Dynamic debug+++++++++++++  Introduction============ Dynamic debug allows you to dynamically enable/disable kerneldebug-print code to obtain additional kernel information. If ``/proc/dynamic_debug/control`` exists, your kernel has dynamicdebug.  You'll need root access (sudo su) to use this. Dynamic debug provides:  * a Catalog of all *prdbgs* in your kernel.   ``cat /proc/dynamic_debug/control`` to see them.  * a Simple query/command language to alter *prdbgs* by selecting on   any combination of 0 or 1 of:    - source filename   - function name   - line number (including ranges of line numbers)   - module name   - format string   - class name (as known/declared by each module) NOTE: To actually get the debug-print output on the console, you mayneed to adjust the kernel ``loglevel=``, or use ``ignore_loglevel``.Read about these kernel parameters inDocumentation/admin-guide/kernel-parameters.rst. Viewing Dynamic Debug Behaviour=============================== You can view the currently configured behaviour in the *prdbg* catalog::   :#> head -n7 /proc/dynamic_debug/control  # filename:lineno [module]function flags format  init/main.c:1179 [main]initcall_blacklist =_ "blacklisting initcall %s\n"  init/main.c:1218 [main]initcall_blacklisted =_ "initcall %s blacklisted\n"  init/main.c:1424 [main]run_init_process =_ "  with arguments:\n"  init/main.c:1426 [main]run_init_process =_ "    %s\n"  init/main.c:1427 [main]run_init_process =_ "  with environment:\n"  init/main.c:1429 [main]run_init_process =_ "    %s\n" The 3rd space-delimited column shows the current flags, preceded bya ``=`` for easy use with grep/cut. ``=p`` shows enabled callsites. Controlling dynamic debug Behaviour=================================== The behaviour of *prdbg* sites are controlled by writingquery/commands to the control file.  Example::   # grease the interface  :#> alias ddcmd='echo $* > /proc/dynamic_debug/control'   :#> ddcmd '-p; module main func run* +p'  :#> grep =p /proc/dynamic_debug/control  init/main.c:1424 [main]run_init_process =p "  with arguments:\n"  init/main.c:1426 [main]run_init_process =p "    %s\n"  init/main.c:1427 [main]run_init_process =p "  with environment:\n"  init/main.c:1429 [main]run_init_process =p "    %s\n" Error messages go to console/syslog::   :#> ddcmd mode foo +p  dyndbg: unknown keyword "mode"  dyndbg: query parse failed  bash: echo: write error: Invalid argument If debugfs is also enabled and mounted, ``dynamic_debug/control`` isalso under the mount-dir, typically ``/sys/kernel/debug/``. Command Language Reference========================== At the basic lexical level, a command is a sequence of words separatedby spaces or tabs.  So these are all equivalent::   :#> ddcmd file svcsock.c line 1603 +p  :#> ddcmd "file svcsock.c line 1603 +p"  :#> ddcmd '  file   svcsock.c     line  1603 +p  ' Command submissions are bounded by a write() system call.Multiple commands can be written together, separated by ``;`` or ``\n``::   :#> ddcmd "func pnpacpi_get_resources +p; func pnp_assign_mem +p"  :#> ddcmd <<"EOC"  func pnpacpi_get_resources +p  func pnp_assign_mem +p  EOC  :#> cat query-batch-file > /proc/dynamic_debug/control You can also use wildcards in each query term. The match rule supports``*`` (matches zero or more characters) and ``?`` (matches exactly onecharacter). For example, you can match all usb drivers::   :#> ddcmd file "drivers/usb/*" +p	# "" to suppress shell expansion Syntactically, a command is pairs of keyword values, followed by aflags change or setting::   command ::= match-spec* flags-spec The match-spec's select *prdbgs* from the catalog, upon which to applythe flags-spec, all constraints are ANDed together.  An absent keywordis the same as keyword "*". Note that since the match-spec can be empty, the flags are checked 1st,then the pairs of keyword and value.  Flag errs will hide keyword errs::   bash-5.2# ddcmd mod bar +foo  dyndbg: read 13 bytes from userspace  dyndbg: query 0: "mod bar +foo" mod:*  dyndbg: unknown flag 'o'  dyndbg: flags parse failed  dyndbg: processed 1 queries, with 0 matches, 1 errs So a match-spec is a keyword, which selects the attribute of thecallsite to be compared, and a value to compare against.  Possiblekeywords are::   match-spec ::= 'func' string |		 'file' string |		 'module' string |		 'format' string |		 'class' string |		 'line' line-range   line-range ::= lineno |		 '-'lineno |		 lineno'-' |		 lineno'-'lineno   lineno ::= unsigned-int .. note::   ``line-range`` cannot contain space, e.g.  "1-30" is valid range but "1 - 30" is not.  The meanings of each keyword are: func    The given string is compared against the function name    of each callsite.  Example:: 	func svc_tcp_accept	func *recv*		# in rfcomm, bluetooth, ping, tcp file    The given string is compared against either the src-root relative    pathname, or the basename of the source file of each callsite.    Examples:: 	file svcsock.c	file kernel/freezer.c	# ie column 1 of control file	file drivers/usb/*	# all callsites under it	file inode.c:start_*	# parse :tail as a func (above)	file inode.c:1-100	# parse :tail as a line-range (above) module    The given string is compared against the module name    of each callsite.  The module name is the string as    seen in ``lsmod``, i.e. without the directory or the ``.ko``    suffix and with ``-`` changed to ``_``.  Examples:: 	module sunrpc	module nfsd	module drm*	# both drm, drm_kms_helper format    The given string is searched for in the dynamic debug format    string.  Note that the string does not need to match the    entire format, only some part.  Whitespace and other    special characters can be escaped using C octal character    escape ``\ooo`` notation, e.g. the space character is ``\040``.    Alternatively, the string can be enclosed in double quote    characters (``"``) or single quote characters (``'``).    Examples:: 	format svcrdma:         // many of the NFS/RDMA server pr_debugs	format readahead        // some pr_debugs in the readahead cache	format nfsd:\040SETATTR // one way to match a format with whitespace	format "nfsd: SETATTR"  // a neater way to match a format with whitespace	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace class    The given class_name is validated against each module, which may    have declared a list of known class_names.  If the class_name is    found for a module, callsite & class matching and adjustment    proceeds.  Examples:: 	class DRM_UT_KMS	# a DRM.debug category	class JUNK		# silent non-match	// class TLD_*		# NOTICE: no wildcard in class names line    The given line number or range of line numbers is compared    against the line number of each ``pr_debug()`` callsite.  A single    line number matches the callsite line number exactly.  A    range of line numbers matches any callsite between the first    and last line number inclusive.  An empty first number means    the first line in the file, an empty last line number means the    last line number in the file.  Examples:: 	line 1603           // exactly line 1603	line 1600-1605      // the six lines from line 1600 to line 1605	line -1605          // the 1605 lines from line 1 to line 1605	line 1600-          // all lines from line 1600 to the end of the file The flags specification comprises a change operation followedby one or more flag characters.  The change operation is oneof the characters::   -    remove the given flags  +    add the given flags  =    set the flags to the given flags The flags are::   p    enables the pr_debug() callsite.  _    enables no flags.   Decorator flags add to the message-prefix, in order:  t    Include thread ID, or <intr>  m    Include module name  f    Include the function name  s    Include the source file name  l    Include line number  d    Include call trace For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, onlythe ``p`` flag has meaning, other flags are ignored. Note the regexp ``^[-+=][fslmptd_]+$`` matches a flags specification.To clear all flags at once, use ``=_`` or ``-fslmptd``.  Debug messages during Boot Process================================== To activate debug messages for core code and built-in modules duringthe boot process, even before userspace and debugfs exists, use``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``.  QUERY followsthe syntax described above, but must not exceed 1023 characters.  Yourbootloader may impose lower limits. These ``dyndbg`` params are processed just after the ddebug tables areprocessed, as part of the early_initcall.  