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Subject[PATCH] proc: Remove races from proc_id_readdir()
Date

Oleg noticed that the call of task_pid_nr_ns() in proc_pid_readdir
is racy with respect to tasks exiting.

After a bit of examination it also appears that the call itself
is completely unnecessary.

So to fix the problem this patch modifies next_tgid() to return
both a tgid and the task struct in question.

A structure is introduced to return these values because it is
slightly cleaner and easier to optimize, and the resulting code
is a little shorter.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

---
fs/proc/base.c | 51 ++++++++++++++++++++++++++++-----------------------
1 files changed, 28 insertions(+), 23 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 5b92b79..7e8eca4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2454,19 +2454,23 @@ out:
* Find the first task with tgid >= tgid
*
*/
-static struct task_struct *next_tgid(unsigned int tgid,
- struct pid_namespace *ns)
-{
+struct tgid_iter {
+ unsigned int tgid;
struct task_struct *task;
+};
+static struct tgid_iter next_tgid(struct pid_namespace *ns, struct tgid_iter iter)
+{
struct pid *pid;

+ if (iter.task)
+ put_task_struct(iter.task);
rcu_read_lock();
retry:
- task = NULL;
- pid = find_ge_pid(tgid, ns);
+ iter.task = NULL;
+ pid = find_ge_pid(iter.tgid, ns);
if (pid) {
- tgid = pid_nr_ns(pid, ns) + 1;
- task = pid_task(pid, PIDTYPE_PID);
+ iter.tgid = pid_nr_ns(pid, ns);
+ iter.task = pid_task(pid, PIDTYPE_PID);
/* What we to know is if the pid we have find is the
* pid of a thread_group_leader. Testing for task
* being a thread_group_leader is the obvious thing
@@ -2479,23 +2483,25 @@ retry:
* found doesn't happen to be a thread group leader.
* As we don't care in the case of readdir.
*/
- if (!task || !has_group_leader_pid(task))
+ if (!iter.task || !has_group_leader_pid(iter.task)) {
+ iter.tgid += 1;
goto retry;
- get_task_struct(task);
+ }
+ get_task_struct(iter.task);
}
rcu_read_unlock();
- return task;
+ return iter;
}

#define TGID_OFFSET (FIRST_PROCESS_ENTRY + ARRAY_SIZE(proc_base_stuff))

static int proc_pid_fill_cache(struct file *filp, void *dirent, filldir_t filldir,
- struct task_struct *task, int tgid)
+ struct tgid_iter iter)
{
char name[PROC_NUMBUF];
- int len = snprintf(name, sizeof(name), "%d", tgid);
+ int len = snprintf(name, sizeof(name), "%d", iter.tgid);
return proc_fill_cache(filp, dirent, filldir, name, len,
- proc_pid_instantiate, task, NULL);
+ proc_pid_instantiate, iter.task, NULL);
}

/* for the /proc/ directory itself, after non-process stuff has been done */
@@ -2503,8 +2509,7 @@ int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
{
unsigned int nr = filp->f_pos - FIRST_PROCESS_ENTRY;
struct task_struct *reaper = get_proc_task(filp->f_path.dentry->d_inode);
- struct task_struct *task;
- int tgid;
+ struct tgid_iter iter;
struct pid_namespace *ns;

if (!reaper)
@@ -2517,14 +2522,14 @@ int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
}

ns = filp->f_dentry->d_sb->s_fs_info;
- tgid = filp->f_pos - TGID_OFFSET;
- for (task = next_tgid(tgid, ns);
- task;
- put_task_struct(task), task = next_tgid(tgid + 1, ns)) {
- tgid = task_pid_nr_ns(task, ns);
- filp->f_pos = tgid + TGID_OFFSET;
- if (proc_pid_fill_cache(filp, dirent, filldir, task, tgid) < 0) {
- put_task_struct(task);
+ iter.task = NULL;
+ iter.tgid = filp->f_pos - TGID_OFFSET;
+ for (iter = next_tgid(ns, iter);
+ iter.task;
+ iter.tgid += 1, iter = next_tgid(ns, iter)) {
+ filp->f_pos = iter.tgid + TGID_OFFSET;
+ if (proc_pid_fill_cache(filp, dirent, filldir, iter) < 0) {
+ put_task_struct(iter.task);
goto out;
}
}
--
1.5.3.rc6.17.g1911
-
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