Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 184 insertions, 0 deletions

diff --git a/Documentation/process/debugging/gdb-kernel-debugging.rst b/Documentation/process/debugging/gdb-kernel-debugging.rst
new file mode 100644
index 000000000..53e225760
--- /dev/null
+++ b/Documentation/process/debugging/gdb-kernel-debugging.rst
@@ -0,0 +1,184 @@
+.. highlight:: none
+
+Debugging kernel and modules via gdb
+====================================
+
+The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
+interfaces allow to debug the Linux kernel and its modules during runtime
+using gdb. Gdb comes with a powerful scripting interface for python. The
+kernel provides a collection of helper scripts that can simplify typical
+kernel debugging steps. This is a short tutorial about how to enable and use
+them. It focuses on QEMU/KVM virtual machines as target, but the examples can
+be transferred to the other gdb stubs as well.
+
+
+Requirements
+------------
+
+- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
+  for distributions)
+
+
+Setup
+-----
+
+- Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
+  www.qemu.org for more details). For cross-development,
+  https://landley.net/aboriginal/bin keeps a pool of machine images and
+  toolchains that can be helpful to start from.
+
+- Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
+  CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
+  CONFIG_FRAME_POINTER, keep it enabled.
+
+- Install that kernel on the guest, turn off KASLR if necessary by adding
+  "nokaslr" to the kernel command line.
+  Alternatively, QEMU allows to boot the kernel directly using -kernel,
+  -append, -initrd command line switches. This is generally only useful if
+  you do not depend on modules. See QEMU documentation for more details on
+  this mode. In this case, you should build the kernel with
+  CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
+
+- Build the gdb scripts (required on kernels v5.1 and above)::
+
+    make scripts_gdb
+
+- Enable the gdb stub of QEMU/KVM, either
+
+    - at VM startup time by appending "-s" to the QEMU command line
+
+  or
+
+    - during runtime by issuing "gdbserver" from the QEMU monitor
+      console
+
+- cd /path/to/linux-build
+
+- Start gdb: gdb vmlinux
+
+  Note: Some distros may restrict auto-loading of gdb scripts to known safe
+  directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
+
+    add-auto-load-safe-path /path/to/linux-build
+
+  to ~/.gdbinit. See gdb help for more details.
+
+- Attach to the booted guest::
+
+    (gdb) target remote :1234
+
+
+Examples of using the Linux-provided gdb helpers
+------------------------------------------------
+
+- Load module (and main kernel) symbols::
+
+    (gdb) lx-symbols
+    loading vmlinux
+    scanning for modules in /home/user/linux/build
+    loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
+    loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
+    loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
+    loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
+    loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
+    ...
+    loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
+
+- Set a breakpoint on some not yet loaded module function, e.g.::
+
+    (gdb) b btrfs_init_sysfs
+    Function "btrfs_init_sysfs" not defined.
+    Make breakpoint pending on future shared library load? (y or [n]) y
+    Breakpoint 1 (btrfs_init_sysfs) pending.
+
+- Continue the target::
+
+    (gdb) c
+
+- Load the module on the target and watch the symbols being loaded as well as
+  the breakpoint hit::
+
+    loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
+    loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
+    loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
+    loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
+
+    Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
+    36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
+
+- Dump the log buffer of the target kernel::
+
+    (gdb) lx-dmesg
+    [     0.000000] Initializing cgroup subsys cpuset
+    [     0.000000] Initializing cgroup subsys cpu
+    [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
+    [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
+    [     0.000000] e820: BIOS-provided physical RAM map:
+    [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
+    [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
+    ....
+
+- Examine fields of the current task struct(supported by x86 and arm64 only)::
+
+    (gdb) p $lx_current().pid
+    $1 = 4998
+    (gdb) p $lx_current().comm
+    $2 = "modprobe\000\000\000\000\000\000\000"
+
+- Make use of the per-cpu function for the current or a specified CPU::
+
+    (gdb) p $lx_per_cpu(runqueues).nr_running
+    $3 = 1
+    (gdb) p $lx_per_cpu(runqueues, 2).nr_running
+    $4 = 0
+
+- Dig into hrtimers using the container_of helper::
+
+    (gdb) set $leftmost = $lx_per_cpu(hrtimer_bases).clock_base[0].active.rb_root.rb_leftmost
+    (gdb) p *$container_of($leftmost, "struct hrtimer", "node")
+    $5 = {
+      node = {
+        node = {
+          __rb_parent_color = 18446612686384860673,
+          rb_right = 0xffff888231da8b00,
+          rb_left = 0x0
+        },
+        expires = 1228461000000
+      },
+      _softexpires = 1228461000000,
+      function = 0xffffffff8137ab20 <tick_nohz_handler>,
+      base = 0xffff888231d9b4c0,
+      state = 1 '\001',
+      is_rel = 0 '\000',
+      is_soft = 0 '\000',
+      is_hard = 1 '\001'
+    }
+
+
+List of commands and functions
+------------------------------
+
+The number of commands and convenience functions may evolve over the time,
+this is just a snapshot of the initial version::
+
+ (gdb) apropos lx
+ function lx_current -- Return current task
+ function lx_module -- Find module by name and return the module variable
+ function lx_per_cpu -- Return per-cpu variable
+ function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
+ function lx_thread_info -- Calculate Linux thread_info from task variable
+ lx-dmesg -- Print Linux kernel log buffer
+ lx-lsmod -- List currently loaded modules
+ lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
+
+Detailed help can be obtained via "help <command-name>" for commands and "help
+function <function-name>" for convenience functions.
+
+Debugging GDB scripts
+---------------------
+
+GDB does not enable a full Python backtrace which can make debugging GDB
+scripts more difficult than necessary. The following will allow for printing a
+full backtrace of the python environment::
+
+ (gdb) set python print-stack full