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Brandon Dyck 2024-08-19 16:30:30 -06:00
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README.md Normal file
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# Go file permissions test
by Brandon Dyck <brandon@dyck.us>
This is an experiment to see how `os.Chmod` and `os.Stat` interact on Windows. The program creates a temporary file, then for each possible set of permissions (00777), sets those permissions on the file with `os.Chmod` and checks the resulting permissions with `os.Stat`.
## Usage
### `go-perm-test`
Prints the results as a CSV in the format
```<expected permissions>,<actual permissions>```
where each set of permissions is an octal integer.
### `go-perm-test <filename>`
Inserts the results into an SQLite database called `filename`.
There are two objects in the DB schema: a table `result_raw(expected INT, actual INT)` and a view `result`.
The `result` view breaks down the permissions into separate fields: The raw data are in the `expected` and `actual` columns, 3-bit fields are in `{expected,actual}_{u,g,o}` columns, and 1-bit fields are in `{expected,actual}_{u,g,o}{r,w,x}` columns.
## Findings
Permissions as observed through `os.Stat` depend entirely upon the owner's write bit set through `os.Chmod`:
```
sqlite> select distinct actual, expected_uw from result;
actual|expected_uw
292|0
438|1
```
This obviously isn't an exhaustive treatment of the subject, and I'm not really interested in doing one, but it corraborates [Michal Pristass findings.](https://archive.is/RZ8WP)

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go.mod Normal file
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module git.codemonkeysoftware.net/b/go-perm-test
go 1.22.1
require (
github.com/dustin/go-humanize v1.0.1 // indirect
github.com/google/uuid v1.3.0 // indirect
github.com/mattn/go-isatty v0.0.16 // indirect
github.com/ncruces/go-strftime v0.1.9 // indirect
github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
golang.org/x/sys v0.16.0 // indirect
modernc.org/libc v1.41.0 // indirect
modernc.org/mathutil v1.6.0 // indirect
modernc.org/memory v1.7.2 // indirect
modernc.org/sqlite v1.29.1 // indirect
zombiezen.com/go/sqlite v1.3.0 // indirect
)

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github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
github.com/google/uuid v1.3.0 h1:t6JiXgmwXMjEs8VusXIJk2BXHsn+wx8BZdTaoZ5fu7I=
github.com/google/uuid v1.3.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
github.com/mattn/go-isatty v0.0.16 h1:bq3VjFmv/sOjHtdEhmkEV4x1AJtvUvOJ2PFAZ5+peKQ=
github.com/mattn/go-isatty v0.0.16/go.mod h1:kYGgaQfpe5nmfYZH+SKPsOc2e4SrIfOl2e/yFXSvRLM=
github.com/ncruces/go-strftime v0.1.9 h1:bY0MQC28UADQmHmaF5dgpLmImcShSi2kHU9XLdhx/f4=
github.com/ncruces/go-strftime v0.1.9/go.mod h1:Fwc5htZGVVkseilnfgOVb9mKy6w1naJmn9CehxcKcls=
github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
golang.org/x/sys v0.16.0 h1:xWw16ngr6ZMtmxDyKyIgsE93KNKz5HKmMa3b8ALHidU=
golang.org/x/sys v0.16.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
modernc.org/libc v1.41.0 h1:g9YAc6BkKlgORsUWj+JwqoB1wU3o4DE3bM3yvA3k+Gk=
modernc.org/libc v1.41.0/go.mod h1:w0eszPsiXoOnoMJgrXjglgLuDy/bt5RR4y3QzUUeodY=
modernc.org/mathutil v1.6.0 h1:fRe9+AmYlaej+64JsEEhoWuAYBkOtQiMEU7n/XgfYi4=
modernc.org/mathutil v1.6.0/go.mod h1:Ui5Q9q1TR2gFm0AQRqQUaBWFLAhQpCwNcuhBOSedWPo=
