path: root/pkg/coverage/coverage.go

// Copyright 2020 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package coverage provides an interface through which Go coverage data can
// be collected, converted to kcov format, and exposed to userspace.
//
// Coverage can be enabled by calling bazel {build,test} with
// --collect_coverage_data and --instrumentation_filter with the desired
// coverage surface. This causes bazel to use the Go cover tool manually to
// generate instrumented files. It injects a hook that registers all coverage
// data with the coverdata package.
package coverage

import (
	"fmt"
	"io"
	"sort"
	"testing"

	"gvisor.dev/gvisor/pkg/sync"
	"gvisor.dev/gvisor/pkg/usermem"

	"github.com/bazelbuild/rules_go/go/tools/coverdata"
)

// coverageMu must be held while accessing coverdata.Cover. This prevents
// concurrent reads/writes from multiple threads collecting coverage data.
var coverageMu sync.RWMutex

// once ensures that globalData is only initialized once.
var once sync.Once

// blockBitLength is the number of bits used to represent coverage block index
// in a synthetic PC (the rest are used to represent the file index). Even
// though a PC has 64 bits, we only use the lower 32 bits because some users
// (e.g., syzkaller) may truncate that address to a 32-bit value.
//
// As of this writing, there are ~1200 files that can be instrumented and at
// most ~1200 blocks per file, so 16 bits is more than enough to represent every
// file and every block.
const blockBitLength = 16

// KcovAvailable returns whether the kcov coverage interface is available. It is
// available as long as coverage is enabled for some files.
func KcovAvailable() bool {
	return len(coverdata.Cover.Blocks) > 0
}

var globalData struct {
	// files is the set of covered files sorted by filename. It is calculated at
	// startup.
	files []string

	// syntheticPCs are a set of PCs calculated at startup, where the PC
	// at syntheticPCs[i][j] corresponds to file i, block j.
	syntheticPCs [][]uint64
}

// ClearCoverageData clears existing coverage data.
//
//go:norace
func ClearCoverageData() {
	coverageMu.Lock()
	defer coverageMu.Unlock()

	// We do not use atomic operations while reading/writing to the counters,
	// which would drastically degrade performance. Slight discrepancies due to
	// racing is okay for the purposes of kcov.
	for _, counters := range coverdata.Cover.Counters {
		for index := 0; index < len(counters); index++ {
			counters[index] = 0
		}
	}
}

var coveragePool = sync.Pool{
	New: func() interface{} {
		return make([]byte, 0)
	},
}

// ConsumeCoverageData builds and writes the collection of covered PCs. It
// returns the number of bytes written.
//
// In Linux, a kernel configuration is set that compiles the kernel with a
// custom function that is called at the beginning of every basic block, which
// updates the memory-mapped coverage information. The Go coverage tool does not
// allow us to inject arbitrary instructions into basic blocks, but it does
// provide data that we can convert to a kcov-like format and transfer them to
// userspace through a memory mapping.
//
// Note that this is not a strict implementation of kcov, which is especially
// tricky to do because we do not have the same coverage tools available in Go
// that that are available for the actual Linux kernel. In Linux, a kernel
// configuration is set that compiles the kernel with a custom function that is
// called at the beginning of every basic block to write program counters to the
// kcov memory mapping. In Go, however, coverage tools only give us a count of
// basic blocks as they are executed. Every time we return to userspace, we
// collect the coverage information and write out PCs for each block that was
// executed, providing userspace with the illusion that the kcov data is always
// up to date. For convenience, we also generate a unique synthetic PC for each
// block instead of using actual PCs. Finally, we do not provide thread-specific
// coverage data (each kcov instance only contains PCs executed by the thread
// owning it); instead, we will supply data for any file specified by --
// instrumentation_filter.
//
// Note that we "consume", i.e. clear, coverdata when this function is run, to
// ensure that each event is only reported once. Due to the limitations of Go
// coverage tools, we reset the global coverage data every time this function is
// run.
//
//go:norace
func ConsumeCoverageData(w io.Writer) int {
	InitCoverageData()

	coverageMu.Lock()
	defer coverageMu.Unlock()

