ioctl handler of the hidraw driver should return ENOIOCTLCMD for numbers it does not want to handle | tcgetattr() ⇔ ioctl(, TCGETS2, …) sets errno to undocumented EINVAL/22

["Дилян Палаузов" <dilyan.palauzov@aegee.org>]

Hello,

With kernel 6.17.6 on x86_64 and glibc 2.42 the below program substitutes tcgetattr() with ioctl(…, TCGETS2, …) and sets errno to 22 / Invalid argument / EINVAL.  The only difference for kernel 6.6.60 on armv7l 2.26 is that the substitution has no 2, tcgetattr() translates to ioctl(…, TCGETS, …).

For tcgetattr() only errno EBADF and ENOTTY are documented at https://sourceware.org/glibc/manual/latest/html_mono/libc.html#index-tcgetattr , at https://man7.org/linux/man-pages/man3/tcgetattr.3p.html and at https://pubs.opengroup.org/onlinepubs/9799919799/functions/tcgetattr.html (Open Group Base Specifications Issue 8/year 2024).

In this concrete case changes to the man7.org documentation must be triggered by the Austin Group - https://lore.kernel.org/linux-man/dsb6oiv7q7ra3gbu4bovy3gah522lgsf3d6h3wxwe4ieuka6fh@g6u5qzlyoxf5/ and but https://sourceware.org/bugzilla/show_bug.cgi?id=33597 suggests this is a kernel bug.

For hidraw devices the possible ioctl commands are mentioned at https://docs.kernel.org/hid/hidraw.html#ioctl, TCGETS and TCGETS2 are not among them, so I was expecting a ENOTTY error.  This problem might also happen for to tcsetattr() - I have not checked it.

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <termios.h>

void main() {
  int fd = open("/dev/hidraw0", O_RDWR | O_NONBLOCK |O_NOCTTY);
  struct termios s;
  errno = 0;
  int ret = tcgetattr(fd, &s);
  printf("Returned fd is %i ret is %i errno is %i %m\n", fd, ret, errno);
}

The above produces:

Returned fd is 3 ret is -1 errno is 22 Invalid argument

Kind regards // Дилян