282 lines
12 KiB
ReStructuredText
282 lines
12 KiB
ReStructuredText
==========================
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Kprobe-based Event Tracing
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==========================
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:Author: Masami Hiramatsu
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Overview
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--------
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These events are similar to tracepoint-based events. Instead of tracepoints,
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this is based on kprobes (kprobe and kretprobe). So it can probe wherever
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kprobes can probe (this means, all functions except those with
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__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
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Unlike the tracepoint-based event, this can be added and removed
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dynamically, on the fly.
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To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
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Similar to the event tracer, this doesn't need to be activated via
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current_tracer. Instead of that, add probe points via
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/sys/kernel/tracing/kprobe_events, and enable it via
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/sys/kernel/tracing/events/kprobes/<EVENT>/enable.
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You can also use /sys/kernel/tracing/dynamic_events instead of
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kprobe_events. That interface will provide unified access to other
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dynamic events too.
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Synopsis of kprobe_events
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-------------------------
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::
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p[:[GRP/][EVENT]] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
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r[MAXACTIVE][:[GRP/][EVENT]] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
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p[:[GRP/][EVENT]] [MOD:]SYM[+0]%return [FETCHARGS] : Set a return probe
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-:[GRP/][EVENT] : Clear a probe
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GRP : Group name. If omitted, use "kprobes" for it.
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EVENT : Event name. If omitted, the event name is generated
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based on SYM+offs or MEMADDR.
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MOD : Module name which has given SYM.
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SYM[+offs] : Symbol+offset where the probe is inserted.
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SYM%return : Return address of the symbol
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MEMADDR : Address where the probe is inserted.
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MAXACTIVE : Maximum number of instances of the specified function that
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can be probed simultaneously, or 0 for the default value
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as defined in Documentation/trace/kprobes.rst section 1.3.1.
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FETCHARGS : Arguments. Each probe can have up to 128 args.
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%REG : Fetch register REG
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@ADDR : Fetch memory at ADDR (ADDR should be in kernel)
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@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
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$stackN : Fetch Nth entry of stack (N >= 0)
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$stack : Fetch stack address.
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$argN : Fetch the Nth function argument. (N >= 1) (\*1)
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$retval : Fetch return value.(\*2)
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$comm : Fetch current task comm.
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+|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
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\IMM : Store an immediate value to the argument.
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NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
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FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
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(u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
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(x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
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and bitfield are supported.
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(\*1) only for the probe on function entry (offs == 0). Note, this argument access
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is best effort, because depending on the argument type, it may be passed on
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the stack. But this only support the arguments via registers.
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(\*2) only for return probe. Note that this is also best effort. Depending on the
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return value type, it might be passed via a pair of registers. But this only
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accesses one register.
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(\*3) this is useful for fetching a field of data structures.
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(\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
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Function arguments at kretprobe
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-------------------------------
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Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This
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is useful to record the function parameter and return value at once, and
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trace the difference of structure fields (for debuging a function whether it
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correctly updates the given data structure or not).
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See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how
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it works.
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.. _kprobetrace_types:
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Types
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-----
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Several types are supported for fetchargs. Kprobe tracer will access memory
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by given type. Prefix 's' and 'u' means those types are signed and unsigned
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respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
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in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
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or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
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x86-64 uses x64).
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These value types can be an array. To record array data, you can add '[N]'
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(where N is a fixed number, less than 64) to the base type.
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E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
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Note that the array can be applied to memory type fetchargs, you can not
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apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
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wrong, but '+8($stack):x8[8]' is OK.)
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Char type can be used to show the character value of traced arguments.
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String type is a special type, which fetches a "null-terminated" string from
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kernel space. This means it will fail and store NULL if the string container
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has been paged out. "ustring" type is an alternative of string for user-space.
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See :ref:`user_mem_access` for more info.
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The string array type is a bit different from other types. For other base
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types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
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as +0(%di):x32.) But string[1] is not equal to string. The string type itself
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represents "char array", but string array type represents "char * array".
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So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
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Bitfield is another special type, which takes 3 parameters, bit-width, bit-
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offset, and container-size (usually 32). The syntax is::
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b<bit-width>@<bit-offset>/<container-size>
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Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
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which shows given pointer in "symbol+offset" style.
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On the other hand, symbol-string type ('symstr') converts the given address to
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"symbol+offset/symbolsize" style and stores it as a null-terminated string.
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With 'symstr' type, you can filter the event with wildcard pattern of the
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symbols, and you don't need to solve symbol name by yourself.
