[RFC,PATCH] cookie based proc_pid_readdir

[Manfred Spraul <manfred@colorfullife.com>]

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proc_dir_readdir skips over tasks if tasks exit between the individual 
get_tgid_list calls. The only approach that guarantees that no tasks are 
skipped is to add a cookie into the task list and restart from that cookie.

Attached is a proof of concept patch that adds such cookies - please 
comment.

--
    Manfred

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// $Header$
// Kernel Version:
//  VERSION = 2
//  PATCHLEVEL = 6
//  SUBLEVEL = 0
//  EXTRAVERSION =
--- 2.6/include/linux/list.h	2003-12-20 09:19:13.000000000 +0100
+++ build-2.6/include/linux/list.h	2004-01-06 12:26:32.000000000 +0100
@@ -39,6 +39,16 @@
 } while (0)
 
 /*
+ * special linked lists that can contain invisible
+ * members. Members with nonzero is_cookie should be
+ * skipped.
+ */
+struct clist_head {
+	struct list_head l;
+	int is_cookie;
+};
+
+/*
  * Insert a new entry between two known consecutive entries. 
  *
  * This is only for internal list manipulation where we know
--- 2.6/include/linux/proc_fs.h	2003-10-25 20:42:42.000000000 +0200
+++ build-2.6/include/linux/proc_fs.h	2004-01-06 12:48:15.000000000 +0100
@@ -95,6 +95,7 @@
 struct dentry *proc_pid_unhash(struct task_struct *p);
 void proc_pid_flush(struct dentry *proc_dentry);
 int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir);
+loff_t proc_pid_llseek(struct file *file, loff_t offset, int origin);
 
 extern struct proc_dir_entry *create_proc_entry(const char *name, mode_t mode,
 						struct proc_dir_entry *parent);
--- 2.6/include/linux/sched.h	2003-12-04 19:44:40.000000000 +0100
+++ build-2.6/include/linux/sched.h	2004-01-06 12:31:57.000000000 +0100
@@ -352,7 +352,7 @@
 	cpumask_t cpus_allowed;
 	unsigned int time_slice, first_time_slice;
 
-	struct list_head tasks;
+	struct clist_head tasks;
 	struct list_head ptrace_children;
 	struct list_head ptrace_list;
 
@@ -719,18 +719,31 @@
 
 #define REMOVE_LINKS(p) do {					\
 	if (thread_group_leader(p))				\
-		list_del_init(&(p)->tasks);			\
+		list_del_init(&(p)->tasks.l);			\
 	remove_parent(p);					\
 	} while (0)
 
 #define SET_LINKS(p) do {					\
 	if (thread_group_leader(p))				\
-		list_add_tail(&(p)->tasks,&init_task.tasks);	\
+		list_add_tail(&(p)->tasks.l,&init_task.tasks.l);	\
 	add_parent(p, (p)->parent);				\
 	} while (0)
 
-#define next_task(p)	list_entry((p)->tasks.next, struct task_struct, tasks)
-#define prev_task(p)	list_entry((p)->tasks.prev, struct task_struct, tasks)
+static inline task_t * next_task(task_t *p)
+{
+	do {
+		p = list_entry((p)->tasks.l.next, struct task_struct, tasks.l);
+	} while(p->tasks.is_cookie);
+	return p;
+}
+
+static inline task_t * prev_task(task_t *p)
+{
+	do {
+		p = list_entry((p)->tasks.l.prev, struct task_struct, tasks.l);
+	} while(p->tasks.is_cookie);
+	return p;
+}
 
