I tested sys_fs_fcntl(0xc0000007) aka cellFsArcadeHddSerialNumber on my real hardware, validated the result, and compared the output values with the ones obtained from some other methods on my PC.
It turned out that padding and alignment (model: left, serial: right) are used by sys_fs_fcntl(0xc0000007) to compose the output.

Following is the information of my HDD obtained through sys_fs_fcntl(0xc0000007) for example:

Model Name: "TOSHIBA MK3265GSX H                     "
Serial Number: "           0A1B2C3D4"

Also, it seems like the syscall doesn't care about the device name in the arguments, though it's supposed to be "CELL_FS_IOS:ATA_HDD".

I also tested sys_fs_fcntl(0xc0000015&0xc000001c) on my real hardware, and it turned out the error handling was a little bit different.
Following is the testing result on the real hardware:

path="/dev_usb000": return: CELL_OK, arg->out_code: CELL_OK
path="/dev_hdd0": return: CELL_OK, arg->out_code: CELL_ENOTMOUNTED
path="/": return: CELL_OK, arg->out_code: CELL_ENOTMOUNTED
path="": return: CELL_ENOMEM, arg->out_code: CELL_OK