pid: add helpers to operate on a struct pid array

We're about to extend af_unix sockets and coredump code with the ability
to hand out thread-specific pidfds. Add a few simple helpers that allow
to operate on multiple struct pids up to PIDTYPE_MAX with automatic
bounds checking.

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Andrew Halaney <andrew@amutable.com>
Link: https://patch.msgid.link/20261005-work-unix-passpidfd-v4-1-350183b6e02a@amutable.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

4 files changed, 84 insertions, 0 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index ddaef0bbc8ba..87635d0cc1f7 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -2,6 +2,9 @@
 #ifndef _LINUX_PID_H
 #define _LINUX_PID_H
 
+#include <linux/array_size.h>
+#include <linux/build_bug.h>
+#include <linux/minmax.h>
 #include <linux/pid_types.h>
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
@@ -92,6 +95,56 @@ static inline struct pid *get_pid(struct pid *pid)
 }
 
 extern void put_pid(struct pid *pid);
+
+/*
+ * Helpers for arrays of struct pid indexed by pid type declared with
+ * DECLARE_PIDS(). The array covers PIDTYPE_PID up to the pid type it
+ * was declared with and the helpers take that bound from the array.
+ */
+static inline void __get_pids(struct pid **dst, struct pid *const *src,
+			      enum pid_type last)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++)
+		dst[type] = get_pid(src[type]);
+}
+
+static inline void __put_pids(struct pid **pids, enum pid_type last)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++) {
+		put_pid(pids[type]);
+		pids[type] = NULL;
+	}
+}
+
+static inline void __swap_pids(struct pid **a, struct pid **b,
+			       enum pid_type last)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++)
+		swap(a[type], b[type]);
+}
+
+static inline bool __pids_equal(struct pid *const *a, struct pid *const *b,
+				enum pid_type last)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++)
+		if (a[type] != b[type])
+			return false;
+	return true;
+}
+
+/* The last pid type an array declared with DECLARE_PIDS() covers. */
+#define pids_last(pids)							\
+	((enum pid_type)(ARRAY_SIZE(pids) - 1 +				\
+			 BUILD_BUG_ON_ZERO(ARRAY_SIZE(pids) > PIDTYPE_MAX)))
+
+#define __pids_last2(a, b)						\
+	(pids_last(a) + BUILD_BUG_ON_ZERO(ARRAY_SIZE(a) != ARRAY_SIZE(b)))
+
+#define get_pids(dst, src)	__get_pids(dst, src, __pids_last2(dst, src))
+#define put_pids(pids)		__put_pids(pids, pids_last(pids))
+#define swap_pids(a, b)		__swap_pids(a, b, __pids_last2(a, b))
+#define pids_equal(a, b)	__pids_equal(a, b, __pids_last2(a, b))
+
 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
 static inline bool pid_has_task(struct pid *pid, enum pid_type type)
 {
diff --git a/include/linux/pid_types.h b/include/linux/pid_types.h
index c2aee1d91dcf..3302690a2a28 100644
--- a/include/linux/pid_types.h
+++ b/include/linux/pid_types.h
@@ -2,6 +2,8 @@
 #ifndef _LINUX_PID_TYPES_H
 #define _LINUX_PID_TYPES_H
 
+#include <linux/build_bug.h>
+
 enum pid_type {
 	PIDTYPE_PID,
 	PIDTYPE_TGID,
@@ -10,6 +12,12 @@ enum pid_type {
 	PIDTYPE_MAX,
 };
 
+struct pid;
+
+/* An array of struct pid indexed by pid type, PIDTYPE_PID up to @last. */
+#define DECLARE_PIDS(name, last)					\
+	struct pid *name[(last) + 1 + BUILD_BUG_ON_ZERO((last) >= PIDTYPE_MAX)]
+
 struct pid_namespace;
 extern struct pid_namespace init_pid_ns;
 
diff --git a/include/linux/pidfs.h b/include/linux/pidfs.h
index 0abf7da9ab23..3c1e82f1a369 100644
--- a/include/linux/pidfs.h
+++ b/include/linux/pidfs.h
@@ -3,6 +3,7 @@
 #define _LINUX_PID_FS_H
 
 #include <linux/gfp_types.h>
+#include <linux/pid.h>
 
 struct coredump_params;
 
@@ -32,5 +33,9 @@ static inline int pidfs_register_pid(struct pid *pid)
 }
 
 void pidfs_free_pid(struct pid *pid);
+int __pidfs_register_pids(struct pid *const *pids, enum pid_type last);
+
+/* Register the pids of an array declared with DECLARE_PIDS(). */
+#define pidfs_register_pids(pids)	__pidfs_register_pids(pids, pids_last(pids))
 
 #endif /* _LINUX_PID_FS_H */
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index d45a5476b97d..3149cdb976c6 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -674,6 +674,24 @@ struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 	return pid;
 }
 
+/* Fill @pids with the pid types of @task up to @last, without references. */
+static inline void __task_pids(struct pid **pids, enum pid_type last,
+			       struct task_struct *task)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++)
+		pids[type] = task_pid_type(task, type);
+}
+
+static inline void __get_task_pids(struct pid **pids, enum pid_type last,
+				   struct task_struct *task)
+{
+	for (enum pid_type type = PIDTYPE_PID; type <= last; type++)
+		pids[type] = get_pid(task_pid_type(task, type));
+}
+
+#define task_pids(pids, task)		__task_pids(pids, pids_last(pids), task)
+#define get_task_pids(pids, task)	__get_task_pids(pids, pids_last(pids), task)
+
 static inline struct pid *task_tgid(struct task_struct *task)
 {
 	return task->signal->pids[PIDTYPE_TGID];