What is ''how to find task_struct by pid without iterating through the task_struct list''?

Question

I'm learning about "How to find task_struct by pid without iterating through the task_struct list", honestly I don't know anything about it. Please give me some knowledge about it.

lyytutoria · Accepted Answer

The best solution for ''how to find task_struct by pid without iterating through the task_struct list'' is doing one of the following commands:

extern struct task_struct *find_task_by_vpid(pid_t nr);
extern struct task_struct *find_task_by_pid_ns(pid_t nr,
struct pid_namespace *ns);

Another way is using the below command in a module:

pid_task(find_vpid(pid), PIDTYPE_PID);

Lina Guillard · Answer

You can find task_struct in a module by using find_task_by_vpid(pid_t nr), etc. These functions cannot be exported so they won't work.

You can substitute the following function in a module:

pid_task(find_vpid(pid), PIDTYPE_PID);

Azriel Lee · Answer

It was not mentioned that the pid_task() function or the pointer, which you get from it, should be used within the RCU critical section because it uses RCU protected data structure. can have use after-free BUG . pid_task() can be used in Linux kernel source code (e.g. in
Take, for example:

rcu_read_lock();
/* search through the global namespace */
task = pid_task(find_pid_ns(pid_num, &init_pid_ns), PIDTYPE_PID);
if (task)
 printk(KERN_INFO "1. pid: %d, state: %#lx
",
 pid_num, task->state); /* valid task dereference */
rcu_read_unlock(); /* after it returns - task pointer becomes invalid! */

if (task)
 printk(KERN_INFO "2. pid: %d, state: %#lx
",
 pid_num, task->state); /* may be successful,
 * but is buggy (task dereference is INVALID!) */

Learn more about RCU API at Kernel.org

P.S. P.S.

This is the first for searching within a particular namespace.

task = find_task_by_pid_ns(pid_num, &init_pid_ns); /* e.g. init namespace */

The second is used to search through the current task's namespace.

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What is ”how to find task_struct by pid without iterating through the task_struct list”?