[PATCH v3 0/2] tracing: Documentation: Add missing config and document for eprobes

[PATCH v3 0/2] tracing: Documentation: Add missing config and document for eprobes

[Steven Rostedt]

Add missing eprobe config option and documentation which should have both been
added back in 5.15.

Changes since v2: https://lore.kernel.org/linux-trace-kernel/20250729161816.678462962@kernel.org/

- Added Kconfig dependency on HAVE_REGS_AND_STACK_ACCESS_API (Masami Hiramatsu)

- Put back s/;/,/ (Randy Dunlap)

Steven Rostedt (2):
      tracing: Have eprobes have their own config option
      Documentation: tracing: Add documentation about eprobes

----
 Documentation/trace/eprobetrace.rst | 269 ++++++++++++++++++++++++++++++++++++
 Documentation/trace/index.rst       |   1 +
 kernel/trace/Kconfig                |  14 ++
 kernel/trace/Makefile               |   2 +-
 4 files changed, 285 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/trace/eprobetrace.rst

[PATCH v3 1/2] tracing: Have eprobes have their own config option

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

Eprobes were added in 5.15 and were selected whenever any of the other
probe events were selected. If kprobe events were enabled (which it is by
default if kprobes are enabled) it would enable eprobe events as well. The
same for uprobes and fprobes.

Have eprobes have its own config and it gets enabled by default if tracing
is enabled.

Link: https://lore.kernel.org/all/20250729102636.b7cce553e7cc263722b12365@kernel.org/

Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v2: https://lore.kernel.org/20250729161912.056641407@kernel.org

- Added Kconfig dependency on HAVE_REGS_AND_STACK_ACCESS_API (Masami Hiramatsu)

 kernel/trace/Kconfig  | 14 ++++++++++++++
 kernel/trace/Makefile |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 93e8e7fc11c0..f80298e6aa16 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -792,6 +792,20 @@ config UPROBE_EVENTS
 	  This option is required if you plan to use perf-probe subcommand
 	  of perf tools on user space applications.
 
+config EPROBE_EVENTS
+	bool "Enable event-based dynamic events"
+	depends on TRACING
+	depends on HAVE_REGS_AND_STACK_ACCESS_API
+	select PROBE_EVENTS
+	select DYNAMIC_EVENTS
+	default y
+	help
+	  Eprobes are dynamic events that can be placed on other existing
+	  events. It can be used to limit what fields are recorded in
+	  an event or even dereference a field of an event. It can
+	  convert the type of an event field. For example, turn an
+	  address into a string.
+
 config BPF_EVENTS
 	depends on BPF_SYSCALL
 	depends on (KPROBE_EVENTS || UPROBE_EVENTS) && PERF_EVENTS
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 057cd975d014..dcb4e02afc5f 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -82,7 +82,7 @@ obj-$(CONFIG_EVENT_TRACING) += trace_event_perf.o
 endif
 obj-$(CONFIG_EVENT_TRACING) += trace_events_filter.o
 obj-$(CONFIG_EVENT_TRACING) += trace_events_trigger.o
-obj-$(CONFIG_PROBE_EVENTS) += trace_eprobe.o
+obj-$(CONFIG_EPROBE_EVENTS) += trace_eprobe.o
 obj-$(CONFIG_TRACE_EVENT_INJECT) += trace_events_inject.o
 obj-$(CONFIG_SYNTH_EVENTS) += trace_events_synth.o
 obj-$(CONFIG_HIST_TRIGGERS) += trace_events_hist.o
-- 
2.47.2

[PATCH v3 2/2] Documentation: tracing: Add documentation about eprobes

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

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.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v2: https://lore.kernel.org/20250729161912.226505358@kernel.org

- Put back s/;/,/ (Randy Dunlap)

 Documentation/trace/eprobetrace.rst | 269 ++++++++++++++++++++++++++++
 Documentation/trace/index.rst       |   1 +
 2 files changed, 270 insertions(+)
 create mode 100644 Documentation/trace/eprobetrace.rst

diff --git a/Documentation/trace/eprobetrace.rst b/Documentation/trace/eprobetrace.rst
new file mode 100644
index 000000000000..89b5157cfab8
--- /dev/null
+++ b/Documentation/trace/eprobetrace.rst
@@ -0,0 +1,269 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Eprobe - Event-based Probe Tracing
+==================================
+
+:Author: Steven Rostedt <rostedt@goodmis.org>
+
+- Written for v6.17
+
+Overview
+========
+
+Eprobes are dynamic events that are placed on existing events to either
+dereference a field that is a pointer, or simply to limit what fields are
+recorded in the trace event.
+
+Eprobes depend on kprobe events so to enable this feature, build your kernel
+with CONFIG_EPROBE_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)
+  $comm		: Fetch current task comm.
+  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
+  \IMM		: Store an immediate value to the argument.
+  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
+  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
+		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
+		  (x8/x16/x32/x64), VFS layer common type(%pd/%pD), "char",
+                  "string", "ustring", "symbol", "symstr" and "bitfield" are
+                  supported.
+
+Types
+-----
+The FETCHARGS above is very similar to the kprobe events as described in
+Documentation/trace/kprobetrace.rst.
+
+The difference between eprobes and kprobes FETCHARGS is that eprobes has a
+$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
+---------------------------
+
+Eprobes may attach to dynamic events as well as to normal events. It may
+attach to a kprobe event, a synthetic event or a fprobe event. This is 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
+           <idle>-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
+           <idle>-0       [001] d..4.  5041.247524: switch: (sched.sched_switch) prev=0 next=90
+           <idle>-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
+           <idle>-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 a specific system call is to be recorded but the syscalls events are not
+enabled, the raw_syscalls can still be used (syscalls are system call
+events are not normal events, but are created from the raw_syscalls events
+within the kernel). In order to trace the openat system call, one can create
+an event probe on top of the raw_syscalls 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.
+::
+
+ # 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
+system call __NR_openat is set at 257.
+::
+
+ # 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 an eprobe tries to read memory that has not been faulted
+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
+system call, create another eprobe to attach to the exit event to show the
+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.
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index cc1dc5a087e8..b4a429dc4f7a 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -36,6 +36,7 @@ the Linux kernel.
    kprobes
    kprobetrace
    fprobetrace
+   eprobetrace
    fprobe
    ring-buffer-design
 
-- 
2.47.2