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== See Also ==
{{Status
|status=Draft
|owner=Knowledge Agent
|last_update=2026-07-16
|review=Pending
}}
 
{{TOC}}
 
== Overview ==
 
Linux file permission에 대한 기술 문서입니다.
 
=== Summary ===
 
* 무엇인가? - Linux file permission
* 왜 필요한가? - HPC 및 서버 환경에서 필수 개념
* 언제 사용하는가? - 서버 구성, 성능 튜닝, 문제 해결 시
 
 
== Purpose ==
 
이 문서가 존재하는 이유
 
* Goal: Linux file permission에 대한 기술 정보 제공
* Scope: Linux file permission의 개념, 사용법, 설정
* Non-goals: 다른 주제로의 확장
 
 
== Key Concepts ==
 
{| class="wikitable"
! Concept
! Description
! Related
|-
| Linux file permission
| HPC/서버 환경에서 중요한 기술 개념
| [[Linux]], [[Server]]
|}
 
 
== Detailed Explanation ==
 
* [[Linux]]
* [[Linux]]
== Linux Regular(default) Permission ==
ls command witl -al option to view the permissions (or '''file mode''') set for the contents of a directory<ref>https://wiki.archlinux.org/title/File_permissions_and_attributes</ref><syntaxhighlight lang="bash">
ls command witl -al option to view the permissions (or '''file mode''') set for the contents of a directory<ref>https://wiki.archlinux.org/title/File_permissions_and_attributes</ref><syntaxhighlight lang="bash">
$ ls -al /
$ ls -al /
Line 22: Line 63:
drwxr-xr-x  10 root    root            4096 Apr  9 12:37 opt
drwxr-xr-x  10 root    root            4096 Apr  9 12:37 opt
dr-xr-xr-x 1264 root    root                0 May 25 08:36 proc
dr-xr-xr-x 1264 root    root                0 May 25 08:36 proc
drwxr-xr-x  35 root    root            1160 May 25 09:26 run
lrwxrwxrwx    1 root    root                8 Aug 31  2022 sbin -> usr/sbin
drwxr-xr-x    2 root    root                0 May 25 08:37 server
drwxr-xr-x    2 root    root            4096 May 18 16:03 srv
-rw-------    1 root    root      2147483648 Aug 31  2022 swapfile
dr-xr-xr-x  13 root    root                0 May 25 08:36 sys
drwxrwxrwt  10 root    root          1441792 May 25 09:40 tmp
drwxr-xr-x  15 root    root            4096 May 18 15:36 usr
drwxr-xr-x  13 root    root            4096 Dec  5 17:31 var
</syntaxhighlight>




[[File:Linux regular permission.png|center|frameless|436x436px|linux regular permission]]
== Best Practices ==
{| class="wikitable"
|+
!Type
!Description
|-
| rowspan="2" |SUID
|When the SUID bit is set on an executable file,
this means that the file will be executed with the same permissions as the owner of the executable file.


The passwd command is the best example for SUID. if we look at the file in detail
* 최신 버전 사용 권장
|-
* 공식 문서 참고
|$ ls -l /usr/bin/passwd
* 테스트 환경에서 먼저 검증
-rwsr-xr-x 1 root root 68208 May 27  2020 /usr/bin/passwd




SUID can be set by
== References ==


<nowiki>#</nowiki>chmod u+s file_name or #chmod 4755 file_name
* [https://wiki.hpcmate.com Linux file permission]




and can be removed by
== Related Pages ==


<nowiki>#</nowiki>chmod u-s file_name or #chmod 0755 file_name
* [[Linux]]
 
* [[Server]]
 
* [[Hardware]]
SUID set files can be found by
* [[Network]]
 
<nowiki>#</nowiki>find / '''-perm''' /4000
 
 
Tip, Capital S as SUID flag means there is an error that you should look into
|-
| rowspan="2" |SGID
|SGID is similar to SUID, With the SGID bit set, any user executing the file will have same permissions as the group owner of the file.
 
*If set on a file, it allows the file to be executed as the '''group''' that owns the file (similar to SUID)
* If set on a directory, any files created in the directory will have their '''group''' ownership set to that of the directory owner
 
 
It’s benefit is in handling the directory. When SGID permission is applied to a directory, all sub directories and files created inside this directory will get the same group ownership as main directory.
 
