[for-linus][PATCH 0/2] tracing: Fixes for 6-6-rc2

[for-linus][PATCH 0/2] tracing: Fixes for 6-6-rc2

[Steven Rostedt]

Tracing fixes for 6.6-rc2:

- Fix the "bytes" output of the per_cpu stat file
  The tracefs/per_cpu/cpu*/stats "bytes" was giving bogus values as the
  accounting was not accurate. It is suppose to show how many used bytes are
  still in the ring buffer, but even when the ring buffer was empty it would
  still show there were bytes used.

- Fix a bug in eventfs where reading a dynamic event directory (open) and then
  creating a dynamic event that goes into that diretory screws up the accounting.
  On close, the newly created event dentry will get a "dput" without ever having
  a "dget" done for it. The fix is to allocate an array on dir open to save what
  dentries were actually "dget" on, and what ones to "dput" on close.

Please pull the latest trace/urgent tree, which can be found at:


  git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
trace/urgent

Head SHA1: ef36b4f92868d66908e235980f74afdfb9742d12


Steven Rostedt (Google) (1):
      eventfs: Remember what dentries were created on dir open

Zheng Yejian (1):
      ring-buffer: Fix bytes info in per_cpu buffer stats

----
 fs/tracefs/event_inode.c   | 87 +++++++++++++++++++++++++++++++++++++---------
 kernel/trace/ring_buffer.c | 28 ++++++++-------
 2 files changed, 85 insertions(+), 30 deletions(-)

[for-linus][PATCH 1/2] ring-buffer: Fix bytes info in per_cpu buffer stats

[Steven Rostedt]

From: Zheng Yejian <zhengyejian1@huawei.com>

The 'bytes' info in file 'per_cpu/cpu<X>/stats' means the number of
bytes in cpu buffer that have not been consumed. However, currently
after consuming data by reading file 'trace_pipe', the 'bytes' info
was not changed as expected.

  # cat per_cpu/cpu0/stats
  entries: 0
  overrun: 0
  commit overrun: 0
  bytes: 568             <--- 'bytes' is problematical !!!
  oldest event ts:  8651.371479
  now ts:  8653.912224
  dropped events: 0
  read events: 8

The root cause is incorrect stat on cpu_buffer->read_bytes. To fix it:
  1. When stat 'read_bytes', account consumed event in rb_advance_reader();
  2. When stat 'entries_bytes', exclude the discarded padding event which
     is smaller than minimum size because it is invisible to reader. Then
     use rb_page_commit() instead of BUF_PAGE_SIZE at where accounting for
     page-based read/remove/overrun.

Also correct the comments of ring_buffer_bytes_cpu() in this patch.

Link: https://lore.kernel.org/linux-trace-kernel/20230921125425.1708423-1-zhengyejian1@huawei.com

Cc: stable@vger.kernel.org
Fixes: c64e148a3be3 ("trace: Add ring buffer stats to measure rate of events")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/ring_buffer.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index a1651edc48d5..28daf0ce95c5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -354,6 +354,11 @@ static void rb_init_page(struct buffer_data_page *bpage)
 	local_set(&bpage->commit, 0);
 }
 
+static __always_inline unsigned int rb_page_commit(struct buffer_page *bpage)
+{
+	return local_read(&bpage->page->commit);
+}
+
 static void free_buffer_page(struct buffer_page *bpage)
 {
 	free_page((unsigned long)bpage->page);
@@ -2003,7 +2008,7 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
 			 * Increment overrun to account for the lost events.
 			 */
 			local_add(page_entries, &cpu_buffer->overrun);
-			local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
+			local_sub(rb_page_commit(to_remove_page), &cpu_buffer->entries_bytes);
 			local_inc(&cpu_buffer->pages_lost);
 		}
 
@@ -2367,11 +2372,6 @@ rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
 			       cpu_buffer->reader_page->read);
 }
 
-static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)
-{
-	return local_read(&bpage->page->commit);
-}
-
 static struct ring_buffer_event *
 rb_iter_head_event(struct ring_buffer_iter *iter)
 {
@@ -2517,7 +2517,7 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
 		 * the counters.
 		 */
 		local_add(entries, &cpu_buffer->overrun);
-		local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
+		local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);
 		local_inc(&cpu_buffer->pages_lost);
 
 		/*
@@ -2660,9 +2660,6 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
 
 	event = __rb_page_index(tail_page, tail);
 
