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pidfd: add PIDFD_SELF* sentinels to refer to own thread/process

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It is useful to be able to utilise the pidfd mechanism to reference the
current thread or process (from a userland point of view - thread group
leader from the kernel's point of view).

Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and
PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader.

For convenience and to avoid confusion from userland's perspective we alias
these:

* PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what
  the user will want to use, as they would find it surprising if for
  instance fd's were unshared()'d and they wanted to invoke pidfd_getfd()
  and that failed.

* PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users
  have no concept of thread groups or what a thread group leader is, and
  from userland's perspective and nomenclature this is what userland
  considers to be a process.

We adjust pidfd_get_task() and the pidfd_send_signal() system call with
specific handling for this, implementing this functionality for
process_madvise(), process_mrelease() (albeit, using it here wouldn't
really make sense) and pidfd_send_signal().

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/24315a16a3d01a548dd45c7515f7d51c767e954e.1738268370.git.lorenzo.stoakes@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Lorenzo Stoakes
2025-01-30 20:40:26 +00:00
committed by Christian Brauner
parent 2014c95afe
commit f08d0c3a71
3 changed files with 112 additions and 53 deletions
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24
kernel/pid.c
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@@ -564,15 +564,29 @@ struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
*/
struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
{
unsigned int f_flags;
unsigned int f_flags = 0;
struct pid *pid;
struct task_struct *task;
enum pid_type type;
pid = pidfd_get_pid(pidfd, &f_flags);
if (IS_ERR(pid))
return ERR_CAST(pid);
switch (pidfd) {
case PIDFD_SELF_THREAD:
type = PIDTYPE_PID;
pid = get_task_pid(current, type);
break;
case PIDFD_SELF_THREAD_GROUP:
type = PIDTYPE_TGID;
pid = get_task_pid(current, type);
break;
default:
pid = pidfd_get_pid(pidfd, &f_flags);
if (IS_ERR(pid))
return ERR_CAST(pid);
type = PIDTYPE_TGID;
break;
}
task = get_pid_task(pid, PIDTYPE_TGID);
task = get_pid_task(pid, type);
put_pid(pid);
if (!task)
return ERR_PTR(-ESRCH);