github.com/rigado/snapd@v2.42.5-go-mod+incompatible/cmd/cmdutil/cmdutil.go (about)

     1  // -*- Mode: Go; indent-tabs-mode: t -*-
     2  
     3  /*
     4   * Copyright (C) 2019 Canonical Ltd
     5   *
     6   * This program is free software: you can redistribute it and/or modify
     7   * it under the terms of the GNU General Public License version 3 as
     8   * published by the Free Software Foundation.
     9   *
    10   * This program is distributed in the hope that it will be useful,
    11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
    12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    13   * GNU General Public License for more details.
    14   *
    15   * You should have received a copy of the GNU General Public License
    16   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
    17   *
    18   */
    19  package cmdutil
    20  
    21  import (
    22  	"bufio"
    23  	"bytes"
    24  	"debug/elf"
    25  	"fmt"
    26  	"io/ioutil"
    27  	"os"
    28  	"os/exec"
    29  	"path/filepath"
    30  	"strings"
    31  
    32  	"github.com/snapcore/snapd/dirs"
    33  	"github.com/snapcore/snapd/osutil"
    34  )
    35  
    36  func elfInterp(cmd string) (string, error) {
    37  	el, err := elf.Open(cmd)
    38  	if err != nil {
    39  		return "", err
    40  	}
    41  	defer el.Close()
    42  
    43  	for _, prog := range el.Progs {
    44  		if prog.Type == elf.PT_INTERP {
    45  			r := prog.Open()
    46  			interp, err := ioutil.ReadAll(r)
    47  			if err != nil {
    48  				return "", nil
    49  			}
    50  
    51  			return string(bytes.Trim(interp, "\x00")), nil
    52  		}
    53  	}
    54  
    55  	return "", fmt.Errorf("cannot find PT_INTERP header")
    56  }
    57  
    58  func parseLdSoConf(root string, confPath string) []string {
    59  	f, err := os.Open(filepath.Join(root, confPath))
    60  	if err != nil {
    61  		return nil
    62  	}
    63  	defer f.Close()
    64  
    65  	var out []string
    66  	scanner := bufio.NewScanner(f)
    67  	for scanner.Scan() {
    68  		line := scanner.Text()
    69  		switch {
    70  		case strings.HasPrefix(line, "#"):
    71  			// nothing
    72  		case strings.TrimSpace(line) == "":
    73  			// nothing
    74  		case strings.HasPrefix(line, "include "):
    75  			l := strings.SplitN(line, "include ", 2)
    76  			files, err := filepath.Glob(filepath.Join(root, l[1]))
    77  			if err != nil {
    78  				return nil
    79  			}
    80  			for _, f := range files {
    81  				out = append(out, parseLdSoConf(root, f[len(root):])...)
    82  			}
    83  		default:
    84  			out = append(out, filepath.Join(root, line))
    85  		}
    86  
    87  	}
    88  	if err := scanner.Err(); err != nil {
    89  		return nil
    90  	}
    91  
    92  	return out
    93  }
    94  
    95  // CommandFromSystemSnap runs a command from the snapd/core snap
    96  // using the proper interpreter and library paths.
    97  //
    98  // At the moment it can only run ELF files, expects a standard ld.so
    99  // interpreter, and can't handle RPATH.
   100  func CommandFromSystemSnap(name string, cmdArgs ...string) (*exec.Cmd, error) {
   101  	from := "snapd"
   102  	root := filepath.Join(dirs.SnapMountDir, "/snapd/current")
   103  	if !osutil.FileExists(root) {
   104  		from = "core"
   105  		root = filepath.Join(dirs.SnapMountDir, "/core/current")
   106  	}
   107  
   108  	cmdPath := filepath.Join(root, name)
   109  	interp, err := elfInterp(cmdPath)
   110  	if err != nil {
   111  		return nil, err
   112  	}
   113  	coreLdSo := filepath.Join(root, interp)
   114  	// we cannot use EvalSymlink here because we need to resolve
   115  	// relative and an absolute symlinks differently. A absolute
   116  	// symlink is relative to root of the snapd/core snap.
   117  	seen := map[string]bool{}
   118  	for osutil.IsSymlink(coreLdSo) {
   119  		link, err := os.Readlink(coreLdSo)
   120  		if err != nil {
   121  			return nil, err
   122  		}
   123  		if filepath.IsAbs(link) {
   124  			coreLdSo = filepath.Join(root, link)
   125  		} else {
   126  			coreLdSo = filepath.Join(filepath.Dir(coreLdSo), link)
   127  		}
   128  		if seen[coreLdSo] {
   129  			return nil, fmt.Errorf("cannot run command from %s: symlink cycle found", from)
   130  		}
   131  		seen[coreLdSo] = true
   132  	}
   133  
   134  	ldLibraryPathForCore := parseLdSoConf(root, "/etc/ld.so.conf")
   135  
   136  	ldSoArgs := []string{"--library-path", strings.Join(ldLibraryPathForCore, ":"), cmdPath}
   137  	allArgs := append(ldSoArgs, cmdArgs...)
   138  	return exec.Command(coreLdSo, allArgs...), nil
   139  }