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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2019-2020 Canonical Ltd
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 3 as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package snapdtool
import (
"bufio"
"bytes"
"debug/elf"
"fmt"
"io"
"os"
"os/exec"
"path/filepath"
"strings"
"github.com/snapcore/snapd/dirs"
"github.com/snapcore/snapd/osutil"
)
var elfInterp = func(cmd string) (string, error) {
el, err := elf.Open(cmd)
if err != nil {
return "", err
}
defer el.Close()
for _, prog := range el.Progs {
if prog.Type == elf.PT_INTERP {
r := prog.Open()
interp, err := io.ReadAll(r)
if err != nil {
return "", nil
}
return string(bytes.Trim(interp, "\x00")), nil
}
}
return "", fmt.Errorf("cannot find PT_INTERP header")
}
func parseLdSoConf(root string, confPath string) []string {
f, err := os.Open(filepath.Join(root, confPath))
if err != nil {
return nil
}
defer f.Close()
var out []string
scanner := bufio.NewScanner(f)
for scanner.Scan() {
line := scanner.Text()
switch {
case strings.HasPrefix(line, "#"):
// nothing
case strings.TrimSpace(line) == "":
// nothing
case strings.HasPrefix(line, "include "):
l := strings.SplitN(line, "include ", 2)
files, err := filepath.Glob(filepath.Join(root, l[1]))
if err != nil {
return nil
}
for _, f := range files {
out = append(out, parseLdSoConf(root, f[len(root):])...)
}
default:
out = append(out, filepath.Join(root, line))
}
}
if err := scanner.Err(); err != nil {
return nil
}
return out
}
// CommandFromSystemSnap runs a command from the snapd/core snap
// using the proper interpreter and library paths if needed.
//
// Files from core need this hack. Files from snapd are executed normally unless
// the snapd snap is not mounted under /snap.
//
// At the moment it can only run ELF files, expects a standard ld.so
// interpreter, and can't handle RPATH.
func CommandFromSystemSnap(name string, cmdArgs ...string) (*exec.Cmd, error) {
from := "snapd"
root := filepath.Join(dirs.SnapMountDir, "/snapd/current")
if !osutil.FileExists(root) {
from = "core"
root = filepath.Join(dirs.SnapMountDir, "/core/current")
}
cmdPath := filepath.Join(root, name)
if from == "snapd" {
// the elf interpreter invoked by the binary will work if snaps are mounted at /snap
// or /snap/snapd/current resolves to <mount dir>/snapd/current so that the interpreter
// locations are correct, otherwise we need to set up a command to invoke it directly
snapdCurrentDir := filepath.Join(dirs.GlobalRootDir, "snap/snapd/current")
if match, err := osutil.ComparePathsByDeviceInode(root, snapdCurrentDir); err == nil && match {
return exec.Command(cmdPath, cmdArgs...), nil
}
interp, err := elfInterp(cmdPath)
if err != nil {
return nil, err
}
slashSnapPrefix := filepath.Join(dirs.GlobalRootDir, "snap") + "/"
interp = filepath.Join(dirs.SnapMountDir, strings.TrimPrefix(interp, slashSnapPrefix))
// all libraries are at the same path as the interpreter
ldLibraryPathForSnapd := filepath.Dir(interp)
ldSoArgs := []string{"--library-path", ldLibraryPathForSnapd, cmdPath}
allArgs := append(ldSoArgs, cmdArgs...)
return exec.Command(interp, allArgs...), nil
}
// We are trying to execute files from core snap. They need
// run with a their interpreter and library paths
interp, err := elfInterp(cmdPath)
if err != nil {
return nil, err
}
coreLdSo := filepath.Join(root, interp)
// we cannot use EvalSymlink here because we need to resolve
// relative and an absolute symlinks differently. A absolute
// symlink is relative to root of the snapd/core snap.
seen := map[string]bool{}
for osutil.IsSymlink(coreLdSo) {
link, err := os.Readlink(coreLdSo)
if err != nil {
return nil, err
}
if filepath.IsAbs(link) {
coreLdSo = filepath.Join(root, link)
} else {
coreLdSo = filepath.Join(filepath.Dir(coreLdSo), link)
}
if seen[coreLdSo] {
return nil, fmt.Errorf("cannot run command from %s: symlink cycle found", from)
}
seen[coreLdSo] = true
}
ldLibraryPathForCore := parseLdSoConf(root, "/etc/ld.so.conf")
ldSoArgs := []string{"--library-path", strings.Join(ldLibraryPathForCore, ":"), cmdPath}
allArgs := append(ldSoArgs, cmdArgs...)
return exec.Command(coreLdSo, allArgs...), nil
}
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