212 lines
8.8 KiB
ReStructuredText
212 lines
8.8 KiB
ReStructuredText
==========================
|
|
Kprobe-based Event Tracing
|
|
==========================
|
|
|
|
:Author: Masami Hiramatsu
|
|
|
|
Overview
|
|
--------
|
|
These events are similar to tracepoint based events. Instead of Tracepoint,
|
|
this is based on kprobes (kprobe and kretprobe). So it can probe wherever
|
|
kprobes can probe (this means, all functions except those with
|
|
__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
|
|
Unlike the Tracepoint based event, this can be added and removed
|
|
dynamically, on the fly.
|
|
|
|
To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
|
|
|
|
Similar to the events tracer, this doesn't need to be activated via
|
|
current_tracer. Instead of that, add probe points via
|
|
/sys/kernel/debug/tracing/kprobe_events, and enable it via
|
|
/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enable.
|
|
|
|
You can also use /sys/kernel/debug/tracing/dynamic_events instead of
|
|
kprobe_events. That interface will provide unified access to other
|
|
dynamic events too.
|
|
|
|
Synopsis of kprobe_events
|
|
-------------------------
|
|
::
|
|
|
|
p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
|
|
r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
|
|
-:[GRP/]EVENT : Clear a probe
|
|
|
|
GRP : Group name. If omitted, use "kprobes" for it.
|
|
EVENT : Event name. If omitted, the event name is generated
|
|
based on SYM+offs or MEMADDR.
|
|
MOD : Module name which has given SYM.
|
|
SYM[+offs] : Symbol+offset where the probe is inserted.
|
|
MEMADDR : Address where the probe is inserted.
|
|
MAXACTIVE : Maximum number of instances of the specified function that
|
|
can be probed simultaneously, or 0 for the default value
|
|
as defined in Documentation/kprobes.txt section 1.3.1.
|
|
|
|
FETCHARGS : Arguments. Each probe can have up to 128 args.
|
|
%REG : Fetch register REG
|
|
@ADDR : Fetch memory at ADDR (ADDR should be in kernel)
|
|
@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
|
|
$stackN : Fetch Nth entry of stack (N >= 0)
|
|
$stack : Fetch stack address.
|
|
$argN : Fetch the Nth function argument. (N >= 1) (\*1)
|
|
$retval : Fetch return value.(\*2)
|
|
$comm : Fetch current task comm.
|
|
+|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(\*3)
|
|
NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
|
|
FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
|
|
(u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
|
|
(x8/x16/x32/x64), "string" and bitfield are supported.
|
|
|
|
(\*1) only for the probe on function entry (offs == 0).
|
|
(\*2) only for return probe.
|
|
(\*3) this is useful for fetching a field of data structures.
|
|
|
|
Types
|
|
-----
|
|
Several types are supported for fetch-args. Kprobe tracer will access memory
|
|
by given type. Prefix 's' and 'u' means those types are signed and unsigned
|
|
respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
|
|
in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
|
|
or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
|
|
x86-64 uses x64).
|
|
These value types can be an array. To record array data, you can add '[N]'
|
|
(where N is a fixed number, less than 64) to the base type.
|
|
E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
|
|
Note that the array can be applied to memory type fetchargs, you can not
|
|
apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
|
|
wrong, but '+8($stack):x8[8]' is OK.)
|
|
String type is a special type, which fetches a "null-terminated" string from
|
|
kernel space. This means it will fail and store NULL if the string container
|
|
has been paged out.
|
|
The string array type is a bit different from other types. For other base
|
|
types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
|
|
as +0(%di):x32.) But string[1] is not equal to string. The string type itself
|
|
represents "char array", but string array type represents "char * array".
|
|
So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
|
|
Bitfield is another special type, which takes 3 parameters, bit-width, bit-
|
|
offset, and container-size (usually 32). The syntax is::
|
|
|
|
b<bit-width>@<bit-offset>/<container-size>
|
|
|
|
Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
|
|
which shows given pointer in "symbol+offset" style.
|
|
For $comm, the default type is "string"; any other type is invalid.
|
|
|
|
|
|
Per-Probe Event Filtering
|
|
-------------------------
|
|
Per-probe event filtering feature allows you to set different filter on each
|
|
probe and gives you what arguments will be shown in trace buffer. If an event
|
|
name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
|
|
under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
|
|
'enable', 'format', 'filter' and 'trigger'.
|
|
|
|
enable:
|
|
You can enable/disable the probe by writing 1 or 0 on it.
|
|
|
|
format:
|
|
This shows the format of this probe event.
|
|
|
|
filter:
|
|
You can write filtering rules of this event.
|
|
|
|
id:
|
|
This shows the id of this probe event.
|
|
|
|
trigger:
|
|
This allows to install trigger commands which are executed when the event is
|
|
hit (for details, see Documentation/trace/events.rst, section 6).
|
|
|
|
Event Profiling
|
|
---------------
|
|
You can check the total number of probe hits and probe miss-hits via
|
|
/sys/kernel/debug/tracing/kprobe_profile.
|
|
The first column is event name, the second is the number of probe hits,
|
|
the third is the number of probe miss-hits.
|
|
|
|
|
|
Usage examples
|
|
--------------
|
|
To add a probe as a new event, write a new definition to kprobe_events
|
|
as below::
|
|
|
|
echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
|
|
|
|
This sets a kprobe on the top of do_sys_open() function with recording
|
|
1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
|
|
assigned to each function argument depends on arch-specific ABI. If you unsure
|
|
the ABI, please try to use probe subcommand of perf-tools (you can find it
|
|
under tools/perf/).
|
|
As this example shows, users can choose more familiar names for each arguments.
|
|
::
|
|
|
|
echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
|
|
|
|
This sets a kretprobe on the return point of do_sys_open() function with
|
|
recording return value as "myretprobe" event.
|
|
You can see the format of these events via
|
|
/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
|
|
::
|
|
|
|
cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
|
|
name: myprobe
|
|
ID: 780
|
|
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:unsigned long __probe_ip; offset:12; size:4; signed:0;
|
|
field:int __probe_nargs; offset:16; size:4; signed:1;
|
|
field:unsigned long dfd; offset:20; size:4; signed:0;
|
|
field:unsigned long filename; offset:24; size:4; signed:0;
|
|
field:unsigned long flags; offset:28; size:4; signed:0;
|
|
field:unsigned long mode; offset:32; size:4; signed:0;
|
|
|
|
|
|
print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
|
|
REC->dfd, REC->filename, REC->flags, REC->mode
|
|
|
|
You can see that the event has 4 arguments as in the expressions you specified.
|
|
::
|
|
|
|
echo > /sys/kernel/debug/tracing/kprobe_events
|
|
|
|
This clears all probe points.
|
|
|
|
Or,
|
|
::
|
|
|
|
echo -:myprobe >> kprobe_events
|
|
|
|
This clears probe points selectively.
|
|
|
|
Right after definition, each event is disabled by default. For tracing these
|
|
events, you need to enable it.
|
|
::
|
|
|
|
echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
|
|
echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
|
|
|
|
And you can see the traced information via /sys/kernel/debug/tracing/trace.
|
|
::
|
|
|
|
cat /sys/kernel/debug/tracing/trace
|
|
# tracer: nop
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
<...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
|
|
<...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
|
|
<...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
|
|
<...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
|
|
<...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
|
|
<...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
|
|
|
|
|
|
Each line shows when the kernel hits an event, and <- SYMBOL means kernel
|
|
returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
|
|
returns from do_sys_open to sys_open+0x1b).
|
|
|