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.

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

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]);
}