Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 180 insertions, 0 deletions

diff --git a/Documentation/trace/debugging.rst b/Documentation/trace/debugging.rst
new file mode 100644
index 000000000..bca1710d9
--- /dev/null
+++ b/Documentation/trace/debugging.rst
@@ -0,0 +1,180 @@
+==============================
+Using the tracer for debugging
+==============================
+
+Copyright 2024 Google LLC.
+
+:Author:   Steven Rostedt <rostedt@goodmis.org>
+:License:  The GNU Free Documentation License, Version 1.2
+          (dual licensed under the GPL v2)
+
+- Written for: 6.12
+
+Introduction
+------------
+The tracing infrastructure can be very useful for debugging the Linux
+kernel. This document is a place to add various methods of using the tracer
+for debugging.
+
+First, make sure that the tracefs file system is mounted::
+
+ $ sudo mount -t tracefs tracefs /sys/kernel/tracing
+
+
+Using trace_printk()
+--------------------
+
+trace_printk() is a very lightweight utility that can be used in any context
+inside the kernel, with the exception of "noinstr" sections. It can be used
+in normal, softirq, interrupt and even NMI context. The trace data is
+written to the tracing ring buffer in a lockless way. To make it even
+lighter weight, when possible, it will only record the pointer to the format
+string, and save the raw arguments into the buffer. The format and the
+arguments will be post processed when the ring buffer is read. This way the
+trace_printk() format conversions are not done during the hot path, where
+the trace is being recorded.
+
+trace_printk() is meant only for debugging, and should never be added into
+a subsystem of the kernel. If you need debugging traces, add trace events
+instead. If a trace_printk() is found in the kernel, the following will
+appear in the dmesg::
+
+  **********************************************************
+  **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
+  **                                                      **
+  ** trace_printk() being used. Allocating extra memory.  **
+  **                                                      **
+  ** This means that this is a DEBUG kernel and it is     **
+  ** unsafe for production use.                           **
+  **                                                      **
+  ** If you see this message and you are not debugging    **
+  ** the kernel, report this immediately to your vendor!  **
+  **                                                      **
+  **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
+  **********************************************************
+
+Debugging kernel crashes
+------------------------
+There is various methods of acquiring the state of the system when a kernel
+crash occurs. This could be from the oops message in printk, or one could
+use kexec/kdump. But these just show what happened at the time of the crash.
+It can be very useful in knowing what happened up to the point of the crash.
+The tracing ring buffer, by default, is a circular buffer that will
+overwrite older events with newer ones. When a crash happens, the content of
+the ring buffer will be all the events that lead up to the crash.
+
+There are several kernel command line parameters that can be used to help in
+this. The first is "ftrace_dump_on_oops". This will dump the tracing ring
+buffer when a oops occurs to the console. This can be useful if the console
+is being logged somewhere. If a serial console is used, it may be prudent to
+make sure the ring buffer is relatively small, otherwise the dumping of the
+ring buffer may take several minutes to hours to finish. Here's an example
+of the kernel command line::
+
+  ftrace_dump_on_oops trace_buf_size=50K
+
+Note, the tracing buffer is made up of per CPU buffers where each of these
+buffers is broken up into sub-buffers that are by default PAGE_SIZE. The
+above trace_buf_size option above sets each of the per CPU buffers to 50K,
+so, on a machine with 8 CPUs, that's actually 400K total.
+
+Persistent buffers across boots
+-------------------------------
+If the system memory allows it, the tracing ring buffer can be specified at
+a specific location in memory. If the location is the same across boots and
+the memory is not modified, the tracing buffer can be retrieved from the
+following boot. There's two ways to reserve memory for the use of the ring
+buffer.
