• Alexei Starovoitov's avatar
    bpf: introduce BPF_RAW_TRACEPOINT · c4f6699d
    Alexei Starovoitov authored
    Introduce BPF_PROG_TYPE_RAW_TRACEPOINT bpf program type to access
    kernel internal arguments of the tracepoints in their raw form.
    
    >From bpf program point of view the access to the arguments look like:
    struct bpf_raw_tracepoint_args {
           __u64 args[0];
    };
    
    int bpf_prog(struct bpf_raw_tracepoint_args *ctx)
    {
      // program can read args[N] where N depends on tracepoint
      // and statically verified at program load+attach time
    }
    
    kprobe+bpf infrastructure allows programs access function arguments.
    This feature allows programs access raw tracepoint arguments.
    
    Similar to proposed 'dynamic ftrace events' there are no abi guarantees
    to what the tracepoints arguments are and what their meaning is.
    The program needs to type cast args properly and use bpf_probe_read()
    helper to access struct fields when argument is a pointer.
    
    For every tracepoint __bpf_trace_##call function is prepared.
    In assembler it looks like:
    (gdb) disassemble __bpf_trace_xdp_exception
    Dump of assembler code for function __bpf_trace_xdp_exception:
       0xffffffff81132080 <+0>:     mov    %ecx,%ecx
       0xffffffff81132082 <+2>:     jmpq   0xffffffff811231f0 <bpf_trace_run3>
    
    where
    
    TRACE_EVENT(xdp_exception,
            TP_PROTO(const struct net_device *dev,
                     const struct bpf_prog *xdp, u32 act),
    
    The above assembler snippet is casting 32-bit 'act' field into 'u64'
    to pass into bpf_trace_run3(), while 'dev' and 'xdp' args are passed as-is.
    All of ~500 of __bpf_trace_*() functions are only 5-10 byte long
    and in total this approach adds 7k bytes to .text.
    
    This approach gives the lowest possible overhead
    while calling trace_xdp_exception() from kernel C code and
    transitioning into bpf land.
    Since tracepoint+bpf are used at speeds of 1M+ events per second
    this is valuable optimization.
    
    The new BPF_RAW_TRACEPOINT_OPEN sys_bpf command is introduced
    that returns anon_inode FD of 'bpf-raw-tracepoint' object.
    
    The user space looks like:
    // load bpf prog with BPF_PROG_TYPE_RAW_TRACEPOINT type
    prog_fd = bpf_prog_load(...);
    // receive anon_inode fd for given bpf_raw_tracepoint with prog attached
    raw_tp_fd = bpf_raw_tracepoint_open("xdp_exception", prog_fd);
    
    Ctrl-C of tracing daemon or cmdline tool that uses this feature
    will automatically detach bpf program, unload it and
    unregister tracepoint probe.
    
    On the kernel side the __bpf_raw_tp_map section of pointers to
    tracepoint definition and to __bpf_trace_*() probe function is used
    to find a tracepoint with "xdp_exception" name and
    corresponding __bpf_trace_xdp_exception() probe function
    which are passed to tracepoint_probe_register() to connect probe
    with tracepoint.
    
    Addition of bpf_raw_tracepoint doesn't interfere with ftrace and perf
    tracepoint mechanisms. perf_event_open() can be used in parallel
    on the same tracepoint.
    Multiple bpf_raw_tracepoint_open("xdp_exception", prog_fd) are permitted.
    Each with its own bpf program. The kernel will execute
    all tracepoint probes and all attached bpf programs.
    
    In the future bpf_raw_tracepoints can be extended with
    query/introspection logic.
    
    __bpf_raw_tp_map section logic was contributed by Steven Rostedt
    Signed-off-by: 's avatarAlexei Starovoitov <ast@kernel.org>
    Signed-off-by: 's avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    Acked-by: 's avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: 's avatarDaniel Borkmann <daniel@iogearbox.net>
    c4f6699d
define_trace.h 3.4 KB