From f08d0c3a71114bb36d1722506d926bd497182781 Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Thu, 30 Jan 2025 20:40:26 +0000 Subject: pidfd: add PIDFD_SELF* sentinels to refer to own thread/process 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 Link: https://lore.kernel.org/r/24315a16a3d01a548dd45c7515f7d51c767e954e.1738268370.git.lorenzo.stoakes@oracle.com Signed-off-by: Christian Brauner --- include/uapi/linux/pidfd.h | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) (limited to 'include/uapi/linux') diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h index 4540f6301b8c..e0abd0b18841 100644 --- a/include/uapi/linux/pidfd.h +++ b/include/uapi/linux/pidfd.h @@ -23,6 +23,30 @@ #define PIDFD_INFO_SIZE_VER0 64 /* sizeof first published struct */ +/* + * The concept of process and threads in userland and the kernel is a confusing + * one - within the kernel every thread is a 'task' with its own individual PID, + * however from userland's point of view threads are grouped by a single PID, + * which is that of the 'thread group leader', typically the first thread + * spawned. + * + * To cut the Gideon knot, for internal kernel usage, we refer to + * PIDFD_SELF_THREAD to refer to the current thread (or task from a kernel + * perspective), and PIDFD_SELF_THREAD_GROUP to refer to the current thread + * group leader... + */ +#define PIDFD_SELF_THREAD -10000 /* Current thread. */ +#define PIDFD_SELF_THREAD_GROUP -20000 /* Current thread group leader. */ + +/* + * ...and for userland we make life simpler - PIDFD_SELF refers to the current + * thread, PIDFD_SELF_PROCESS refers to the process thread group leader. + * + * For nearly all practical uses, a user will want to use PIDFD_SELF. + */ +#define PIDFD_SELF PIDFD_SELF_THREAD +#define PIDFD_SELF_PROCESS PIDFD_SELF_THREAD_GROUP + struct pidfd_info { /* * This mask is similar to the request_mask in statx(2). -- cgit v1.2.3