+
+The more reliable way (on x86) is to reserve memory with the "memmap" kernel
+command line option and then use that memory for the trace_instance. This
+requires a bit of knowledge of the physical memory layout of the system. The
+advantage of using this method, is that the memory for the ring buffer will
+always be the same::
+
+  memmap==12M$0x284500000 trace_instance=boot_map@0x284500000:12M
+
+The memmap above reserves 12 megabytes of memory at the physical memory
+location 0x284500000. Then the trace_instance option will create a trace
+instance "boot_map" at that same location with the same amount of memory
+reserved. As the ring buffer is broke up into per CPU buffers, the 12
+megabytes will be broken up evenly between those CPUs. If you have 8 CPUs,
+each per CPU ring buffer will be 1.5 megabytes in size. Note, that also
+includes meta data, so the amount of memory actually used by the ring buffer
+will be slightly smaller.
+
+Another more generic but less robust way to allocate a ring buffer mapping
+at boot is with the "reserve_mem" option::
+
+  reserve_mem=12M:4096:trace trace_instance=boot_map@trace
+
+The reserve_mem option above will find 12 megabytes that are available at
+boot up, and align it by 4096 bytes. It will label this memory as "trace"
+that can be used by later command line options.
+
+The trace_instance option creates a "boot_map" instance and will use the
+memory reserved by reserve_mem that was labeled as "trace". This method is
+more generic but may not be as reliable. Due to KASLR, the memory reserved
+by reserve_mem may not be located at the same location. If this happens,
+then the ring buffer will not be from the previous boot and will be reset.
+
+Sometimes, by using a larger alignment, it can keep KASLR from moving things
+around in such a way that it will move the location of the reserve_mem. By
+using a larger alignment, you may find better that the buffer is more
+consistent to where it is placed::
+
+  reserve_mem=12M:0x2000000:trace trace_instance=boot_map@trace
+
+On boot up, the memory reserved for the ring buffer is validated. It will go
+through a series of tests to make sure that the ring buffer contains valid
+data. If it is, it will then set it up to be available to read from the
+instance. If it fails any of the tests, it will clear the entire ring buffer
+and initialize it as new.
+
+The layout of this mapped memory may not be consistent from kernel to
+kernel, so only the same kernel is guaranteed to work if the mapping is
+preserved. Switching to a different kernel version may find a different
+layout and mark the buffer as invalid.
+
+NB: Both the mapped address and size must be page aligned for the architecture.
+
+Using trace_printk() in the boot instance
+-----------------------------------------
+By default, the content of trace_printk() goes into the top level tracing
+instance. But this instance is never preserved across boots. To have the
+trace_printk() content, and some other internal tracing go to the preserved
+buffer (like dump stacks), either set the instance to be the trace_printk()
+destination from the kernel command line, or set it after boot up via the
+trace_printk_dest option.
+
+After boot up::
+
+  echo 1 > /sys/kernel/tracing/instances/boot_map/options/trace_printk_dest
+
+From the kernel command line::
+
+  reserve_mem=12M:4096:trace trace_instance=boot_map^traceprintk^traceoff@trace
+
+If setting it from the kernel command line, it is recommended to also
+disable tracing with the "traceoff" flag, and enable tracing after boot up.
+Otherwise the trace from the most recent boot will be mixed with the trace
+from the previous boot, and may make it confusing to read.
+
+Using a backup instance for keeping previous boot data
+------------------------------------------------------
+
+It is also possible to record trace data at system boot time by specifying
+events with the persistent ring buffer, but in this case the data before the
+reboot will be lost before it can be read. This problem can be solved by a
+backup instance. From the kernel command line::
+
+  reserve_mem=12M:4096:trace trace_instance=boot_map@trace,sched,irq trace_instance=backup=boot_map
+
+On boot up, the previous data in the "boot_map" is copied to the "backup"
+instance, and the "sched:*" and "irq:*" events for the current boot are traced
+in the "boot_map". Thus the user can read the previous boot data from the "backup"
+instance without stopping the trace.
+
+Note that this "backup" instance is readonly, and will be removed automatically
+if you clear the trace data or read out all trace data from the "trace_pipe"
+or the "trace_pipe_raw" files.