[PATCH 0/4] eventfs: More updates to eventfs_iterate()

[PATCH 0/4] eventfs: More updates to eventfs_iterate()

[Steven Rostedt]

With the ongoing descussion of eventfs iterator, a few more changes
are required and some changes are just enhancements.

- Stop immediately in the loop if the ei is found to be in the process
  of being freed.

- Make the ctx->pos update consistent with the skipped previous read index.
  This fixes a bug with duplicate files being showned by 'ls'.

- Swap reading ei->entries with ei->children to make the next change
  easier to read

- Add a "shortcut" in the ei->entries array to skip over already read
  entries.


Steven Rostedt (Google) (4):
      eventfs: Have eventfs_iterate() stop immediately if ei->is_freed is set
      eventfs: Do ctx->pos update for all iterations in  eventfs_iterate()
      eventfs: Read ei->entries before ei->children in eventfs_iterate()
      eventfs: Shortcut eventfs_iterate() by skipping entries already read

----
 fs/tracefs/event_inode.c | 67 ++++++++++++++++++++++++++----------------------
 1 file changed, 36 insertions(+), 31 deletions(-)

[PATCH 1/4] eventfs: Have eventfs_iterate() stop immediately if ei->is_freed is set

[Steven Rostedt]

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

If ei->is_freed is set in eventfs_iterate(), it means that the directory
that is being iterated on is in the process of being freed. Just exit the
loop immediately when that is ever detected, and separate out the return
of the entry->callback() from ei->is_freed.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 72912b5f9a90..0aca6910efb3 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -788,11 +788,12 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 		name = entry->name;
 
 		mutex_lock(&eventfs_mutex);
-		/* If ei->is_freed, then the event itself may be too */
-		if (!ei->is_freed)
-			r = entry->callback(name, &mode, &cdata, &fops);
-		else
-			r = -1;
+		/* If ei->is_freed then just bail here, nothing more to do */
+		if (ei->is_freed) {
+			mutex_unlock(&eventfs_mutex);
+			goto out;
+		}
+		r = entry->callback(name, &mode, &cdata, &fops);
 		mutex_unlock(&eventfs_mutex);
 		if (r <= 0)
 			continue;
-- 
2.42.0

[PATCH 2/4] eventfs: Do ctx->pos update for all iterations in eventfs_iterate()

[Steven Rostedt]

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

The ctx->pos was only updated when it added an entry, but the "skip to
current pos" check (c--) happened for every loop regardless of if the
entry was added or not. This inconsistency caused readdir to be incorrect.

It was due to:

	for (i = 0; i < ei->nr_entries; i++) {

		if (c > 0) {
			c--;
			continue;
		}

		mutex_lock(&eventfs_mutex);
		/* If ei->is_freed then just bail here, nothing more to do */
		if (ei->is_freed) {
			mutex_unlock(&eventfs_mutex);
			goto out;
		}
		r = entry->callback(name, &mode, &cdata, &fops);
		mutex_unlock(&eventfs_mutex);

		[..]
		ctx->pos++;
	}

But this can cause the iterator to return a file that was already read.
That's because of the way the callback() works. Some events may not have
all files, and the callback can return 0 to tell eventfs to skip the file
for this directory.

for instance, we have:

 # ls /sys/kernel/tracing/events/ftrace/function
format  hist  hist_debug  id  inject

and

 # ls /sys/kernel/tracing/events/sched/sched_switch/
enable  filter  format  hist  hist_debug  id  inject  trigger

Where the function directory is missing "enable", "filter" and
"trigger". That's because the callback() for events has:

static int event_callback(const char *name, umode_t *mode, void **data,
			  const struct file_operations **fops)
{
	struct trace_event_file *file = *data;
	struct trace_event_call *call = file->event_call;

[..]

	/*
	 * Only event directories that can be enabled should have
	 * triggers or filters, with the exception of the "print"
	 * event that can have a "trigger" file.
	 */
	if (!(call->flags & TRACE_EVENT_FL_IGNORE_ENABLE)) {
		if (call->class->reg && strcmp(name, "enable") == 0) {
			*mode = TRACE_MODE_WRITE;
			*fops = &ftrace_enable_fops;
			return 1;
		}

		if (strcmp(name, "filter") == 0) {
			*mode = TRACE_MODE_WRITE;
			*fops = &ftrace_event_filter_fops;
			return 1;
		}
	}

	if (!(call->flags & TRACE_EVENT_FL_IGNORE_ENABLE) ||
	    strcmp(trace_event_name(call), "print") == 0) {
		if (strcmp(name, "trigger") == 0) {
			*mode = TRACE_MODE_WRITE;
			*fops = &event_trigger_fops;
			return 1;
		}
	}
[..]
	return 0;
}

Where the function event has the TRACE_EVENT_FL_IGNORE_ENABLE set.

