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1e4624eb5a
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/
Link: https://lore.kernel.org/linux-trace-kernel/20240104220048.172295263@goodmis.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 493ec81a8f ("eventfs: Stop using dcache_readdir() for getdents()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.