LinuxQuestions.org
Latest LQ Deal: Latest LQ Deals
Go Back   LinuxQuestions.org > Forums > Non-*NIX Forums > Programming
User Name
Password
Programming This forum is for all programming questions.
The question does not have to be directly related to Linux and any language is fair game.

Notices


Reply
  Search this Thread
Old 01-25-2023, 12:07 AM   #1
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Rep: Reputation: 0
file open permission denied


Hello,

I am using Linux 5.15.84 running on Raspberry Pi 4B and trying to use the function open() to open /dev/mem. But got the error because of permission denied.
crw-r----- 1 root kmem 1, 1 Jan 17 21:44 /dev/mem

As /dev/mem is owned by root, is giving root permission temporarily the only way out?

At the same time, in another program, I am able to open a file in /proc. That file is also owned by root.
-rw-rw-rw- 1 root root 0 Jan 23 22:19 /proc/driver-gpio

What is the difference when use open() to open these two files?

Thanks for your help!

Crane
 
Old 01-25-2023, 12:47 AM   #2
chrism01
LQ Guru
 
Registered: Aug 2004
Location: Sydney
Distribution: Centos 7.7 (?), Centos 8.1
Posts: 18,145

Rep: Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672
If you look at the permission settings, you'll see

First char = file type if any. In your case that's c (char type stream) and '-' = 'regular'

Then we have 3 groups of (potentially rwx) for user, group, other = world.

Filetypes https://www.computernetworkingnotes....-in-linux.html
File Perms https://www.computernetworkingnotes....-language.html
 
1 members found this post helpful.
Old 01-25-2023, 01:43 AM   #3
pan64
LQ Addict
 
Registered: Mar 2012
Location: Hungary
Distribution: debian/ubuntu/suse ...
Posts: 19,594

Rep: Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629
why do you want to do that at all?
 
2 members found this post helpful.
Old 01-25-2023, 11:22 PM   #4
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Original Poster
Rep: Reputation: 0
Quote:
Originally Posted by chrism01 View Post
If you look at the permission settings, you'll see

First char = file type if any. In your case that's c (char type stream) and '-' = 'regular'

Then we have 3 groups of (potentially rwx) for user, group, other = world.

Filetypes https://www.computernetworkingnotes....-in-linux.html
File Perms https://www.computernetworkingnotes....-language.html
Thank you Chris. Clear now!
 
Old 01-25-2023, 11:25 PM   #5
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Original Poster
Rep: Reputation: 0
Quote:
Originally Posted by pan64 View Post
why do you want to do that at all?
Thank you Pan64 for raising this question. I am looking to write a user space driver and find an example which use this way to access to GPIO registers. Just want to test this method to write an user space driver.

It looks it is not a decent way to write a user space driver. Any suggestions?

Thanks!
Crane
 
Old 01-26-2023, 02:22 AM   #6
pan64
LQ Addict
 
Registered: Mar 2012
Location: Hungary
Distribution: debian/ubuntu/suse ...
Posts: 19,594

Rep: Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629
Here is a book about it: https://www.oreilly.com/library/view...1/ch02s07.html
But actually you do not need to buy it, just read what is available:
Quote:
Direct access to memory is possible only by mmapping /dev/mem, and only a privileged user can do that.
Otherwise you can find a lot of links on the net about it:
https://tldp.org/LDP/khg/HyperNews/g...ices/fake.html
https://www.kernel.org/doc/html/v5.0...uio-howto.html
(although they are old, you can start with them)
And here is something for GPIO: https://blog.lxsang.me/post/id/33
 
1 members found this post helpful.
Old 01-26-2023, 10:12 PM   #7
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Original Poster
Rep: Reputation: 0
Quote:
Originally Posted by pan64 View Post
Here is a book about it: https://www.oreilly.com/library/view...1/ch02s07.html
But actually you do not need to buy it, just read what is available:
Otherwise you can find a lot of links on the net about it:
https://tldp.org/LDP/khg/HyperNews/g...ices/fake.html
https://www.kernel.org/doc/html/v5.0...uio-howto.html
(although they are old, you can start with them)
And here is something for GPIO: https://blog.lxsang.me/post/id/33
Great stuff. Thank you so much pan64. Will start from here.

Regards,
Crane
 
Old 01-28-2023, 10:40 PM   #8
sundialsvcs
LQ Guru
 
Registered: Feb 2004
Location: SE Tennessee, USA
Distribution: Gentoo, LFS
Posts: 10,102
Blog Entries: 4

Rep: Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662
Something else to realize is that /dev, /proc, /sys are non-existent "filesystems."

Each of these are, in fact, "Linux kernel APIs." There is absolutely nothing "physically real" about them.

And so, even though the metaphor of "directories, device-nodes and files," with associated "permission masks" and "content," has generally been retained, there might be exceptions to the rule. It is entirely up to the kernel itself. Fundamentally, they are all APIs.