 
A practical example of SGID is with '''[[Samba]] server''' for sharing files on your local network. It’s guaranteed that all new files will not lose the permissions desired, no matter who created it.
|-
|$ ls -ld /var/local
drwxrwsr-x 2 root staff 4096 Apr 15  2020 /var/local
 
 
SGID bit can be set by
 
<nowiki>#</nowiki>chmod g+s directory_name or #chmod 2755 directory_name
 
 
SGID bit can be removed by
 
<nowiki>#</nowiki>chmod g-s directory_name or #chmod 0755 directory_name
 
 
Or find by
 
find . -perm /2000
|-
| rowspan="2" |Sticky Bit
|The sticky bit works on the directory (Under [[linux]], this permission does not affect individual files).  With sticky bit set on a directory, all the files in the directory can only be deleted or renamed by the file owners only or the root.
 
 
This is typically used in the '''/tmp directory''' that works as the trash can of temporary files.
|-
|$ ls -ld /tmp
drwxrwxrwt 11 root root 4096 Feb 27 18:40 /tmp
 
sticky bit can be set by
 
<nowiki>#</nowiki>chmod +t directory_name  or #chmod 1755 directory_name
 
Sticky bit can be removed by


<nowiki>#</nowiki> chmod -t directory_name or #chmod 0755 directory_name


And find by
[[Category:Linux]]
== Knowledge Graph ==


<nowiki>#</nowiki>find . -perm /1000
Related
|}


→ [[Linux Kernel Tuning]]
→ [[systemd]]
→ [[LVM]]
→ [[iptables]]
→ [[Ubuntu]]
→ [[fstab]]
→ [[Linux Kernel]]


In numerical method,  need to pass a fourth, preceding pre-defined special digit in our <code>chmod</code> command
[[Category:Reference]]
* Default 0, SUID = 4, SGID = 2, Sticky = 1
 
The syntax is:
<code>$ chmod X### file | directory</code>
 
== Linux Extended ACL ==
Extended ACLs allow us to apply a more specific set of permissions to a file or directory without (necessarily) changing the base ownership and permissions. Default permissions can be set only for one owner, one group and Other users. So different permissions cannot be configured for different users and groups.
 
 
On Linux, Extended ACL shows "+" information when do $ls -al command. we can manage the ACL with "'''getfacl'''" and "'''setfacl'''" on the command line.
[[File:Linux extended acl.png|center|frameless|400x400px]]
 
=== Listing access control lists ===
'''The getfacl command is used on Linux to print a complete listing of all regular permissions and access control lists permissions on a file or directory.'''
 
'''$ getfacl <file, directory>'''<syntaxhighlight lang="bash">
# getfacl /accounting
getfacl: Removing leading '/' from absolute path names
# file: accounting
# owner: accounting
# group: accounting
user::rwx
group::rwx
other::---
#The first three output lines display the name, owner, and owning group of the directory. The next three lines contain the three ACL entries
</syntaxhighlight>
 
=== Setting access control lists using setfacl ===
The setfacl has the following syntax,
 
 
$ setfacl [action] <what>:<user/group>:<permissions>
 
'''$ setfacl {-m, -x, -d, -b}  {u, g}:<name>:[r, w, x] <file, directory>'''
 
* '''-m''' : (set or modify) means that you want to '''modify''' one or several ACL entries on the file or directory.
* '''-x''' : (delete) means that you want to '''remove''' one or several ACL entries on a file or directory.
* -d : (default) means specific type of permission assigned to a directory, that doesn’t change the permissions of the directory itself
* -b :  with -b option, you can erase all the ACLs set<ref>https://linuxconfig.org/how-to-manage-acls-on-linux</ref>
* '''{u, g}''' : if you want to modify the ACL for a user or for a group.
* '''name''' : this is an optional parameter, it can be omitted if you want to set the ACL entries for every user or for every group on your host.
* '''[r, w, x]''' : in order to set read, write or execute permissions on the file or directory.
 
<syntaxhighlight lang="bash">
Example,
 
To set specific write permissions for a user on a file
$ setfacl -m u:user:w <file, directory>  
 
To set execute permissions for all users on your host
$ setfacl -m u::x <file, directory>    
 
To set full permissions for a specific group on your host
$ setfacl -m g:group:rwx <file, directory>
 
To remove a user specific entry from a file,
$ setfacl -x u:<user> <file, directory>
 
To remove ACL related to groups on your host
$ setfacl -x g:<group> <file, directory>
 
To allow rwx to accouting group for accounting folder
$ setfacl -d -m accounting:rwx /accounting
 
To modify user2 be rwx, group admin rwx for myfolder
$ setfacl -m "u:user2:rwx,g:admin:rwx" myfolder
 
