Provided by: ply_2.4.0-1_amd64 
NAME
ply - dynamically instrument the kernel
SYNOPSIS
ply program-file
ply program-text
DESCRIPTION
ply dynamically instruments the running kernel to aggregate and extract user-defined data. It compiles an
input program to one or more Linux bpf(2) binaries and attaches them to arbitrary points in the kernel
using kprobes and tracepoints.
OPTIONS
-c command, --command=command
When all probes are running, run command. When the command exits, stop all probes and exit. The
command is run as if invoked with sh -c <command>.
-d, --debug
Enable debugging output.
-e, --dry-run
Exit after compilation, without actually instrumenting the system. Typically used in conjunction
with --dump.
-h, --help
Print usage message.
-k, --keep-going
Instead of printing a warning and exiting whenever any trace data is lost, only print the warning
and keep going.
-S, --dump
After compilation, dump the internal AST, generated BPF instructions and other internal
information. This is very useful to include when reporting a bug.
-T, --self-test
Run the built-in self-test which verifies that the required features in the kernel are available
and that all providers are operational.
-u, --unbuffer
Unconditionally disable buffering of stdout/stderr - even when they are not connected to a TTY.
This is useful in interactive sessions where the output is futher processed by another program.
-v, --version
Print version information.
SYNTAX
The syntax is C-like in general, taking its inspiration dtrace(1) and, by extension, from awk(1).
Probes
A program consists of one or more probes, which are analogous to awk's pattern-action statements. The
syntax for a probe is as follows:
provider:probe-definition ['/' predicate '/']
{
statement ';'
[statement ';' ... ]
}
The provider selects which probe interface to use. See the PROVIDERS section for more information about
each provider. It is then up to the provider to parse the probe-definition to determine the point(s) of
instrumentation.
When tracing, it is often desirable to filter events to match some criteria. Because of this, ply allows
you to provide a predicate, i.e. an expression that must evaluate to a non-zero value in order for the
probe to be executed.
Then follows a block of statements that perform the actual information gathering.
A provider may define a default probe clause to be used if the user does not supply one.
Control of Flow
Probes support basic conditional control of flow via an if-statement, which conforms to the same rules as
C's equivalent:
'if' '(' expr ')'
statement ';' | block
[else
statement ';' | block]
In order to ensure that a probe will have a finite run-time the kernel does not allow backwards
branching. As a result, ply does not have any loop construct like for or while. A simple for statement
with an invariant that is known at compile-time could be added later. In that case we could unroll the
loop when generating BPF.
Type System
The type system is modeled after C. As such ply understands the difference between signed and unsigned
integers, the difference between a short and a long long, what separates an integer from a pointer, how a
struct is laid out in memory and so on. It is not complete though, notably floating point numbers and
unions are missing.
Programs are statically typed, but all types are inferred automatically. Thus, the type system is mostly
hidden from the user. Plans are to expose more of it in the future by allowing casts, type declarations
and so on.
Numbers and string literals are specified in the same way as in C.
Maps
The primary way to extract information is to store it in a map, i.e. in a hash table. Like awk(1), ply
dynamically creates any referenced maps and their key and value types are inferred from the context in
which they are used. All maps are in the global scope and can thus be used both for extracting data to
the end-user, and for carrying data between probes. Map names follow the rules of identifiers from C.
mapname[exprs]
Data can be stored in a map by assigning a value to a given key:
mapname[exprs] = expr
The delete keyword can be used to remove an association from a map:
delete mapname[exprs]
You can also remove all elements in the map using clear function.
Aggregations
More often than not, looking at each individual datum from a trace is not nearly as helpful as an
aggregation of the data. Therefore ply supports aggregating data at the source, thereby reducing tracing
overhead. Aggregations are syntactically similar to maps, indeed they are a kind of map, but they are
distinguished by a leading '@'. Also, they can only be assigned the result of one of the following
aggregation functions:
@agg[exprs] = count()
Bump a counter.
@agg[exprs] = sum(scalr-expr)
Evaluates the argument and aggregates the result.
@agg[exprs] = quantize(scalar-expr)
Evaluates the argument and aggregates on the most significant bit of the result. In other words,
it stores the distribution of the expression.
PROVIDERS
A provider makes data available to the user by exporting functions and variables to the probe. Function
calls use the same syntax as most languages that inherit from C. In addition to the provider-specific
functions, all providers inherits a set of common functions and variables:
• char[16] comm, char[16] execname name of the running process's executable.
• u32 cpu CPU ID of the processor on which the probe fired.
• u32 gid Group ID of the running process.
• u32 kpid: Kernel PID of the running process. Also known as pid by the kernel. For a single-threaded
process kpid is equal to pid. For multi-threaded processes, kpid will be unique while pid will be the
same across all threads.
• char[N] mem(void *address [, int size]) Copy size bytes from address. If size is omitted, 64 bytes
will be copied.
• s64 time, s64 walltime: Nanoseconds elaped since system boot. time is intended for time deltas and
walltime should be used for timestamps. They refer to the same data, but with different default
output formats.
• u32 pid: Process ID of the running process. Also known as thread group ID (tgid) by the kernel.
• void print(...): Print each expression with its default output format, separated by commas and
terminated with a newline, to ply's standard out.
