Getting user process pid when writing Linux Kernel Module

Question

How can I get the PID of the user process which triggered my Kernel module's file_operation.read routine (i.e., which process is reading /dev/mydev) ?

Answer 1

When your read function is executing, it's doing so in the context of the process that issued the system call. You should thus pe able to use current, i.e. current->pid.

Answer 2

These days, we have some helper functions defined in sched.h. In the case of pid, you can use:

pid = task_pid_nr(current);

to get the current task's pid.

here is the comment taken from include/linux/sched.h as of v3.8.

the helpers to get the task's different pids as they are seen from various namespaces

• task_xid_nr() : global id, i.e. the id seen from the init namespace;
• task_xid_vnr() : virtual id, i.e. the id seen from the pid namespace of current.
• task_xid_nr_ns() : id seen from the ns specified;
• set_task_vxid() : assigns a virtual id to a task;

see also pid_nr() etc in include/linux/pid.h

Answer 3

On a kernel 2.6.39 arm build, if current->pid does not work then it may be done by:

    pid_nr(get_task_pid(current, PIDTYPE_PID))

The PIDTYPE_PID can be substituted by PIDTYPE_PGID or PIDTYPE_SID. The header source is at include/linux/pid.h as Yasushi pointed out.

Which of the approaches work depends on what header files the code uses.