Thus you can enable debugmessages in all code run after this early_initcall via this bootparameter. On an x86 system for example ACPI enablement is a subsys_initcall and::    dyndbg="file ec.c +p" will show early Embedded Controller transactions during ACPI setup ifyour machine (typically a laptop) has an Embedded Controller.PCI (or other devices) initialization also is a hot candidate for usingthis boot parameter for debugging purposes. If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed atboot time, without effect, but will be reprocessed when module isloaded later. Bare ``dyndbg=`` is only processed at boot.  Debug Messages at Module Initialization Time============================================ When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for``foo.params``, strips ``foo.``, and passes them to the kernel along withparams given in modprobe args or ``/etc/modprobe.d/*.conf`` files,in the following order: 1. parameters given via ``/etc/modprobe.d/*.conf``:: 	options foo dyndbg=+pt	options foo dyndbg # defaults to +p 2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed:: 	foo.dyndbg=" func bar +p; func buz +mp" 3. args to modprobe:: 	modprobe foo dyndbg==pmf # override previous settings These ``dyndbg`` queries are applied in order, with last having final say.This allows boot args to override or modify those from ``/etc/modprobe.d``(sensible, since 1 is system wide, 2 is kernel or boot specific), andmodprobe args to override both. In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.``foo`` is extracted from the param-name, and applied to each query in``QUERY``, and only 1 match-spec of each type is allowed. The ``dyndbg`` option is a "fake" module parameter, which means: - modules do not need to define it explicitly- every module gets it tacitly, whether they use pr_debug or not- it doesn't appear in ``/sys/module/$module/parameters/``  To see it, grep the control file, or inspect ``/proc/cmdline.`` For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (orenabled by ``-DDEBUG`` flag during compilation) can be disabled later viathe debugfs interface if the debug messages are no longer needed::    echo "module module_name -p" > /proc/dynamic_debug/control Examples======== ::   // enable the message at line 1603 of file svcsock.c  :#> ddcmd 'file svcsock.c line 1603 +p'   // enable all the messages in file svcsock.c  :#> ddcmd 'file svcsock.c +p'   // enable all the messages in the NFS server module  :#> ddcmd 'module nfsd +p'   // enable all 12 messages in the function svc_process()  :#> ddcmd 'func svc_process +p'   // disable all 12 messages in the function svc_process()  :#> ddcmd 'func svc_process -p'   // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.  :#> ddcmd 'format "nfsd: READ" +p'   // enable messages in files of which the paths include string "usb"  :#> ddcmd 'file *usb* +p'   // enable all messages  :#> ddcmd '+p'   // add module, function to all enabled messages  :#> ddcmd '+mf'   // boot-args example, with newlines and comments for readability  Kernel command line: ...    // see what's going on in dyndbg=value processing    dynamic_debug.verbose=3    // enable pr_debugs in the btrfs module (can be builtin or loadable)    btrfs.dyndbg="+p"    // enable pr_debugs in all files under init/    // and the function parse_one, #cmt is stripped    dyndbg="file init/* +p #cmt ; func parse_one +p"    // enable pr_debugs in 2 functions in a module loaded later    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p" Kernel Configuration==================== Dynamic Debug is enabled via kernel config items::   CONFIG_DYNAMIC_DEBUG=y	# build catalog, enables CORE  CONFIG_DYNAMIC_DEBUG_CORE=y	# enable mechanics only, skip catalog If you do not want to enable dynamic debug globally (i.e. in some embeddedsystem), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamicdebug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of anymodules which you'd like to dynamically debug later.  Kernel *prdbg* API================== The following functions are cataloged and controllable when dynamicdebug is enabled::   pr_debug()  dev_dbg()  print_hex_dump_debug()  print_hex_dump_bytes() Otherwise, they are off by default; ``ccflags += -DDEBUG`` or``#define DEBUG`` in a source file will enable them appropriately. If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` isjust a shortcut for ``print_hex_dump(KERN_DEBUG)``. For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string isits ``prefix_str`` argument, if it is constant string; or ``hexdump``in case ``prefix_str`` is built dynamically.