 #define for_each_process(p) \
 	for (p = &init_task ; (p = next_task(p)) != &init_task ; )
--- 2.6/include/linux/init_task.h	2003-12-04 19:44:40.000000000 +0100
+++ build-2.6/include/linux/init_task.h	2004-01-06 12:32:51.000000000 +0100
@@ -75,7 +75,7 @@
 	.active_mm	= &init_mm,					\
 	.run_list	= LIST_HEAD_INIT(tsk.run_list),			\
 	.time_slice	= HZ,						\
-	.tasks		= LIST_HEAD_INIT(tsk.tasks),			\
+	.tasks		= { LIST_HEAD_INIT(tsk.tasks.l), 0},		\
 	.ptrace_children= LIST_HEAD_INIT(tsk.ptrace_children),		\
 	.ptrace_list	= LIST_HEAD_INIT(tsk.ptrace_list),		\
 	.real_parent	= &tsk,						\
--- 2.6/fs/proc/base.c	2003-12-20 09:19:13.000000000 +0100
+++ build-2.6/fs/proc/base.c	2004-01-06 14:17:20.000000000 +0100
@@ -1627,33 +1627,6 @@
 #define PROC_MAXPIDS 20
 
 /*
- * Get a few tgid's to return for filldir - we need to hold the
- * tasklist lock while doing this, and we must release it before
- * we actually do the filldir itself, so we use a temp buffer..
- */
-static int get_tgid_list(int index, unsigned int *tgids)
-{
-	struct task_struct *p;
-	int nr_tgids = 0;
-
-	index--;
-	read_lock(&tasklist_lock);
-	for_each_process(p) {
-		int tgid = p->pid;
-		if (!pid_alive(p))
-			continue;
-		if (--index >= 0)
-			continue;
-		tgids[nr_tgids] = tgid;
-		nr_tgids++;
-		if (nr_tgids >= PROC_MAXPIDS)
-			break;
-	}
-	read_unlock(&tasklist_lock);
-	return nr_tgids;
-}
-
-/*
  * Get a few tid's to return for filldir - we need to hold the
  * tasklist lock while doing this, and we must release it before
  * we actually do the filldir itself, so we use a temp buffer..
@@ -1685,35 +1658,134 @@
 	return nr_tids;
 }
 
+loff_t proc_pid_llseek(struct file *file, loff_t offset, int origin)
+{
+	long long retval;
+	struct inode *inode = file->f_dentry->d_inode->i_mapping->host;
+
+	down(&inode->i_sem);
+	switch (origin) {
+		case 2:
+			offset += inode->i_size;
+			break;
+		case 1:
+			offset += file->f_pos;
+	}
+	retval = -EINVAL;
+	if (offset>=0 && offset<=inode->i_sb->s_maxbytes) {
+		if (offset != file->f_pos) {
+			file->f_pos = offset;
+			if (file->f_version) {
+				struct clist_head *cl;
+				cl = (struct clist_head *)file->f_version;
+				write_lock_irq(&tasklist_lock);
+				list_del(&cl->l);
+				write_unlock_irq(&tasklist_lock);
+				kfree(cl);
+				file->f_version = 0;
+			}
+		}
+		retval = offset;
+	}
+	up(&inode->i_sem);
+	return retval;
+}
+
 /* for the /proc/ directory itself, after non-process stuff has been done */
 int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
 {
-	unsigned int tgid_array[PROC_MAXPIDS];
-	char buf[PROC_NUMBUF];
 	unsigned int nr = filp->f_pos - FIRST_PROCESS_ENTRY;
-	unsigned int nr_tgids, i;
+	struct clist_head fcl; /* fallback if kmalloc fails - -ENOMEM is not permitted */
+	struct clist_head *pcl;
 
 	if (!nr) {
 		ino_t ino = fake_ino(0,PROC_TGID_INO);
 		if (filldir(dirent, "self", 4, filp->f_pos, ino, DT_LNK) < 0)
 			return 0;
 		filp->f_pos++;
-		nr++;
+	} else {
+		nr--;
 	}
 