• void printf(format, ...): Prints formatted output to ply's standard out. In addition to the formats
recognized by the printf sitting in your
• int strcmp(char *a, char *b): Returns -1, 0 or 1 if the first argument is less than, equal to or
greater than the second argument respectively. Strings are compared by their lexicographical order.
• u32 uid: User ID of the running process.
kprobe and kretprobe
These providers use the corresponding kernel features to instrument arbitrary instructions in the kernel.
The probe-definition may be either an address or a symbol name. When using a symbol name, glob expansion
is performed allowing a single probe to be inserted at multiple locations. An offset relative to a symbol
may also be specfied for kprobes.
Examples:
• kretprobe:schedule: Trace every time schedule returns.
• kprobe:SyS_*: Trace every time a syscall is made.
• kprobe:dev_hard_start_xmit+8: Trace function with offset.
Shared variables:
struct pt_regs *regs
Hardware register contents from when the probe was triggered. This matches the definition in
<sys/ptrace.h> on your system.
u32 stack
Stack trace ID of the current probe. This is just returns an index into a separate map containing
the actual instruction pointers. As a user though, you can think of this function as returning a
string containing the stack trace at the current location. Indeed print(stack) will produce
exactly that.
CAUTION: On some architectures (looking at you, ARM), capturing stack traces at the entry of a
function, before the prologue has run, does not work. Setting your probe after the prologue will
work around the issue (typically two instructions, or +8, on ARM).
kprobe specific functions:
arg0, arg1 ... argN:
void *caller
The program counter, as recorded in regs, at the time the probe was triggered. was attached. The
default output format will resolve it to a symbolic name if one is available.
kretprobe specific function:
retval Return value of the probed function.
tracepoint
The tracepoint provider can instrument all stable tracepoints in the kernel. They are identified by their
relative path from the /sys/kernel/debug/tracing/events directory, where each leaf directory corresponds
to a tracepoint.
Examples:
• tracepoint:sched/sched_wakeup: Trace every time a process is awoken.
• tracepoint:irq/irq_handler_entry: Trace every time an interrupt is handled.
Variables:
struct <X> *data
A struct is dynamically generated for each tracepoint by parsing its format file. I.e. if the
contents of the format file looks like the following:
name: tcp_send_reset
ID: 1304
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:const void * skbaddr; offset:8; size:8; signed:0;
field:const void * skaddr; offset:16; size:8; signed:0;
field:__u16 sport; offset:24; size:2; signed:0;
field:__u16 dport; offset:26; size:2; signed:0;
field:__u8 saddr[4]; offset:28; size:4; signed:0;
field:__u8 daddr[4]; offset:32; size:4; signed:0;
field:__u8 saddr_v6[16]; offset:36; size:16; signed:0;
field:__u8 daddr_v6[16]; offset:52; size:16; signed:0;
Then data would point to a struct of the following type:
struct data {
unsigned short common_type;
unsigned char common_flags;
unsigned char common_preempt_count;
int common_pid;
const void * skbaddr;
const void * skaddr;
__u16 sport;
__u16 dport;
__u8 saddr[4];
__u8 daddr[4];
__u8 saddr_v6[16];
__u8 daddr_v6[16];
};
Functions:
char[N] dyn(void *address [, int size])
Copy size bytes from a dynamic data pointer in data, i.e. a member marked with __data_loc. If size
is omitted, the default string size determines the number of bytes to be copied.
BEGIN and END
These special providers are called at the beginning and the end of the tracing session like awk and
bpftrace. The names are case sensitive. Users can print some messages or fill maps to known info.
interval
The interval provider will be trigger at each given interval. Users can specify time and unit (optional).
If unit is omitted, then second is used. The supported units are:
• m: minutes
• s: seconds (default)
• ms: milli-seconds
• us: micro-seconds
• ns: nano-seconds
Examples:
• interval:1: Called for every second
• interval:500ms: Called for every 500 milli-second
profile
The profile provider supports profiling by allowing the user to specify how many times it will fire
per-second. Values of 1-1000 are supported, and the profile provider supports two probe formats:
• profile:[N]hz: Profile on all CPUs N times per second
• profile:[C]:[N]hz: Profile on CPU C N times per second
EXAMPLE
Extracting data
Print all openated files on the system, and who opened them:
kprobe:SyS_openat
{
print(comm, pid, str(arg1));
}
Quantize
Record the distribution of the return value of read(2):
kretprobe:SyS_read
{
@["dist"] = quantize(retval);
}
Wildcards
Count all syscalls made on the system, grouped by function:
kprobe:SyS_*
{
@[caller] = count();
}
Count all syscalls made by every dd(1) process, grouped by function:
kprobe:SyS_* / !strcmp(execname, "dd") /
{
@[caller] = count();
}
Object Tracking
Record the distribution of the time it takes an skb to go from netif_receive to ip_rcv:
kprobe:__netif_receive_skb_core
{
rx[arg0] = time;
}
kprobe:ip_rcv / rx[arg0] /
{
@["diff"] = quantize(time - rx[arg0]);
}
RETURN VALUE
0 Program was successfully compiled and loaded into the kernel.
Non-Zero
An error occurred during compilation or during kernel setup.
AUTHORS
Tobias Waldekranz tobias@waldekranz.com
COPYRIGHT
Copyright 2018 Tobias Waldekranz
License: GPLv2
SEE ALSO
awk(1) dtrace(1) bpf(2)
February 2025 PLY(1)