MKNOD "Function not implemented" but is now used by nfs-kernel-server

[gdelafond]

Hello,

When exporting through NFS with at least nfs-kernel-server from Debian stretch (1.3.4-2.1), it uses mknod() system call to create a file.

Create a file locally (touch /cg/test):

unique: 4, opcode: LOOKUP (1), nodeid: 1, insize: 48, pid: 107156
   unique: 4, success, outsize: 144
unique: 5, opcode: LOOKUP (1), nodeid: 1932, insize: 48, pid: 107156
   unique: 5, error: -2 (No such file or directory), outsize: 16
unique: 6, opcode: CREATE (35), nodeid: 1932, insize: 64, pid: 107156
   unique: 6, success, outsize: 160
unique: 7, opcode: FLUSH (25), nodeid: 1942, insize: 64, pid: 107156
   unique: 7, error: -38 (Function not implemented), outsize: 16
unique: 8, opcode: SETATTR (4), nodeid: 1942, insize: 128, pid: 107156
   unique: 8, success, outsize: 120
unique: 9, opcode: RELEASE (18), nodeid: 1942, insize: 64, pid: 0
   unique: 9, success, outsize: 16
unique: 10, opcode: STATFS (17), nodeid: 1, insize: 40, pid: 72040
   unique: 10, success, outsize: 96

Create a file through an NFS mount point (touch /mnt/cg/test) :

unique: 70, opcode: LOOKUP (1), nodeid: 1932, insize: 46, pid: 107327
   unique: 70, error: -2 (No such file or directory), outsize: 16
unique: 71, opcode: LOOKUP (1), nodeid: 1932, insize: 46, pid: 107327
   unique: 71, error: -2 (No such file or directory), outsize: 16
unique: 72, opcode: MKNOD (8), nodeid: 1932, insize: 62, pid: 107327
   unique: 72, error: -38 (Function not implemented), outsize: 16
unique: 73, opcode: LOOKUP (1), nodeid: 1932, insize: 46, pid: 107327
   unique: 73, error: -2 (No such file or directory), outsize: 16
unique: 74, opcode: STATFS (17), nodeid: 1, insize: 40, pid: 72040

As far as I know, nfs was not using mknod() on Debian Wheezy (and jessie I think).

Is there a way to implement it (for S_IFREG flag only of course)?

[rgacogne]

Great, using mknod() to create a regular file does not only seem very weird, it's also not a portable use of mknod() according to POSIX:

POSIX.1-2001 says: "The only portable use of mknod() is to create a
FIFO-special file.  If mode is not S_IFIFO or dev is not 0, the
behavior of mknod() is unspecified.