	total := 0
	var pcBuffer [8]byte
	for fileNum, file := range globalData.files {
		counters := coverdata.Cover.Counters[file]
		for index := 0; index < len(counters); index++ {
			// We do not use atomic operations while reading/writing to the counters,
			// which would drastically degrade performance. Slight discrepancies due to
			// racing is okay for the purposes of kcov.
			if counters[index] == 0 {
				continue
			}
			// Non-zero coverage data found; consume it and report as a PC.
			counters[index] = 0
			pc := globalData.syntheticPCs[fileNum][index]
			usermem.ByteOrder.PutUint64(pcBuffer[:], pc)
			n, err := w.Write(pcBuffer[:])
			if err != nil {
				if err == io.EOF {
					// Simply stop writing if we encounter EOF; it's ok if we attempted to
					// write more than we can hold.
					return total + n
				}
				panic(fmt.Sprintf("Internal error writing PCs to kcov area: %v", err))
			}
			total += n
		}
	}

	if total == 0 {
		// An empty profile indicates that coverage is not enabled, in which case
		// there shouldn't be any task work registered.
		panic("kcov task work is registered, but no coverage data was found")
	}
	return total
}

// InitCoverageData initializes globalData. It should be called before any kcov
// data is written.
func InitCoverageData() {
	once.Do(func() {
		// First, order all files. Then calculate synthetic PCs for every block
		// (using the well-defined ordering for files as well).
		for file := range coverdata.Cover.Blocks {
			globalData.files = append(globalData.files, file)
		}
		sort.Strings(globalData.files)

		for fileNum, file := range globalData.files {
			blocks := coverdata.Cover.Blocks[file]
			pcs := make([]uint64, 0, len(blocks))
			for blockNum := range blocks {
				pcs = append(pcs, calculateSyntheticPC(fileNum, blockNum))
			}
			globalData.syntheticPCs = append(globalData.syntheticPCs, pcs)
		}
	})
}

// Symbolize prints information about the block corresponding to pc.
func Symbolize(out io.Writer, pc uint64) error {
	fileNum, blockNum := syntheticPCToIndexes(pc)
	file, err := fileFromIndex(fileNum)
	if err != nil {
		return err
	}
	block, err := blockFromIndex(file, blockNum)
	if err != nil {
		return err
	}
	writeBlock(out, pc, file, block)
	return nil
}

// WriteAllBlocks prints all information about all blocks along with their
// corresponding synthetic PCs.
func WriteAllBlocks(out io.Writer) {
	for fileNum, file := range globalData.files {
		for blockNum, block := range coverdata.Cover.Blocks[file] {
			writeBlock(out, calculateSyntheticPC(fileNum, blockNum), file, block)
		}
	}
}

func calculateSyntheticPC(fileNum int, blockNum int) uint64 {
	return (uint64(fileNum) << blockBitLength) + uint64(blockNum)
}

func syntheticPCToIndexes(pc uint64) (fileNum int, blockNum int) {
	return int(pc >> blockBitLength), int(pc & ((1 << blockBitLength) - 1))
}

// fileFromIndex returns the name of the file in the sorted list of instrumented files.
func fileFromIndex(i int) (string, error) {
	total := len(globalData.files)
	if i < 0 || i >= total {
		return "", fmt.Errorf("file index out of range: [%d] with length %d", i, total)
	}
	return globalData.files[i], nil
}

// blockFromIndex returns the i-th block in the given file.
func blockFromIndex(file string, i int) (testing.CoverBlock, error) {
	blocks, ok := coverdata.Cover.Blocks[file]
	if !ok {
		return testing.CoverBlock{}, fmt.Errorf("instrumented file %s does not exist", file)
	}
	total := len(blocks)
	if i < 0 || i >= total {
		return testing.CoverBlock{}, fmt.Errorf("block index out of range: [%d] with length %d", i, total)
	}
	return blocks[i], nil
}

func writeBlock(out io.Writer, pc uint64, file string, block testing.CoverBlock) {
	io.WriteString(out, fmt.Sprintf("%#x\n", pc))
	io.WriteString(out, fmt.Sprintf("%s:%d.%d,%d.%d\n", file, block.Line0, block.Col0, block.Line1, block.Col1))
}