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SubjectRe: [PATCH v4 2/5] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes
On Thu, Dec 19, 2019 at 12:55 AM Sargun Dhillon <> wrote:

> +#define PIDFD_IOCTL_GETFD _IOWR('p', 0xb0, __u32)

This describes an ioctl command that reads and writes a __u32 variable
using a pointer passed as the argument, which doesn't match the

> +static long pidfd_getfd(struct pid *pid, u32 fd)
> +{
> + return retfd;

This function passes an fd as the argument and returns a new
fd, so the command number would be

#define PIDFD_IOCTL_GETFD _IO('p', 0xb0)

While this implementation looks easy enough, and it is roughly what
I would do in case of a system call, I would recommend for an ioctl
implementation to use the __u32 pointer instead:

static long pidfd_getfd_ioctl(struct pid *pid, u32 __user *arg)
int their_fd, new_fd;
int ret;

ret = get_user(their_fd, arg);
if (ret)
return ret;

new_fd = pidfd_getfd(pid, their_fd);
if (new_fd < 0)
return new_fd;

return put_user(new_fd, arg);

Direct argument passing in ioctls may confuse readers because it
is fairly unusual, and it doesn't work with this:

> const struct file_operations pidfd_fops = {
> .release = pidfd_release,
> .poll = pidfd_poll,
> + .unlocked_ioctl = pidfd_ioctl,
> + .compat_ioctl = compat_ptr_ioctl,

compat_ptr_ioctl() only works if the argument is a pointer, as it
mangles the argument to turn it from a 32-bit pointer value into
a 64-bit pointer value. These are almost always the same
(arch/s390 being the sole exception), but you should not rely
on it. For now it would be find to do '.compat_ioctl = pidfd_ioctl',
but that in turn is wrong if you ever add another ioctl command
that does pass a pointer.


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