Commit 66ed5944 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

bpf/task_iter: In task_file_seq_get_next use task_lookup_next_fd_rcu 

When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Using task_lookup_next_fd_rcu simplifies task_file_seq_get_next, by
moving the checking for the maximum file descritor into the generic
code, and by remvoing the need for capturing and releasing a reference
on files_struct.  As the reference count of files_struct no longer
needs to be maintained bpf_iter_seq_task_file_info can have it's files
member removed and task_file_seq_get_next no longer needs it's fstruct
argument.

The curr_fd local variable does need to become unsigned to be used
with fnext_task.  As curr_fd is assigned from and assigned a u32
making curr_fd an unsigned int won't cause problems and might prevent
them.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com


Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-11-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-16-ebiederm@xmission.com


Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
parent 5b17b618

kernel/bpf/task_iter.c

+10 −34
Original line number Diff line number Diff line
@@ -130,45 +130,33 @@ struct bpf_iter_seq_task_file_info { 
	 */

	struct bpf_iter_seq_task_common common;

	struct task_struct *task;

	struct files_struct *files;

	u32 tid;

	u32 fd;

};



static struct file *

task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info,

		       struct task_struct **task, struct files_struct **fstruct)

		       struct task_struct **task)

{

	struct pid_namespace *ns = info->common.ns;

	u32 curr_tid = info->tid, max_fds;

	struct files_struct *curr_files;

	u32 curr_tid = info->tid;

	struct task_struct *curr_task;

	int curr_fd = info->fd;

	unsigned int curr_fd = info->fd;



	/* If this function returns a non-NULL file object,

	 * it held a reference to the task/files_struct/file.

	 * it held a reference to the task/file.

	 * Otherwise, it does not hold any reference.

	 */

again:

	if (*task) {

		curr_task = *task;

		curr_files = *fstruct;

		curr_fd = info->fd;

	} else {

		curr_task = task_seq_get_next(ns, &curr_tid, true);

		if (!curr_task)

			return NULL;



		curr_files = get_files_struct(curr_task);

		if (!curr_files) {

			put_task_struct(curr_task);

			curr_tid = ++(info->tid);

			info->fd = 0;

			goto again;

		}



		/* set *fstruct, *task and info->tid */

		*fstruct = curr_files;

		/* set *task and info->tid */

		*task = curr_task;

		if (curr_tid == info->tid) {

			curr_fd = info->fd;
@@ -179,13 +167,11 @@ again: 
	}



	rcu_read_lock();

	max_fds = files_fdtable(curr_files)->max_fds;

	for (; curr_fd < max_fds; curr_fd++) {

	for (;; curr_fd++) {

		struct file *f;



		f = files_lookup_fd_rcu(curr_files, curr_fd);

		f = task_lookup_next_fd_rcu(curr_task, &curr_fd);

		if (!f)

			continue;

			break;

		if (!get_file_rcu(f))

			continue;
@@ -197,10 +183,8 @@ again: 


	/* the current task is done, go to the next task */

	rcu_read_unlock();

	put_files_struct(curr_files);

	put_task_struct(curr_task);

	*task = NULL;

	*fstruct = NULL;

	info->fd = 0;

	curr_tid = ++(info->tid);

	goto again;
@@ -209,13 +193,11 @@ again: 
static void *task_file_seq_start(struct seq_file *seq, loff_t *pos)

{

	struct bpf_iter_seq_task_file_info *info = seq->private;

	struct files_struct *files = NULL;

	struct task_struct *task = NULL;

	struct file *file;



	file = task_file_seq_get_next(info, &task, &files);

	file = task_file_seq_get_next(info, &task);

	if (!file) {

		info->files = NULL;

		info->task = NULL;

		return NULL;

	}
@@ -223,7 +205,6 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos) 
	if (*pos == 0)

		++*pos;

	info->task = task;

	info->files = files;



	return file;

}
@@ -231,22 +212,19 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos) 
static void *task_file_seq_next(struct seq_file *seq, void *v, loff_t *pos)

{

	struct bpf_iter_seq_task_file_info *info = seq->private;

	struct files_struct *files = info->files;

	struct task_struct *task = info->task;

	struct file *file;



	++*pos;

	++info->fd;

	fput((struct file *)v);

	file = task_file_seq_get_next(info, &task, &files);

	file = task_file_seq_get_next(info, &task);

	if (!file) {

		info->files = NULL;

		info->task = NULL;

		return NULL;

	}



	info->task = task;

	info->files = files;



	return file;

}
@@ -295,9 +273,7 @@ static void task_file_seq_stop(struct seq_file *seq, void *v) 
		(void)__task_file_seq_show(seq, v, true);

	} else {

		fput((struct file *)v);

		put_files_struct(info->files);

		put_task_struct(info->task);

		info->files = NULL;

		info->task = NULL;

	}

}

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