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For $comm, the default type is "string"; any other type is invalid.
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.. _user_mem_access:
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User Memory Access
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------------------
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Kprobe events supports user-space memory access. For that purpose, you can use
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either user-space dereference syntax or 'ustring' type.
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The user-space dereference syntax allows you to access a field of a data
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structure in user-space. This is done by adding the "u" prefix to the
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dereference syntax. For example, +u4(%si) means it will read memory from the
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address in the register %si offset by 4, and the memory is expected to be in
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user-space. You can use this for strings too, e.g. +u0(%si):string will read
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a string from the address in the register %si that is expected to be in user-
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space. 'ustring' is a shortcut way of performing the same task. That is,
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+0(%si):ustring is equivalent to +u0(%si):string.
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Note that kprobe-event provides the user-memory access syntax but it doesn't
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use it transparently. This means if you use normal dereference or string type
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for user memory, it might fail, and may always fail on some architectures. The
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user has to carefully check if the target data is in kernel or user space.
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Per-Probe Event Filtering
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-------------------------
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Per-probe event filtering feature allows you to set different filter on each
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probe and gives you what arguments will be shown in trace buffer. If an event
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name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
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under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
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'enable', 'format', 'filter' and 'trigger'.
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enable:
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You can enable/disable the probe by writing 1 or 0 on it.
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format:
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This shows the format of this probe event.
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filter:
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You can write filtering rules of this event.
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id:
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This shows the id of this probe event.
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trigger:
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This allows to install trigger commands which are executed when the event is
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hit (for details, see Documentation/trace/events.rst, section 6).
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Event Profiling
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---------------
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You can check the total number of probe hits and probe miss-hits via
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/sys/kernel/tracing/kprobe_profile.
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The first column is event name, the second is the number of probe hits,
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the third is the number of probe miss-hits.
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Kernel Boot Parameter
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---------------------
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You can add and enable new kprobe events when booting up the kernel by
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"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
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kprobe events, which format is similar to the kprobe_events.
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The difference is that the probe definition parameters are comma-delimited
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instead of space. For example, adding myprobe event on do_sys_open like below::
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p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
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should be below for kernel boot parameter (just replace spaces with comma)::
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p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
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Usage examples
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--------------
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To add a probe as a new event, write a new definition to kprobe_events
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as below::
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echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
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This sets a kprobe on the top of do_sys_open() function with recording
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1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
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assigned to each function argument depends on arch-specific ABI. If you unsure
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the ABI, please try to use probe subcommand of perf-tools (you can find it
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under tools/perf/).
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As this example shows, users can choose more familiar names for each arguments.
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::
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echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
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This sets a kretprobe on the return point of do_sys_open() function with
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recording return value as "myretprobe" event.
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You can see the format of these events via
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/sys/kernel/tracing/events/kprobes/<EVENT>/format.
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::
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cat /sys/kernel/tracing/events/kprobes/myprobe/format
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name: myprobe
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ID: 780
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format:
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field:unsigned short common_type; offset:0; size:2; signed:0;
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field:unsigned char common_flags; offset:2; size:1; signed:0;
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field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
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field:int common_pid; offset:4; size:4; signed:1;
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field:unsigned long __probe_ip; offset:12; size:4; signed:0;
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field:int __probe_nargs; offset:16; size:4; signed:1;
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field:unsigned long dfd; offset:20; size:4; signed:0;
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field:unsigned long filename; offset:24; size:4; signed:0;
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field:unsigned long flags; offset:28; size:4; signed:0;
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field:unsigned long mode; offset:32; size:4; signed:0;
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print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
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REC->dfd, REC->filename, REC->flags, REC->mode
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You can see that the event has 4 arguments as in the expressions you specified.
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::
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echo > /sys/kernel/tracing/kprobe_events
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This clears all probe points.
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Or,
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::
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echo -:myprobe >> kprobe_events
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This clears probe points selectively.
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Right after definition, each event is disabled by default. For tracing these
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events, you need to enable it.
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::
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echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
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echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
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Use the following command to start tracing in an interval.
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::
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# echo 1 > tracing_on
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Open something...
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# echo 0 > tracing_on
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And you can see the traced information via /sys/kernel/tracing/trace.
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::
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cat /sys/kernel/tracing/trace
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# tracer: nop
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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<...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
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<...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
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<...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
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<...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
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<...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
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<...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
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Each line shows when the kernel hits an event, and <- SYMBOL means kernel
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returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
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returns from do_sys_open to sys_open+0x1b).
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