1// Copyright 2020 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package framepointer defines an Analyzer that reports assembly code
6// that clobbers the frame pointer before saving it.
7package framepointer
8
9import (
10	"go/build"
11	"regexp"
12	"strings"
13
14	"golang.org/x/tools/go/analysis"
15	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
16)
17
18const Doc = "report assembly that clobbers the frame pointer before saving it"
19
20var Analyzer = &analysis.Analyzer{
21	Name: "framepointer",
22	Doc:  Doc,
23	URL:  "https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/framepointer",
24	Run:  run,
25}
26
27var (
28	re             = regexp.MustCompile
29	asmWriteBP     = re(`,\s*BP$`) // TODO: can have false positive, e.g. for TESTQ BP,BP. Seems unlikely.
30	asmMentionBP   = re(`\bBP\b`)
31	asmControlFlow = re(`^(J|RET)`)
32)
33
34func run(pass *analysis.Pass) (interface{}, error) {
35	if build.Default.GOARCH != "amd64" { // TODO: arm64 also?
36		return nil, nil
37	}
38	if build.Default.GOOS != "linux" && build.Default.GOOS != "darwin" {
39		return nil, nil
40	}
41
42	// Find assembly files to work on.
43	var sfiles []string
44	for _, fname := range pass.OtherFiles {
45		if strings.HasSuffix(fname, ".s") && pass.Pkg.Path() != "runtime" {
46			sfiles = append(sfiles, fname)
47		}
48	}
49
50	for _, fname := range sfiles {
51		content, tf, err := analysisutil.ReadFile(pass, fname)
52		if err != nil {
53			return nil, err
54		}
55
56		lines := strings.SplitAfter(string(content), "\n")
57		active := false
58		for lineno, line := range lines {
59			lineno++
60
61			// Ignore comments and commented-out code.
62			if i := strings.Index(line, "//"); i >= 0 {
63				line = line[:i]
64			}
65			line = strings.TrimSpace(line)
66
67			// We start checking code at a TEXT line for a frameless function.
68			if strings.HasPrefix(line, "TEXT") && strings.Contains(line, "(SB)") && strings.Contains(line, "$0") {
69				active = true
70				continue
71			}
72			if !active {
73				continue
74			}
75
76			if asmWriteBP.MatchString(line) { // clobber of BP, function is not OK
77				pass.Reportf(analysisutil.LineStart(tf, lineno), "frame pointer is clobbered before saving")
78				active = false
79				continue
80			}
81			if asmMentionBP.MatchString(line) { // any other use of BP might be a read, so function is OK
82				active = false
83				continue
84			}
85			if asmControlFlow.MatchString(line) { // give up after any branch instruction
86				active = false
87				continue
88			}
89		}
90	}
91	return nil, nil
92}
93