-	/* account for padding bytes */
-	local_add(BUF_PAGE_SIZE - tail, &cpu_buffer->entries_bytes);
-
 	/*
 	 * Save the original length to the meta data.
 	 * This will be used by the reader to add lost event
@@ -2676,7 +2673,8 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
 	 * write counter enough to allow another writer to slip
 	 * in on this page.
 	 * We put in a discarded commit instead, to make sure
-	 * that this space is not used again.
+	 * that this space is not used again, and this space will
+	 * not be accounted into 'entries_bytes'.
 	 *
 	 * If we are less than the minimum size, we don't need to
 	 * worry about it.
@@ -2701,6 +2699,9 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
 	/* time delta must be non zero */
 	event->time_delta = 1;
 
+	/* account for padding bytes */
+	local_add(BUF_PAGE_SIZE - tail, &cpu_buffer->entries_bytes);
+
 	/* Make sure the padding is visible before the tail_page->write update */
 	smp_wmb();
 
@@ -4215,7 +4216,7 @@ u64 ring_buffer_oldest_event_ts(struct trace_buffer *buffer, int cpu)
 EXPORT_SYMBOL_GPL(ring_buffer_oldest_event_ts);
 
 /**
- * ring_buffer_bytes_cpu - get the number of bytes consumed in a cpu buffer
+ * ring_buffer_bytes_cpu - get the number of bytes unconsumed in a cpu buffer
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to read from.
  */
@@ -4723,6 +4724,7 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
 
 	length = rb_event_length(event);
 	cpu_buffer->reader_page->read += length;
+	cpu_buffer->read_bytes += length;
 }
 
 static void rb_advance_iter(struct ring_buffer_iter *iter)
@@ -5816,7 +5818,7 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
 	} else {
 		/* update the entry counter */
 		cpu_buffer->read += rb_page_entries(reader);
-		cpu_buffer->read_bytes += BUF_PAGE_SIZE;
+		cpu_buffer->read_bytes += rb_page_commit(reader);
 
 		/* swap the pages */
 		rb_init_page(bpage);
-- 
2.40.1

[for-linus][PATCH 2/2] eventfs: Remember what dentries were created on dir open

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Using the following code with libtracefs:

	int dfd;

	// create the directory events/kprobes/kp1
	tracefs_kprobe_raw(NULL, "kp1", "schedule_timeout", "time=$arg1");

	// Open the kprobes directory
	dfd = tracefs_instance_file_open(NULL, "events/kprobes", O_RDONLY);

	// Do a lookup of the kprobes/kp1 directory (by looking at enable)
	tracefs_file_exists(NULL, "events/kprobes/kp1/enable");

	// Now create a new entry in the kprobes directory
	tracefs_kprobe_raw(NULL, "kp2", "schedule_hrtimeout", "expires=$arg1");

	// Do another lookup to create the dentries
	tracefs_file_exists(NULL, "events/kprobes/kp2/enable"))

	// Close the directory
	close(dfd);

What happened above, the first open (dfd) will call
dcache_dir_open_wrapper() that will create the dentries and up their ref
counts.

Now the creation of "kp2" will add another dentry within the kprobes
directory.

Upon the close of dfd, eventfs_release() will now do a dput for all the
entries in kprobes. But this is where the problem lies. The open only
upped the dentry of kp1 and not kp2. Now the close is decrementing both
kp1 and kp2, which causes kp2 to get a negative count.

Doing a "trace-cmd reset" which deletes all the kprobes cause the kernel
to crash! (due to the messed up accounting of the ref counts).

To solve this, save all the dentries that are opened in the
dcache_dir_open_wrapper() into an array, and use this array to know what
dentries to do a dput on in eventfs_release().

Since the dcache_dir_open_wrapper() calls dcache_dir_open() which uses the
file->private_data, we need to also add a wrapper around dcache_readdir()
that uses the cursor assigned to the file->private_data. This is because
the dentries need to also be saved in the file->private_data. To do this
create the structure:

  struct dentry_list {
	void		*cursor;
	struct dentry	**dentries;
  };

Which will hold both the cursor and the dentries. Some shuffling around is
needed to make sure that dcache_dir_open() and dcache_readdir() only see
the cursor.

Link: https://lore.kernel.org/linux-trace-kernel/20230919211804.230edf1e@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20230922163446.1431d4fa@gandalf.local.home

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ajay Kaher <akaher@vmware.com>
Fixes: 63940449555e7 ("eventfs: Implement eventfs lookup, read, open functions")
Reported-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 87 ++++++++++++++++++++++++++++++++--------
 1 file changed, 70 insertions(+), 17 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 9f64e7332796..5f1714089884 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -70,6 +70,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 					  struct dentry *dentry,
 					  unsigned int flags);
 static int dcache_dir_open_wrapper(struct inode *inode, struct file *file);
+static int dcache_readdir_wrapper(struct file *file, struct dir_context *ctx);
 static int eventfs_release(struct inode *inode, struct file *file);
 
 static const struct inode_operations eventfs_root_dir_inode_operations = {
@@ -79,7 +80,7 @@ static const struct inode_operations eventfs_root_dir_inode_operations = {
 static const struct file_operations eventfs_file_operations = {
 	.open		= dcache_dir_open_wrapper,
 	.read		= generic_read_dir,
-	.iterate_shared	= dcache_readdir,
+	.iterate_shared	= dcache_readdir_wrapper,
 	.llseek		= generic_file_llseek,
 	.release	= eventfs_release,
 };
@@ -396,6 +397,11 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 	return ret;
 }
 
