v8_tracer.ebpf.c

// This file contains the code and map definitions for the V8 tracer
//
// Core unwinding of frames is simple, as all the generated code uses frame pointers,
// and all the interesting data is directly accessible via FP. The only additional
// task needed in EBPF code is to collect a Code* or SharedFunctionInfo* and potentially
// the current bytecode offset when in interpreted mode. Rest of the processing can
// be done from host agent.
//
// See the host agent interpreterv8.go for more references.

#include "bpfdefs.h"
#include "tracemgmt.h"
#include "types.h"
#include "v8_tracer.h"

#define v8Ver(x,y,z) (((x)<<24)+((y)<<16)+(z))

// The number of V8 frames to unwind per frame-unwinding eBPF program.
#define V8_FRAMES_PER_PROGRAM   8

// The maximum V8 frame length used in heuristic to validate FP
#define V8_MAX_FRAME_LENGTH     8192

// Map from V8 process IDs to a structure containing addresses of variables
// we require in order to build the stack trace
bpf_map_def SEC("maps") v8_procs = {
  .type = BPF_MAP_TYPE_HASH,
  .key_size = sizeof(pid_t),
  .value_size = sizeof(V8ProcInfo),
  .max_entries = 1024,
};

// Record a V8 frame
static inline __attribute__((__always_inline__))
ErrorCode push_v8(Trace *trace, unsigned long pointer_and_type, unsigned long delta_or_marker) {
  DEBUG_PRINT("Pushing v8 frame delta_or_marker=%lx, pointer_and_type=%lx",
    delta_or_marker, pointer_and_type);
  return _push(trace, pointer_and_type, delta_or_marker, FRAME_MARKER_V8);
}

// Verify a V8 tagged pointer
static inline __attribute__((__always_inline__))
uintptr_t v8_verify_pointer(uintptr_t maybe_pointer) {
  if ((maybe_pointer & HeapObjectTagMask) != HeapObjectTag) {
    return 0;
  }
  return maybe_pointer & ~HeapObjectTagMask;
}

// Read and verify a V8 tagged pointer from given memory location.
static inline __attribute__((__always_inline__))
uintptr_t v8_read_object_ptr(uintptr_t addr) {
  uintptr_t maybe_pointer;
  if (bpf_probe_read(&maybe_pointer, sizeof(maybe_pointer), (void*)addr)) {
    return 0;
  }
  return v8_verify_pointer(maybe_pointer);
}

// Verify and parse a V8 SMI  ("SMall Integer") value.
// On 64-bit systems: SMI is the upper 32-bits of a 64-bit word, and the lowest bit is the tag.
// Returns the SMI value, or def_value in case of errors.
static inline __attribute__((__always_inline__))
uintptr_t v8_parse_smi(uintptr_t maybe_smi, uintptr_t def_value) {
  if ((maybe_smi & SmiTagMask) != SmiTag) {
    return def_value;
  }
  return maybe_smi >> SmiValueShift;
}

// Read the type tag of a Heap Object at given memory location.
// Returns zero on error (valid object type IDs are non-zero).
static inline __attribute__((__always_inline__))
u16 v8_read_object_type(V8ProcInfo *vi, uintptr_t addr) {
  if (!addr) {
    return 0;
  }
  uintptr_t map = v8_read_object_ptr(addr + vi->off_HeapObject_map);
  u16 type;
  if (!map || bpf_probe_read(&type, sizeof(type), (void*)(map + vi->off_Map_instancetype))) {
    return 0;
  }
  return type;
}

// Unwind one V8 frame
static inline __attribute__((__always_inline__))
ErrorCode unwind_one_v8_frame(PerCPURecord *record, V8ProcInfo *vi, bool top) {
  UnwindState *state = &record->state;
  Trace *trace = &record->trace;
  unsigned long regs[2], sp = state->sp, fp = state->fp, pc = state->pc;
  V8UnwindScratchSpace *scratch = &record->v8UnwindScratch;

  // All V8 frames have frame pointer. Check that the FP looks valid.
  DEBUG_PRINT("v8: pc: %lx, sp: %lx, fp: %lx", pc, sp, fp);
  if (fp < sp || fp >= sp + V8_MAX_FRAME_LENGTH) {
    DEBUG_PRINT("v8: frame pointer too far off %lx / %lx", fp, sp);
    increment_metric(metricID_UnwindV8ErrBadFP);
    return ERR_V8_BAD_FP;
  }

  // Read FP pointer data
  if (bpf_probe_read(scratch->fp_ctx, V8_FP_CONTEXT_SIZE, (void*)(fp - V8_FP_CONTEXT_SIZE))) {
    DEBUG_PRINT("v8:  -> failed to read frame pointer context");
    increment_metric(metricID_UnwindV8ErrBadFP);
    return ERR_V8_BAD_FP;
  }