This means that the entries array elements for "enable", "filter" and
"trigger" when called on the function event will have the callback return
0 and not 1, to tell eventfs to skip these files for it.

Because the "skip to current ctx->pos" check happened for all entries, but
the ctx->pos++ only happened to entries that exist, it would confuse the
reading of a directory. Which would cause:

 # ls /sys/kernel/tracing/events/ftrace/function/
format  hist  hist  hist_debug  hist_debug  id  inject  inject

The missing "enable", "filter" and "trigger" caused ls to show "hist",
"hist_debug" and "inject" twice.

Update the ctx->pos for every iteration to keep its update and the "skip"
update consistent. This also means that on error, the ctx->pos needs to be
decremented if it was incremented without adding something.

Link: https://lore.kernel.org/all/20240104150500.38b15a62@gandalf.local.home/

Fixes: 493ec81a8fb8e ("eventfs: Stop using dcache_readdir() for getdents()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 0aca6910efb3..c73fb1f7ddbc 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -760,6 +760,8 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 			continue;
 		}
 
+		ctx->pos++;
+
 		if (ei_child->is_freed)
 			continue;
 
@@ -767,13 +769,12 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		dentry = create_dir_dentry(ei, ei_child, ei_dentry);
 		if (!dentry)
-			goto out;
+			goto out_dec;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
 		if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR))
-			goto out;
-		ctx->pos++;
+			goto out_dec;
 	}
 
 	for (i = 0; i < ei->nr_entries; i++) {
@@ -784,6 +785,8 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 			continue;
 		}
 
+		ctx->pos++;
+
 		entry = &ei->entries[i];
 		name = entry->name;
 
@@ -791,7 +794,7 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 		/* If ei->is_freed then just bail here, nothing more to do */
 		if (ei->is_freed) {
 			mutex_unlock(&eventfs_mutex);
-			goto out;
+			goto out_dec;
 		}
 		r = entry->callback(name, &mode, &cdata, &fops);
 		mutex_unlock(&eventfs_mutex);
@@ -800,19 +803,23 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		dentry = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
 		if (!dentry)
-			goto out;
+			goto out_dec;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
 		if (!dir_emit(ctx, name, strlen(name), ino, DT_REG))
-			goto out;
-		ctx->pos++;
+			goto out_dec;
 	}
 	ret = 1;
  out:
 	srcu_read_unlock(&eventfs_srcu, idx);
 
 	return ret;
+
+ out_dec:
+	/* Incremented ctx->pos without adding something, reset it */
+	ctx->pos--;
+	goto out;
 }
 
 /**
-- 
2.42.0

[PATCH 3/4] eventfs: Read ei->entries before ei->children in eventfs_iterate()

[Steven Rostedt]

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

In order to apply a shortcut to skip over the current ctx->pos
immediately, by using the ei->entries array, the reading of that array
should be first. Moving the array reading before the linked list reading
will make the shortcut change diff nicer to read.

Link: https://lore.kernel.org/all/CAHk-=wiKwDUDv3+jCsv-uacDcHDVTYsXtBR9=6sGM5mqX+DhOg@mail.gmail.com/

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 46 ++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 23 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index c73fb1f7ddbc..a1934e0eea3b 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -752,8 +752,8 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 	 * Need to create the dentries and inodes to have a consistent
 	 * inode number.
 	 */
-	list_for_each_entry_srcu(ei_child, &ei->children, list,
-				 srcu_read_lock_held(&eventfs_srcu)) {
+	for (i = 0; i < ei->nr_entries; i++) {
+		void *cdata = ei->data;
 
 		if (c > 0) {
 			c--;
@@ -762,23 +762,32 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		ctx->pos++;
 
-		if (ei_child->is_freed)
-			continue;
+		entry = &ei->entries[i];
+		name = entry->name;
 