+struct dentry_list {
+	void			*cursor;
+	struct dentry		**dentries;
+};
+
 /**
  * eventfs_release - called to release eventfs file/dir
  * @inode: inode to be released
@@ -404,26 +410,25 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 static int eventfs_release(struct inode *inode, struct file *file)
 {
 	struct tracefs_inode *ti;
-	struct eventfs_inode *ei;
-	struct eventfs_file *ef;
-	struct dentry *dentry;
-	int idx;
+	struct dentry_list *dlist = file->private_data;
+	void *cursor;
+	int i;
 
 	ti = get_tracefs(inode);
 	if (!(ti->flags & TRACEFS_EVENT_INODE))
 		return -EINVAL;
 
-	ei = ti->private;
-	idx = srcu_read_lock(&eventfs_srcu);
-	list_for_each_entry_srcu(ef, &ei->e_top_files, list,
-				 srcu_read_lock_held(&eventfs_srcu)) {
-		mutex_lock(&eventfs_mutex);
-		dentry = ef->dentry;
-		mutex_unlock(&eventfs_mutex);
-		if (dentry)
-			dput(dentry);
+	if (WARN_ON_ONCE(!dlist))
+		return -EINVAL;
+
+	for (i = 0; dlist->dentries[i]; i++) {
+		dput(dlist->dentries[i]);
 	}
-	srcu_read_unlock(&eventfs_srcu, idx);
+
+	cursor = dlist->cursor;
+	kfree(dlist->dentries);
+	kfree(dlist);
+	file->private_data = cursor;
 	return dcache_dir_close(inode, file);
 }
 
@@ -442,22 +447,70 @@ static int dcache_dir_open_wrapper(struct inode *inode, struct file *file)
 	struct tracefs_inode *ti;
 	struct eventfs_inode *ei;
 	struct eventfs_file *ef;
+	struct dentry_list *dlist;
+	struct dentry **dentries = NULL;
 	struct dentry *dentry = file_dentry(file);
+	struct dentry *d;
 	struct inode *f_inode = file_inode(file);
+	int cnt = 0;
 	int idx;
+	int ret;
 
 	ti = get_tracefs(f_inode);
 	if (!(ti->flags & TRACEFS_EVENT_INODE))
 		return -EINVAL;
 
+	if (WARN_ON_ONCE(file->private_data))
+		return -EINVAL;
+
+	dlist = kmalloc(sizeof(*dlist), GFP_KERNEL);
+	if (!dlist)
+		return -ENOMEM;
+
 	ei = ti->private;
 	idx = srcu_read_lock(&eventfs_srcu);
 	list_for_each_entry_srcu(ef, &ei->e_top_files, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {
-		create_dentry(ef, dentry, false);
+		d = create_dentry(ef, dentry, false);
+		if (d) {
+			struct dentry **tmp;
+
+			tmp = krealloc(dentries, sizeof(d) * (cnt + 2), GFP_KERNEL);
+			if (!tmp)
+				break;
+			tmp[cnt] = d;
+			tmp[cnt + 1] = NULL;
+			cnt++;
+			dentries = tmp;
+		}
 	}
 	srcu_read_unlock(&eventfs_srcu, idx);
-	return dcache_dir_open(inode, file);
+	ret = dcache_dir_open(inode, file);
+
+	/*
+	 * dcache_dir_open() sets file->private_data to a dentry cursor.
+	 * Need to save that but also save all the dentries that were
+	 * opened by this function.
+	 */
+	dlist->cursor = file->private_data;
+	dlist->dentries = dentries;
+	file->private_data = dlist;
+	return ret;
+}
+
+/*
+ * This just sets the file->private_data back to the cursor and back.
+ */
+static int dcache_readdir_wrapper(struct file *file, struct dir_context *ctx)
+{
+	struct dentry_list *dlist = file->private_data;
+	int ret;
+
+	file->private_data = dlist->cursor;
+	ret = dcache_readdir(file, ctx);
+	dlist->cursor = file->private_data;
+	file->private_data = dlist;
+	return ret;
 }
 
 /**
-- 
2.40.1