Fix/workaround posted upstream, see [1].

[1] https://github.com/pengutronix/genimage/pull/116

Many thanks!