</syntaxhighlight>
 
=== Creating defaults access control lists  on directories ===
'''defaults access control lists  are used to create ACL entries on a directory that will be inherited by objects in this directory like files or subdirectories. it is recommended to specify default permissions for all three categories (user, group and other)'''
 
 
To create default ACL entries syntax,
 
$ setfacl -d -m {u, g}:<name>:[r, w, x] <directory>
 
<syntaxhighlight lang="bash">
For example,
 
to assign read permissions to all files created in a directory,
$ setfacl -d -m u::r directory
 
To delete default existing access control lists on directories, use the -k flag
$ setfacl -k <directory>
 
To remove default ACL entries in a directory and all subdirectories, you would have to use a recursive option (-R)
$ setfacl -kR <directory>
</syntaxhighlight>
 
=== Working with the access control lists mask ===
The ACL mask is different from the file creation mask (umask) and it is used in order to restrict existing ACL entries existing on a file or directory.
 
The ACL mask is '''updated every time you run a setfacl command unless you specify that you don’t want to update the mask with the -n flag.'''<syntaxhighlight lang="bash">
To prevent the mask from being updated,
$ setfacl -n -m u:antoine:rwx <file, directory>
</syntaxhighlight>
 
Following example show how mask is mapping to user and group<ref>https://www.learnitguide.net/2015/12/setting-up-access-control-lists-acl-on.html</ref><syntaxhighlight lang="bash">
# file: myfolder
# owner: user1
# group: [[test]]
user::rwx
user:user2:rwx          # effective: r-x due to mask
group::r-x
group:admin:rwx        # effective: r-x due to mask
mask::r-x              # mask effective to user and group
other::---
</syntaxhighlight>
 
== Umask ==
The umask utility is used to control the file-creation mode mask, which determines the initial value of file permission bits for newly created files since all new file permission calculated by
 
new file permission = default permission & (~mask)
 
* default permission on Linux  - the default creation permissions are <code>777</code> for directories and only <code>666</code> for files.
* only the file permission bits of the mask are used - see umask(2). The ''suid'', ''sgid'' and ''sticky'' bits of the mask are ignored.
 
assumming that the file-creation mode mask is <code>027</code>
 
* <code>0</code> stands for the ''user'' permission bits not set on a newly created file
* <code>2</code> stands for the ''group'' permission bits not set on a newly created file
* <code>7</code> stands for the ''other'' permission bits not set on a newly created file
 
To display the current mask, simply invoke <code>umask</code> without specifying any arguments<syntaxhighlight lang="bash">
$ umask
0027
 
#When the -S option, standardized by POSIX
$ umask -S
u=rwx,g=rx,o=
 
</syntaxhighlight>
 
 
 
To set the umask value through the ''umask'' command,
 
System-wide umask value can be set in <code>/etc/profile or in the default shell configuration files (e.g. /etc/bash.bashrc) in most Linux distributions.</code>
 
User specific umask value can be set in <code>~/.bashrc</code> to only change your umask, however these changes will only take effect after the next login
 
under GUI environment like KDE, The umask value can be set via <code>pam_umask.so</code> or a systemd<syntaxhighlight lang="bash">
/etc/systemd/system/user@.service.d/override.conf
 
[Service]
UMask=0002
 
</syntaxhighlight><code>pam_umask.so</code> allows to set the system-wide umask value for both, text console and graphical environment.
 
<code>pam_umask.so</code> needs to be enabled in a configuration file that is included by both, <code>/etc/pam.d/login</code> and <code>/etc/pam.d/systemd-user</code>. Add the following line to <code>/etc/pam.d/system-login</code>:
# session    optional  pam_umask.so        umask=022
 
== File Attributes ==
Apart from the file mode bits that control user and group read, write and execute permissions, several file systems file attributes that enable further customization of file operations.
 
Extended attributes can be changed with chattr and listing them with lsattr which is included in e2fsprogs package. [https://ngelinux.com/what-is-chattr-and-lsattr-command-in-linux-and-how-to-use-them/ this page] explained details how to use lsattr and chattr
 
<lsattr | chattr> [+|-] [attributes] <filename>
 
These are a few useful attributes. Not all filesystems [[support]] every attribute.
 