-	nr_tgids = get_tgid_list(nr, tgid_array);
+	if (filp->f_version) {
+		pcl = (struct clist_head *)filp->f_version;
+	} else {
+		task_t *p;
+		pcl = kmalloc(sizeof(struct clist_head), GFP_KERNEL);
+		if (pcl)
+			filp->f_version = (unsigned long)pcl;
+		else
+			pcl = &fcl;
+
+		INIT_LIST_HEAD(&pcl->l);
+		pcl->is_cookie = 1;
+		
+		write_lock_irq(&tasklist_lock);
+		p = &init_task;
+		while(nr) {
+			if (pid_alive(p))
+				nr--;
+			p = next_task(p);
+			if (p == &init_task) {
+				write_unlock_irq(&tasklist_lock);
+				if (pcl != &fcl) {
+					filp->f_version = 0;
+					kfree(pcl);
+				}
+				return 0;
+			}
+		}
+		list_add(&pcl->l, &p->tasks.l);
+		write_unlock_irq(&tasklist_lock);
+	}
 
-	for (i = 0; i < nr_tgids; i++) {
-		int tgid = tgid_array[i];
-		ino_t ino = fake_ino(tgid,PROC_TGID_INO);
-		unsigned long j = PROC_NUMBUF;
+	for (;;) {
+		unsigned int tgid;
+		int j;
+		char buf[PROC_NUMBUF];
+		ino_t ino;
+		task_t *p;
+		struct list_head *walk;
+
+		write_lock_irq(&tasklist_lock);
+		walk = &pcl->l;
+		do {
+			walk = walk->next;
+			p = list_entry(walk, struct task_struct, tasks.l);
+		} while (p->tasks.is_cookie);
+		if (p == &init_task) {
+			write_unlock_irq(&tasklist_lock);
+			break;
+		}
+		tgid = p->pid;
+		list_del(&pcl->l);
+		list_add(&pcl->l, &p->tasks.l);
+		write_unlock_irq(&tasklist_lock);
+
+		ino = fake_ino(tgid,PROC_TGID_INO);
 
+		j = PROC_NUMBUF;
 		do buf[--j] = '0' + (tgid % 10); while (tgid/=10);
 
-		if (filldir(dirent, buf+j, PROC_NUMBUF-j, filp->f_pos, ino, DT_DIR) < 0)
+		if (filldir(dirent, buf+j, PROC_NUMBUF-j, filp->f_pos, ino, DT_DIR) < 0) {
+			write_lock_irq(&tasklist_lock);
+			walk = &pcl->l;
+			do {
+				walk = walk->prev;
+				p = list_entry(walk, struct task_struct, tasks.l);
+			} while (p->tasks.is_cookie);
+			list_del(&pcl->l);
+			list_add_tail(&pcl->l, &p->tasks.l);
+			write_unlock_irq(&tasklist_lock);
 			break;
+		}
 		filp->f_pos++;
 	}
+	if (pcl == &fcl) {
+		write_lock_irq(&tasklist_lock);
+		list_del(&fcl.l);
+		write_unlock_irq(&tasklist_lock);
+	}
 	return 0;
 }
 
--- 2.6/fs/proc/root.c	2003-10-25 20:44:17.000000000 +0200
+++ build-2.6/fs/proc/root.c	2004-01-06 13:19:31.000000000 +0100
@@ -127,6 +127,7 @@
 static struct file_operations proc_root_operations = {
 	.read		 = generic_read_dir,
 	.readdir	 = proc_root_readdir,
+	.llseek		 = proc_pid_llseek,
 };
 
 /*
--- 2.6/kernel/pid.c	2003-12-04 19:44:40.000000000 +0100
+++ build-2.6/kernel/pid.c	2004-01-06 12:29:34.000000000 +0100
@@ -255,7 +255,7 @@
 	attach_pid(thread, PIDTYPE_TGID, thread->tgid);
 	attach_pid(thread, PIDTYPE_PGID, leader->__pgrp);
 	attach_pid(thread, PIDTYPE_SID, thread->session);
-	list_add_tail(&thread->tasks, &init_task.tasks);
+	list_add_tail(&thread->tasks.l, &init_task.tasks.l);
 
 	attach_pid(leader, PIDTYPE_PID, leader->pid);
 	attach_pid(leader, PIDTYPE_TGID, leader->tgid);