  // Make the verifier happy to access fpctx using the HA provided fp_* variables
  if (vi->fp_marker          > V8_FP_CONTEXT_SIZE - sizeof(unsigned long) ||
      vi->fp_function        > V8_FP_CONTEXT_SIZE - sizeof(unsigned long) ||
      vi->fp_bytecode_offset > V8_FP_CONTEXT_SIZE - sizeof(unsigned long)) {
    return ERR_UNREACHABLE;
  }
  unsigned long fp_marker = *(unsigned long*)(scratch->fp_ctx + vi->fp_marker);
  unsigned long fp_function = *(unsigned long*)(scratch->fp_ctx + vi->fp_function);
  unsigned long fp_bytecode_offset = *(unsigned long*)(scratch->fp_ctx + vi->fp_bytecode_offset);

  // Data that will be sent to HA is in these variables.
  uintptr_t pointer_and_type = 0, delta_or_marker = 0;

  // Before V8 5.8.261 the frame marker was a SMI. Now it has the tag, but it's not shifted fully.
  // The special coding was done to reduce the frame marker push <immed64> to <immed32>.
  if ((fp_marker & SmiTagMask) == SmiTag) {
    // Shift with the tag length only (shift on normal SMI is different).
    pointer_and_type = V8_FILE_TYPE_MARKER;
    delta_or_marker = fp_marker >> SmiTagShift;
    DEBUG_PRINT("v8:  -> stub frame, tag %ld", delta_or_marker);
    goto frame_done;
  }

  // Extract the JSFunction being executed
  uintptr_t jsfunc = v8_verify_pointer(fp_function);
  u16 jsfunc_tag = v8_read_object_type(vi, jsfunc);
  if (jsfunc_tag < vi->type_JSFunction_first || jsfunc_tag > vi->type_JSFunction_last) {
    DEBUG_PRINT("v8:  -> not a JSFunction: %x <= %x <= %x",
      vi->type_JSFunction_first, jsfunc_tag, vi->type_JSFunction_last);
    increment_metric(metricID_UnwindV8ErrBadJSFunc);
    return ERR_V8_BAD_JS_FUNC;
  }

  // Read the SFI to identify the function.
  uintptr_t sfi = v8_read_object_ptr(jsfunc + vi->off_JSFunction_shared);
  if (v8_read_object_type(vi, sfi) != vi->type_SharedFunctionInfo) {
    DEBUG_PRINT("v8:  -> no SharedFunctionInfo");
    increment_metric(metricID_UnwindV8ErrBadJSFunc);
    return ERR_V8_BAD_JS_FUNC;
  }

  // First determine if we are in interpreter mode. The simplest way to check
  // is if fp_bytecode_offset holds a SMI (the bytecode delta). The delta is
  // relative to the object pointer (not the actual bytecode data), so it is
  // always positive. In native mode, the same slot contains a Feedback Vector
  // tagged pointer.
  delta_or_marker = v8_parse_smi(fp_bytecode_offset, 0);
  if (delta_or_marker != 0) {
    DEBUG_PRINT("v8:  -> bytecode_delta %lx", delta_or_marker);
    pointer_and_type = V8_FILE_TYPE_BYTECODE | sfi;
    goto frame_done;
  }

  // Executing native code. At this point we can at least report the SFI if
  // other things fail.
  pointer_and_type = V8_FILE_TYPE_NATIVE_SFI | sfi;

  // Try to determine the Code object from JSFunction.
  uintptr_t code = v8_read_object_ptr(jsfunc + vi->off_JSFunction_code);
  u16 code_type = v8_read_object_type(vi, code);
  if (code_type != vi->type_Code) {
    // If the object type tag does not match, it might be some new functionality
    // in the VM. Report the JSFunction for function name, but report no line
    // number information. This allows to get a complete trace even if this one
    // frame will have some missing information.
    DEBUG_PRINT("v8: jsfunc = %lx, code = %lx, code_type = %x", jsfunc, code, code_type);
    increment_metric(metricID_UnwindV8ErrBadCode);
    goto frame_done;
  }

  // Read the Code blob type and size
  if (bpf_probe_read(scratch->code, sizeof(scratch->code), (void*) code)) {
    increment_metric(metricID_UnwindV8ErrBadCode);
    goto frame_done;
  }
  // Make the verifier happy to access fpctx using the HA provided fp_* variables
  if (vi->off_Code_instruction_size > sizeof(scratch->code) - sizeof(u32) ||
      vi->off_Code_flags            > sizeof(scratch->code) - sizeof(u32)) {
    return ERR_UNREACHABLE;
  }

  uintptr_t code_start;
  if (vi->version >= v8Ver(11, 1, 204)) {
    // Starting V8 11.1.204 the instruction/code start is a pointer field instead
    // of offset where the code starts.
    code_start = *(uintptr_t*)(scratch->code + vi->off_Code_instruction_start);
  } else {
    code_start = code + vi->off_Code_instruction_start;
  }
  u32 code_size  = *(u32*)(scratch->code + vi->off_Code_instruction_size);
  u32 code_flags = *(u32*)(scratch->code + vi->off_Code_flags);
  u8 code_kind = (code_flags & vi->codekind_mask) >> vi->codekind_shift;

  uintptr_t code_end = code_start + code_size;
  DEBUG_PRINT("v8: func = %lx / sfi = %lx / code = %lx", jsfunc, sfi, code);
  DEBUG_PRINT("v8:  -> instructions: %lx..%lx (%d)", code_start, code_end, code_size);

  if (!(pc >= code_start && pc < code_end)) {
    // PC is not inside the Code object's code area. This can happen due to:
    // - on top frame when we are executing prologue/epilogue of called function,
    //   in this case we can try to recover original PC from the stack
    // - the JSFunction's Code object was changed due to On-Stack-Replacement or
    //   or other deoptimization reasons. This case is currently not handled.

