From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 30 Aug 2026 09:22:00 -0700 Subject: Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace 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 --- Documentation/admin-guide/dynamic-debug-howto.rst | 392 ++++++++++++++++++++++ 1 file changed, 392 insertions(+) create mode 100644 Documentation/admin-guide/dynamic-debug-howto.rst (limited to 'Documentation/admin-guide/dynamic-debug-howto.rst') diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst new file mode 100644 index 000000000..9c2f096ed --- /dev/null +++ b/Documentation/admin-guide/dynamic-debug-howto.rst @@ -0,0 +1,392 @@ +Dynamic debug ++++++++++++++ + + +Introduction +============ + +Dynamic debug allows you to dynamically enable/disable kernel +debug-print code to obtain additional kernel information. + +If ``/proc/dynamic_debug/control`` exists, your kernel has dynamic +debug. 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 may +need to adjust the kernel ``loglevel=``, or use ``ignore_loglevel``. +Read about these kernel parameters in +Documentation/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 by +a ``=`` for easy use with grep/cut. ``=p`` shows enabled callsites. + +Controlling dynamic debug Behaviour +=================================== + +The behaviour of *prdbg* sites are controlled by writing +query/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`` is +also under the mount-dir, typically ``/sys/kernel/debug/``. + +Command Language Reference +========================== + +At the basic lexical level, a command is a sequence of words separated +by 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 one +character). 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 a +flags change or setting:: + + command ::= match-spec* flags-spec + +The match-spec's select *prdbgs* from the catalog, upon which to apply +the flags-spec, all constraints are ANDed together. An absent keyword +is 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 the +callsite to be compared, and a value to compare against. Possible +keywords 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 followed +by one or more flag characters. The change operation is one +of 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 + 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()``, only +the ``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 during +the boot process, even before userspace and debugfs exists, use +``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``. QUERY follows +the syntax described above, but must not exceed 1023 characters. Your +bootloader may impose lower limits. + +These ``dyndbg`` params are processed just after the ddebug tables are +processed, as part of the early_initcall. Thus you can enable debug +messages in all code run after this early_initcall via this boot +parameter. + +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 if +your machine (typically a laptop) has an Embedded Controller. +PCI (or other devices) initialization also is a hot candidate for using +this boot parameter for debugging purposes. + +If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at +boot time, without effect, but will be reprocessed when module is +loaded 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 with +params 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), and +modprobe 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 (or +enabled by ``-DDEBUG`` flag during compilation) can be disabled later via +the 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 embedded +system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic +debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any +modules which you'd like to dynamically debug later. + + +Kernel *prdbg* API +================== + +The following functions are cataloged and controllable when dynamic +debug 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()`` is +just a shortcut for ``print_hex_dump(KERN_DEBUG)``. + +For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is +its ``prefix_str`` argument, if it is constant string; or ``hexdump`` +in case ``prefix_str`` is built dynamically. -- cgit v1.3.1