From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 89E198F54; Tue, 29 Jul 2025 01:02:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.133 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753750963; cv=none; b=B6Z1JqO6HfMV5287mft8Tyv7FsEZoGP6tHghW7lBpf8VXbirx0gjcIPWnQAIXzBGYqJXOwRmKH50W/s+Aajt2/vQHjwHuc6VEWhzLfHcZVK9w3AaxfR9gOVG+doNZxGybRwD61Onidt91VbDFpKUVX/SvRzWH/oT49KOya465KU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753750963; c=relaxed/simple; bh=TgEGiwf6rruNP3HGR3nvXfXCvQIv4ayNFx92So0hmC8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Y4yTrg6f01PaHZUaKhhimyj8yS3qOfi/op23exsaRAV/zG//FU3o0appE630gQuLIdIoGfQn9Vo3XubqrJR9ykFLt67bd3Q8GXXRe/oqn7U+rZv0nbaXWKij/VRKGEMqtBHmCUj/NqbLkO/+fzXMH3EBNBHu6EnCuS/ufWWEjpg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=J1JK/Edz; arc=none smtp.client-ip=198.137.202.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="J1JK/Edz" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Sender:Reply-To:Content-ID:Content-Description; bh=ghgay6/aPEBbQ06oHoX3n84kOJW4qgBGJMDtfOt2W8s=; b=J1JK/EdzifBtyPBQnXG54zO1SI U3ekq9nhz3f3FG3zCPhULMtuIaFK6IMItw8d+H8T7BvfJ6X9+y1tL3F+rqSwpjIxIcqlMVf/zO6kO /awhnY+TiFZLreHAIhgEIDtOzjPOKddq9D5YqY6ClA+DJk970Bdc4SCii8Abf+bPseV2zCUlenpjN ylJBNGm7qRpSJp/ZP22Dj+v1d34jYPNkV9yVBxxMtoOnH+V2VWboX5tS8XtB/FZyF6xTWHLACGLyI LZ9Yt3G+KbfX5TvYgmAIT2cgAlZmIwVThXuiwLKZi3QSgVXoBI4UoUU84B8wYv3c3H78nolx8yoMy fOZ36R0g==; Received: from [50.53.25.54] (helo=[192.168.254.17]) by bombadil.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1ugYjm-0000000FfnB-3UqD; Tue, 29 Jul 2025 01:02:38 +0000 Message-ID: Date: Mon, 28 Jul 2025 18:02:37 -0700 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] Documentation: tracing: Add documentation about eprobes To: Steven Rostedt , LKML , Linux trace kernel , linux-doc@vger.kernel.org Cc: Masami Hiramatsu , Mathieu Desnoyers , Mark Rutland , Andrew Morton , Namhyung Kim , Jonathan Corbet References: <20250728171522.7d54e116@batman.local.home> Content-Language: en-US From: Randy Dunlap In-Reply-To: <20250728171522.7d54e116@batman.local.home> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/28/25 2:15 PM, Steven Rostedt wrote: > From: Steven Rostedt > > Eprobes was added back in 5.15, but was never documented. It became a > "secret" interface even though it has been a topic of several > presentations. For some reason, when eprobes was added, documenting it > never became a priority, until now. > > Signed-off-by: Steven Rostedt (Google) > --- > Documentation/trace/eprobes.rst | 268 ++++++++++++++++++++++++++++++++ > Documentation/trace/index.rst | 1 + > 2 files changed, 269 insertions(+) > create mode 100644 Documentation/trace/eprobes.rst > > diff --git a/Documentation/trace/eprobes.rst b/Documentation/trace/eprobes.rst > new file mode 100644 > index 000000000000..c7aa7c867e9e > --- /dev/null > +++ b/Documentation/trace/eprobes.rst > @@ -0,0 +1,268 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +===================== > +Eprobe - Event probes > +===================== > + > +:Author: Steven Rostedt > + > +- Written for v6.17 > + > +Overview > +======== > + > +Eprobes are dynamic events that are placed on existing events to eiter either > +dereference a field that is a pointer, or simply to limit what fields get (preference:) are > +recorded in the trace event. > + > +Eprobes depend on kprobe events so to enable this feature, build your kernel s/,/;/ > +with CONFIG_KPROBE_EVENTS=y. > + > +Eprobes are created via the /sys/kernel/tracing/dynamic_events file. > + > +Synopsis of eprobe_events > +------------------------- > +:: > + > + e[:[EGRP/][EEVENT]] GRP.EVENT [FETCHARGS] : Set a probe > + -:[EGRP/][EEVENT] : Clear a probe > + > + EGRP : Group name of the new event. If omitted, use "eprobes" for it. > + EEVENT : Event name. If omitted, the event name is generated and will > + be the same event name as the event it attached to. > + GRP : Group name of the event to attach to. > + EVENT : Event name of the event to attach to. > + > + FETCHARGS : Arguments. Each probe can have up to 128 args. > + $FIELD : Fetch the value of the event field called FIELD. > + @ADDR : Fetch memory at ADDR (ADDR should be in kernel) > + @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) > + $comm : Fetch current task comm. > + +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) > + \IMM : Store an immediate value to the argument. > + NAME=FETCHARG : Set NAME as the argument name of FETCHARG. > + FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types > + (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types > + (x8/x16/x32/x64), VFS layer common type(%pd/%pD), "char", > + "string", "ustring", "symbol", "symstr" and bitfield are Should bitfield be quoted? > + supported. > + > +Types > +----- > +The FETCHARGS above is very similar to the kprobe events as described in > +Documentation/trace/kprobetrace.rst. > + > +The difference between eprobes and kprobes FETCHARGS is that eprobes has a > +$FIELD command that returns the content of the event field of the event > +that is attached. Eprobes do not have access to registers, stacks and function > +arguments that kprobes has. > + > +If a field argument is a pointer, it may be dereferenced just like a memory > +address using the FETCHARGS syntax. > + > + > +Attaching to dynamic events > +--------------------------- > + > +Note that eprobes may attach to dynamic events as well as to normal events. It Don't need "Note that". > +may attach to a kprobe event, a synthetic event or a fprobe event. This is I would say: an fprobe event. > +useful if the type of a field needs to be changed. See Example 2 below. > + > +Usage examples > +============== > + > +Example 1 > +--------- > + > +The basic usage of eprobes is to limit the data that is being recorded into > +the tracing buffer. For example, a common event to trace is the sched_switch > +trace event. That has a format of:: > + > + field:unsigned short common_type; offset:0; size:2; signed:0; > + field:unsigned char common_flags; offset:2; size:1; signed:0; > + field:unsigned char common_preempt_count; offset:3; size:1; signed:0; > + field:int common_pid; offset:4; size:4; signed:1; > + > + field:char prev_comm[16]; offset:8; size:16; signed:0; > + field:pid_t prev_pid; offset:24; size:4; signed:1; > + field:int prev_prio; offset:28; size:4; signed:1; > + field:long prev_state; offset:32; size:8; signed:1; > + field:char next_comm[16]; offset:40; size:16; signed:0; > + field:pid_t next_pid; offset:56; size:4; signed:1; > + field:int next_prio; offset:60; size:4; signed:1; > + > +The first four fields are common to all events and can not be limited. But the > +rest of the event has 60 bytes of information. It records the names of the > +previous and next tasks being scheduled out and in, as well as their pids and > +priorities. It also records the state of the previous task. If only the pids > +of the tasks are of interest, why waste the ring buffer with all the other > +fields? > + > +An eprobe can limit what gets recorded. Note, it does not help in performance, > +as all the fields are recorded in a temporary buffer to process the eprobe. > +:: > + > + # echo 'e:sched/switch sched.sched_switch prev=$prev_pid:u32 next=$next_pid:u32' >> /sys/kernel/tracing/dynamic_events > + # echo 1 > /sys/kernel/tracing/events/sched/switch/enable > + # cat /sys/kernel/tracing/trace > + > + # tracer: nop > + # > + # entries-in-buffer/entries-written: 2721/2721 #P:8 > + # > + # _-----=> irqs-off/BH-disabled > + # / _----=> need-resched > + # | / _---=> hardirq/softirq > + # || / _--=> preempt-depth > + # ||| / _-=> migrate-disable > + # |||| / delay > + # TASK-PID CPU# ||||| TIMESTAMP FUNCTION > + # | | | ||||| | | > + sshd-session-1082 [004] d..4. 