-		name = ei_child->name;
+		mutex_lock(&eventfs_mutex);
+		/* If ei->is_freed then just bail here, nothing more to do */
+		if (ei->is_freed) {
+			mutex_unlock(&eventfs_mutex);
+			goto out_dec;
+		}
+		r = entry->callback(name, &mode, &cdata, &fops);
+		mutex_unlock(&eventfs_mutex);
+		if (r <= 0)
+			continue;
 
-		dentry = create_dir_dentry(ei, ei_child, ei_dentry);
+		dentry = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
 		if (!dentry)
 			goto out_dec;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
-		if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR))
+		if (!dir_emit(ctx, name, strlen(name), ino, DT_REG))
 			goto out_dec;
 	}
 
-	for (i = 0; i < ei->nr_entries; i++) {
-		void *cdata = ei->data;
+	list_for_each_entry_srcu(ei_child, &ei->children, list,
+				 srcu_read_lock_held(&eventfs_srcu)) {
 
 		if (c > 0) {
 			c--;
@@ -787,27 +796,18 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		ctx->pos++;
 
-		entry = &ei->entries[i];
-		name = entry->name;
-
-		mutex_lock(&eventfs_mutex);
-		/* If ei->is_freed then just bail here, nothing more to do */
-		if (ei->is_freed) {
-			mutex_unlock(&eventfs_mutex);
-			goto out_dec;
-		}
-		r = entry->callback(name, &mode, &cdata, &fops);
-		mutex_unlock(&eventfs_mutex);
-		if (r <= 0)
+		if (ei_child->is_freed)
 			continue;
 
-		dentry = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
+		name = ei_child->name;
+
+		dentry = create_dir_dentry(ei, ei_child, ei_dentry);
 		if (!dentry)
 			goto out_dec;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
-		if (!dir_emit(ctx, name, strlen(name), ino, DT_REG))
+		if (!dir_emit(ctx, name, strlen(name), ino, DT_DIR))
 			goto out_dec;
 	}
 	ret = 1;
-- 
2.42.0

[PATCH 4/4] eventfs: Shortcut eventfs_iterate() by skipping entries already read

[Steven Rostedt]

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

As the ei->entries array is fixed for the duration of the eventfs_inode,
it can be used to skip over already read entries in eventfs_iterate().

That is, if ctx->pos is greater than zero, there's no reason in doing the
loop across the ei->entries array for the entries less than ctx->pos.
Instead, start the lookup of the entries at the current ctx->pos.

Link: https://lore.kernel.org/all/CAHk-=wiKwDUDv3+jCsv-uacDcHDVTYsXtBR9=6sGM5mqX+DhOg@mail.gmail.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index a1934e0eea3b..fdff53d5a1f8 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -746,21 +746,15 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 	if (!ei || !ei_dentry)
 		goto out;
 
-	ret = 0;
-
 	/*
 	 * Need to create the dentries and inodes to have a consistent
 	 * inode number.
 	 */
-	for (i = 0; i < ei->nr_entries; i++) {
-		void *cdata = ei->data;
-
-		if (c > 0) {
-			c--;
-			continue;
-		}
+	ret = 0;
 
-		ctx->pos++;
+	/* Start at 'c' to jump over already read entries */
+	for (i = c; i < ei->nr_entries; i++, ctx->pos++) {
+		void *cdata = ei->data;
 
 		entry = &ei->entries[i];
 		name = entry->name;
@@ -769,7 +763,7 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 		/* If ei->is_freed then just bail here, nothing more to do */
 		if (ei->is_freed) {
 			mutex_unlock(&eventfs_mutex);
-			goto out_dec;
+			goto out;
 		}
 		r = entry->callback(name, &mode, &cdata, &fops);
 		mutex_unlock(&eventfs_mutex);
@@ -778,14 +772,17 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 
 		dentry = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
 		if (!dentry)
-			goto out_dec;
+			goto out;
 		ino = dentry->d_inode->i_ino;
 		dput(dentry);
 
 		if (!dir_emit(ctx, name, strlen(name), ino, DT_REG))
-			goto out_dec;
+			goto out;
 	}
 
+	/* Subtract the skipped entries above */
+	c -= min((unsigned int)c, (unsigned int)ei->nr_entries);
+
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {
 
-- 
2.42.0