modernc.org/memory v1.7.2 h1:Klh90S215mmH8c9gO98QxQFsY+W451E8AnzjoE2ee1E=
modernc.org/memory v1.7.2/go.mod h1:NO4NVCQy0N7ln+T9ngWqOQfi7ley4vpwvARR+Hjw95E=
modernc.org/sqlite v1.29.1 h1:19GY2qvWB4VPw0HppFlZCPAbmxFU41r+qjKZQdQ1ryA=
modernc.org/sqlite v1.29.1/go.mod h1:hG41jCYxOAOoO6BRK66AdRlmOcDzXf7qnwlwjUIOqa0=
zombiezen.com/go/sqlite v1.3.0 h1:98g1gnCm+CNz6AuQHu0gqyw7gR2WU3O3PJufDOStpUs=
zombiezen.com/go/sqlite v1.3.0/go.mod h1:yRl27//s/9aXU3RWs8uFQwjkTG9gYNGEls6+6SvrclY=

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package main
import (
"fmt"
"io/fs"
"os"
"strings"
"zombiezen.com/go/sqlite"
"zombiezen.com/go/sqlite/sqlitex"
)
type Result struct{ Expected, Actual fs.FileMode }
type SQLiteEmitter struct {
conn *sqlite.Conn
}
func (s *SQLiteEmitter) Close() error {
return s.conn.Close()
}
func sqlViewColSpec() string {
var cols []string
var whos = []rune{'o', 'g', 'u'}
var whats = []rune{'x', 'w', 'r'}
for _, fieldName := range []string{"expected", "actual"} {
cols = append(cols, fmt.Sprintf("%s", fieldName))
for iwho, who := range whos {
cols = append(cols, fmt.Sprintf("%[1]s & (7 << %[3]d * 3) >> (%[3]d * 3) AS %[1]s_%[2]c", fieldName, who, iwho))
for iwhat, what := range whats {
cols = append(cols, fmt.Sprintf("%[1]s & (1 << %[3]d * 3 + %[5]d) >> (%[3]d * 3 + %[5]d) AS %[1]s_%[2]c%[4]c", fieldName, who, iwho, what, iwhat))
}
}
}
return strings.Join(cols, ",\n")
}
const createTable = `CREATE TABLE result_raw (
expected INT NOT NULL,
actual INT NOT NULL
);`
var createView string = `CREATE VIEW result AS
SELECT
` + sqlViewColSpec() + `
FROM result_raw;`
func NewSQLite(path string) (*SQLiteEmitter, error) {
conn, err := sqlite.OpenConn(path, sqlite.OpenCreate|sqlite.OpenReadWrite|sqlite.OpenWAL)
if err != nil {
return nil, err
}
fmt.Println(createView)
err = sqlitex.Execute(conn, createTable, &sqlitex.ExecOptions{})
if err != nil {
return nil, fmt.Errorf("NewSQLite: cannot create table: %w", err)
}
err = sqlitex.Execute(conn, createView, &sqlitex.ExecOptions{})
if err != nil {
return nil, fmt.Errorf("NewSQLite: cannot create view: %w", err)
}
return &SQLiteEmitter{
conn: conn,
}, nil
}
func (s *SQLiteEmitter) Emit(r Result) error {
return sqlitex.Execute(s.conn, "INSERT INTO result_raw(expected, actual) VALUES (?, ?)", &sqlitex.ExecOptions{
Args: []any{r.Expected, r.Actual},
})
}
func emitCSV(r Result) error {
_, err := fmt.Printf("%03o,%03o\n", r.Expected, r.Actual)
if err != nil {
return fmt.Errorf("emitCSV: %w", err)
}
return nil
}
func getData(emit func(Result) error) (err error) {
f, err := os.CreateTemp("", "go-perm-test-*")
if err != nil {
return err
}
path := f.Name()
f.Close()
for mode := fs.FileMode(0); mode <= os.ModePerm; mode++ {
err := os.Chmod(path, mode)
if err != nil {
return fmt.Errorf("cannot set mode 0%03o on %s: %w", mode, path, err)
}
info, err := os.Stat(path)
if err != nil {
return fmt.Errorf("cannot stat %s: %w", path, err)
}
result := Result{
Expected: mode,
Actual: info.Mode(),
}
err = emit(result)
if err != nil {
return err
}
}
return nil
}
func run() error {
var emit func(Result) error
if len(os.Args) < 2 {
emit = emitCSV
} else {
dbPath := os.Args[1]
db, err := NewSQLite(dbPath)
if err != nil {
return err
}
defer db.Close()
emit = db.Emit
}
return getData(emit)
}
func main() {
err := run()
if err != nil {
fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
os.Exit(1)
}
}

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