5041.239906: switch: (sched.sched_switch) prev=1082 next=0 > + bash-1085 [001] d..4. 5041.240198: switch: (sched.sched_switch) prev=1085 next=141 > + kworker/u34:5-141 [001] d..4. 5041.240259: switch: (sched.sched_switch) prev=141 next=1085 > + -0 [004] d..4. 5041.240354: switch: (sched.sched_switch) prev=0 next=1082 > + bash-1085 [001] d..4. 5041.240385: switch: (sched.sched_switch) prev=1085 next=141 > + kworker/u34:5-141 [001] d..4. 5041.240410: switch: (sched.sched_switch) prev=141 next=1085 > + bash-1085 [001] d..4. 5041.240478: switch: (sched.sched_switch) prev=1085 next=0 > + sshd-session-1082 [004] d..4. 5041.240526: switch: (sched.sched_switch) prev=1082 next=0 > + -0 [001] d..4. 5041.247524: switch: (sched.sched_switch) prev=0 next=90 > + -0 [002] d..4. 5041.247545: switch: (sched.sched_switch) prev=0 next=16 > + kworker/1:1-90 [001] d..4. 5041.247580: switch: (sched.sched_switch) prev=90 next=0 > + rcu_sched-16 [002] d..4. 5041.247591: switch: (sched.sched_switch) prev=16 next=0 > + -0 [002] d..4. 5041.257536: switch: (sched.sched_switch) prev=0 next=16 > + rcu_sched-16 [002] d..4. 5041.257573: switch: (sched.sched_switch) prev=16 next=0 > + > +Note, without adding the "u32" after the prev_pid and next_pid, the values > +would default showing in hexadecimal. > + > +Example 2 > +--------- > + > +If syscall events are not enabled but the raw syscall are (systemcall syscalls are (system call > +events are not normal events, but are created from the raw_syscall events > +within the kernel). In order to trace the openat system call, one can create ^^ not a complete sentence. > +an event probe on top of the raw_syscall event: > +:: > + > + # cd /sys/kernel/tracing > + # cat events/raw_syscalls/sys_enter/format > + name: sys_enter > + ID: 395 > + format: > + field:unsigned short common_type; offset:0; size:2; signed:0; > + field:unsigned char common_flags; offset:2; size:1; signed:0; > + field:unsigned char common_preempt_count; offset:3; size:1; signed:0; > + field:int common_pid; offset:4; size:4; signed:1; > + > + field:long id; offset:8; size:8; signed:1; > + field:unsigned long args[6]; offset:16; size:48; signed:0; > + > + print fmt: "NR %ld (%lx, %lx, %lx, %lx, %lx, %lx)", REC->id, REC->args[0], REC->args[1], REC->args[2], REC->args[3], REC->args[4], REC->args[5] > + > +From the source code, the sys_openat() has: > +:: > + > + int sys_openat(int dirfd, const char *path, int flags, mode_t mode) > + { > + return my_syscall4(__NR_openat, dirfd, path, flags, mode); > + } > + > +The path is the second parameter, and that is what is wanted. s/wanted/want/ > +:: > + > + # echo 'e:openat raw_syscalls.sys_enter nr=$id filename=+8($args):ustring' >> dynamic_events > + > +This is being run on x86_64 where the word size is 8 bytes and the openat > +systemcall __NR_openat is set at 257. system call > +:: > + > + # echo 'nr == 257' > events/eprobes/openat/filter > + > +Now enable the event and look at the trace. > +:: > + > + # echo 1 > events/eprobes/openat/enable > + # cat trace > + > + # tracer: nop > + # > + # entries-in-buffer/entries-written: 4/4 #P:8 > + # > + # _-----=> irqs-off/BH-disabled > + # / _----=> need-resched > + # | / _---=> hardirq/softirq > + # || / _--=> preempt-depth > + # ||| / _-=> migrate-disable > + # |||| / delay > + # TASK-PID CPU# ||||| TIMESTAMP FUNCTION > + # | | | ||||| | | > + cat-1298 [003] ...2. 