    if (top && trace->stack_len == 0) {
      unsigned long stk[3];
      if (bpf_probe_read(stk, sizeof(stk), (void*)(sp - sizeof(stk)))) {
        DEBUG_PRINT("v8:  --> bad stack pointer");
        increment_metric(metricID_UnwindV8ErrBadFP);
        return ERR_V8_BAD_FP;
      }

      int i;
#pragma unroll
      for (i = sizeof(stk)/sizeof(stk[0])-1; i >= 0; i--) {
        if (stk[i] >= code_start && stk[i] < code_end) {
          break;
        }
      }
      if (i < 0) {
        // Not able to recover PC.
        // TODO: investigate why this seems to happen occasionally
        DEBUG_PRINT("v8:  --> outside code blob: stack top %lx %lx %lx",
          stk[2], stk[1], stk[0]);
        goto frame_done;
      }

      // Recover the PC for the function which is in FP.
      pc = stk[i];
      DEBUG_PRINT("v8:  --> pc recovered from stack: %lx", pc);
    } else {
      DEBUG_PRINT("v8:  --> outside code blob (not topmost frame)");
      goto frame_done;
    }
  }

  // Code matches RIP, report it.
  if (code_kind == vi->codekind_baseline) {
    // Baseline Code does not have backpointer to SFI, so give the JSFunc.
    pointer_and_type = V8_FILE_TYPE_NATIVE_JSFUNC | jsfunc;
  } else {
    pointer_and_type = V8_FILE_TYPE_NATIVE_CODE | code;
  }

  // Use cookie that differentiates different types of Code objects
  u32 cookie = (code_size << 4) | code_kind;
  delta_or_marker = (pc - code_start) | ((uintptr_t)cookie << V8_LINE_COOKIE_SHIFT);

frame_done:
  // Unwind with frame pointer
  if (bpf_probe_read(regs, sizeof(regs), (void*)fp)) {
    DEBUG_PRINT("v8:  --> bad frame pointer");
    increment_metric(metricID_UnwindV8ErrBadFP);
    return ERR_V8_BAD_FP;
  }
  state->sp = fp + sizeof(regs);
  state->fp = regs[0];
  state->pc = regs[1];

  ErrorCode error = push_v8(trace, pointer_and_type, delta_or_marker);
  if (error) {
    return error;
  }

  DEBUG_PRINT("v8: pc: %lx, sp: %lx, fp: %lx",
              (unsigned long) state->pc, (unsigned long) state->sp,
              (unsigned long) state->fp);

  increment_metric(metricID_UnwindV8Frames);
  return ERR_OK;
}

// unwind_v8 is the entry point for tracing when invoked from the native tracer
// or interpreter dispatcher. It does not reset the trace object and will append the
// V8 stack frames to the trace object for the current CPU.
SEC("perf_event/unwind_v8")
int unwind_v8(struct pt_regs *ctx) {
  PerCPURecord *record = get_per_cpu_record();
  if (!record) {
    return -1;
  }

  Trace *trace = &record->trace;
  u32 pid = trace->pid;
  DEBUG_PRINT("==== unwind_v8 %d ====", trace->stack_len);

  int unwinder = PROG_UNWIND_STOP;
  ErrorCode error = ERR_OK;
  V8ProcInfo *vi = bpf_map_lookup_elem(&v8_procs, &pid);
  if (!vi) {
    DEBUG_PRINT("v8: no V8ProcInfo for this pid");
    error = ERR_V8_NO_PROC_INFO;
    increment_metric(metricID_UnwindV8ErrNoProcInfo);
    goto exit;
  }

  increment_metric(metricID_UnwindV8Attempts);

#pragma unroll
  for (int i = 0; i < V8_FRAMES_PER_PROGRAM; i++) {
    unwinder = PROG_UNWIND_STOP;

    error = unwind_one_v8_frame(record, vi, i == 0);
    if (error) {
      break;
    }

    error = get_next_unwinder_after_native_frame(record, &unwinder);
    if (error || unwinder != PROG_UNWIND_V8) {
      break;
    }
  }

exit:
  record->state.unwind_error = error;
  tail_call(ctx, unwinder);
  DEBUG_PRINT("v8: tail call for next frame unwinder (%d) failed", unwinder);
  return -1;
}