* <code>a</code> - append only: File can only be opened for appending. like i, but can only be appended.
* <code>c</code> - compressed: Enable filesystem-level compression for the file.
* <code>i</code> - immutable: Cannot be modified, deleted, renamed, linked to. Can only be set by root. with this bit set, the file becomes the unmovable object. No writes, not from the kernel or daemons, and not from the root user.
* <code>j</code> - data journaling: Use the journal for file data writes as well as metadata.
* <code>m</code> - no compression: Disable filesystem-level compression for the file.
* <code>A</code> - no atime update: The file's atime will not be modified.
* <code>C</code> - no copy on write: Disable copy-on-write, for filesystems that support it.
 
<nowiki>**</nowiki>If root cannot delete or modify some file, that file should have a and i file attributes. some virus or malware use this attributes to execute process under Linux. 
 
== Extended Attributes (xattrs) ==
xattr - Extended attributes are name:value pairs associated permanently with files and directories defines four extended attribute classes: security, system, trusted and user.
 
archlinux provides great information about [https://man.archlinux.org/man/xattr.7 xattr] and [https://wiki.archlinux.org/title/Capabilities capability]
 
<nowiki>**</nowiki>'''unlike ACLs, xattrs do not show up anywhere but with lsattr'''<ref>https://www.linkedin.com/pulse/linux-permissions-anything-simple-jack-barber</ref>
 
== POSIX Capabilities ==
POSIX 1003.1e capabilities are role/action based capabilities that that allow certain root-only tasks to be performed by regular users. They can be set in the XATTR space for a file in the filesystem to allow a program to perform such capabilities if run non-root. OR, users on a per user basis can be given capability access by editing/adding /etc/security/capabilities.conf.
 
The major use for capabilities is to let network tools, especially packet capture tools run as non-root to prevent exploits run against them from gaining root, or otherwise exploited as a SUID binary. Yes, you can use /dev/udp and /dev/tcp to open connections, but not set permissions
 
If setting per file/program you can check with
 
getcap <filename>
 
and you can set capabilities with
 
setcap <capabilities> <filename>
 
== References ==
<references />
 
[[Category: Terminology]]
[[Category: Linux]]

Latest revision as of 11:29, 17 July 2026

Template:Status

Template:TOC

Overview

Linux file permission에 대한 기술 문서입니다.

Summary

  • 무엇인가? - Linux file permission
  • 왜 필요한가? - HPC 및 서버 환경에서 필수 개념
  • 언제 사용하는가? - 서버 구성, 성능 튜닝, 문제 해결 시


Purpose

이 문서가 존재하는 이유

  • Goal: Linux file permission에 대한 기술 정보 제공
  • Scope: Linux file permission의 개념, 사용법, 설정
  • Non-goals: 다른 주제로의 확장


Key Concepts

Concept Description Related
Linux file permission HPC/서버 환경에서 중요한 기술 개념 Linux, Server


Detailed Explanation

ls command witl -al option to view the permissions (or file mode) set for the contents of a directory[1]<syntaxhighlight lang="bash"> $ ls -al / total 2098648 drwxr-xr-x 20 root root 4096 May 25 08:40 . drwxr-xr-x 20 root root 4096 May 25 08:40 .. lrwxrwxrwx 1 root root 7 Aug 31 2022 bin -> usr/bin drwxr-xr-x 4 root root 4096 May 25 09:40 boot drwxr-xr-x 20 root root 4820 May 25 08:37 dev drwxr-xr-x 130 root root 12288 May 25 09:42 etc drwxr-xr-x 6 root root 4096 May 15 10:36 home lrwxrwxrwx 1 root root 7 Aug 31 2022 lib -> usr/lib lrwxrwxrwx 1 root root 9 Aug 31 2022 lib32 -> usr/lib32 lrwxrwxrwx 1 root root 9 Aug 31 2022 lib64 -> usr/lib64 lrwxrwxrwx 1 root root 10 Aug 31 2022 libx32 -> usr/libx32 drwx------ 2 root root 16384 Aug 31 2022 lost+found drwxr-xr-x 3 root root 4096 Aug 31 2022 media drwxr-xr-x 5 root root 4096 Dec 13 10:03 mnt drwxr-xr-x 10 root root 4096 Apr 9 12:37 opt dr-xr-xr-x 1264 root root 0 May 25 08:36 proc


Best Practices

  • 최신 버전 사용 권장
  • 공식 문서 참고
  • 테스트 환경에서 먼저 검증


References


Related Pages

Knowledge Graph

Related

→ Linux Kernel Tuning → systemd → LVM → iptables → Ubuntu → fstab → Linux Kernel

  1. ↑ https://wiki.archlinux.org/title/File_permissions_and_attributes
Retrieved from "http://wiki.hpcmate.com/index.php?title=Linux_file_permission&oldid=3367"