2060.875970: openat: (raw_syscalls.sys_enter) nr=0x101 filename=(fault) > + cat-1298 [003] ...2. 2060.876197: openat: (raw_syscalls.sys_enter) nr=0x101 filename=(fault) > + cat-1298 [003] ...2. 2060.879126: openat: (raw_syscalls.sys_enter) nr=0x101 filename=(fault) > + cat-1298 [003] ...2. 2060.879639: openat: (raw_syscalls.sys_enter) nr=0x101 filename=(fault) > + > +The filename shows "(fault)". This is likely because the filename has not been > +pulled into memory yet and currently trace events cannot fault in memory that > +is not present. When a eprobe tries to read memory that has not been faulted an eprobe > +in yet, it will show the "(fault)" text. > + > +To get around this, as the kernel will likely pull in this filename and make > +it present, attaching it to a synthetic event that can pass the address of the > +filename from the entry of the event to the end of the event, this can be used > +to show the filename when the system call returns. > + > +Remove the old eprobe:: > + > + # echo 1 > events/eprobes/openat/enable > + # echo '-:openat' >> dynamic_events > + > +This time make an eprobe where the address of the filename is saved:: > + > + # echo 'e:openat_start raw_syscalls.sys_enter nr=$id filename=+8($args):x64' >> dynamic_events > + > +Create a synthetic event that passes the address of the filename to the > +end of the event:: > + > + # echo 's:filename u64 file' >> dynamic_events > + # echo 'hist:keys=common_pid:f=filename if nr == 257' > events/eprobes/openat_start/trigger > + # echo 'hist:keys=common_pid:file=$f:onmatch(eprobes.openat_start).trace(filename,$file) if id == 257' > events/raw_syscalls/sys_exit/trigger > + > +Now that the address of the filename has been passed to the end of the > +systemcall, create another eprobe to attach to the exit event to show the system call, > +string:: > + > + # echo 'e:openat synthetic.filename filename=+0($file):ustring' >> dynamic_events > + # echo 1 > events/eprobes/openat/enable > + # cat trace > + > + # tracer: nop > + # > + # entries-in-buffer/entries-written: 4/4 #P:8 > + # > + # _-----=> irqs-off/BH-disabled > + # / _----=> need-resched > + # | / _---=> hardirq/softirq > + # || / _--=> preempt-depth > + # ||| / _-=> migrate-disable > + # |||| / delay > + # TASK-PID CPU# ||||| TIMESTAMP FUNCTION > + # | | | ||||| | | > + cat-1331 [001] ...5. 2944.787977: openat: (synthetic.filename) filename="/etc/ld.so.cache" > + cat-1331 [001] ...5. 2944.788480: openat: (synthetic.filename) filename="/lib/x86_64-linux-gnu/libc.so.6" > + cat-1331 [001] ...5. 2944.793426: openat: (synthetic.filename) filename="/usr/lib/locale/locale-archive" > + cat-1331 [001] ...5. 2944.831362: openat: (synthetic.filename) filename="trace" > + > +Example 3 > +--------- > + > +If syscall trace events are available, the above would not need the first > +eprobe, but it would still need the last one:: > + > + # echo 's:filename u64 file' >> dynamic_events > + # echo 'hist:keys=common_pid:f=filename' > events/syscalls/sys_enter_openat/trigger > + # echo 'hist:keys=common_pid:file=$f:onmatch(syscalls.sys_enter_openat).trace(filename,$file)' > events/syscalls/sys_exit_openat/trigger > + # echo 'e:openat synthetic.filename filename=+0($file):ustring' >> dynamic_events > + # echo 1 > events/eprobes/openat/enable > + > +And this would produce the same result as Example 2. Thanks for the new documentation. -- ~Randy