Last edited by sundialsvcs; 01-28-2023 at 10:42 PM.
 
1 members found this post helpful.
Old 02-01-2023, 10:43 PM   #9
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Original Poster
Rep: Reputation: 0
Quote:
Originally Posted by sundialsvcs View Post
Something else to realize is that /dev, /proc, /sys are non-existent "filesystems."

Each of these are, in fact, "Linux kernel APIs." There is absolutely nothing "physically real" about them.

And so, even though the metaphor of "directories, device-nodes and files," with associated "permission masks" and "content," has generally been retained, there might be exceptions to the rule. It is entirely up to the kernel itself. Fundamentally, they are all APIs.
Thank you sundialsvcs!

One thing to clarify. When you say "up to the kernel itself", what does this mean? Does it mean its accessibility is up to kernel, not anything like normal file?
 
Old 02-01-2023, 11:31 PM   #10
chrism01
LQ Guru
 
Registered: Aug 2004
Location: Sydney
Distribution: Centos 7.7 (?), Centos 8.1
Posts: 18,145

Rep: Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672Reputation: 2672
It means those data structures are created/maintained by the kernel & could change any time.
 
Old 02-02-2023, 01:13 AM   #11
pan64
LQ Addict
 
Registered: Mar 2012
Location: Hungary
Distribution: debian/ubuntu/suse ...
Posts: 19,594

Rep: Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629Reputation: 6629
Quote:
Originally Posted by Cranegsh View Post
Thank you sundialsvcs!

One thing to clarify. When you say "up to the kernel itself", what does this mean? Does it mean its accessibility is up to kernel, not anything like normal file?
Those are sometimes called virtual filesystems, they look like a filesystem. When you try to access them (like /dev/mem or /proc/<whatever> the kernel will recognize it, do something and actually will simulate a filesystem-like behavior.
(for example cat /proc/<PID>/cmdline will display the command - how the process <PID> was started, but that file actually does not exist at all).
 
Old 02-02-2023, 01:43 PM   #12
sundialsvcs
LQ Guru
 
Registered: Feb 2004
Location: SE Tennessee, USA
Distribution: Gentoo, LFS
Posts: 10,102
Blog Entries: 4

Rep: Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662Reputation: 3662
Quote:
One thing to clarify. When you say "up to the kernel itself", what does this mean? Does it mean its accessibility is up to kernel, not anything like normal file?
What I literally mean is that: "this is a kernel API" (API = Application Program Interface) which merely presents itself as "files and directories." (Part of this implementation is to provide "ownership and permission-masks.") But, at the end of the day, everything that you "think you see," and everything that you can "do," is determined and implemented solely by the kernel itself.

Yes: "it is not anything like 'a normal file,' yet it perfectly appears to be."

You "ask to open, read or write a 'file' in some 'directory.'" The kernel interprets your request, decides if you can do it (and if so, just what you should see ...), and if so presents the results to you "as a 'file.'" But it is "100% an illusion." There is no physical file, and there is no directory.

"/dev, /proc, /sys ..." they don't exist. (Well, some entries in "/dev" do correspond to physical devices, but some don't.)

Personally, I think that this is a brilliant approach because, "in Unix/Linux, 'everything is a file.'" Kernel API's are often very difficult to deal with, but this one is ... elegant.

Last edited by sundialsvcs; 02-02-2023 at 01:54 PM.
 
1 members found this post helpful.
Old 02-02-2023, 02:03 PM   #13
astrogeek
Moderator
 
Registered: Oct 2008
Distribution: Slackware [64]-X.{0|1|2|37|-current} ::12<=X<=15, FreeBSD_12{.0|.1}
Posts: 6,005
Blog Entries: 23

Rep: Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967Reputation: 3967
Quote:
Originally Posted by sundialsvcs View Post
Personally, I think that this is a brilliant approach because, "in Unix/Linux, 'everything is a file.'" Kernel API's are often very difficult to deal with, but this one is ... elegant.
It is indeed brilliant and elegant! For comparison look at [put anything else here].
 
Old 02-02-2023, 05:39 PM   #14
dugan
LQ Guru
 
Registered: Nov 2003
Location: Canada
Distribution: distro hopper
Posts: 10,773

Rep: Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097Reputation: 5097
I have a better link for that device drivers book

https://lwn.net/Kernel/LDD3/
 
Old 02-02-2023, 10:07 PM   #15
Cranegsh
LQ Newbie
 
Registered: Jan 2023
Posts: 14

Original Poster
Rep: Reputation: 0
Thanks Chris, pan64, sundialsvcs, astrogeek, dugan for your reply and sharing the link.

I found this article that explains the details of /dev and /proc.
https://www.linux.com/news/using-dev...-file-systems/
It looks they are the ways to interact with the Linux kernel, not serving the purpose of access to the devices.
 
  


Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off



Similar Threads
Thread Thread Starter Forum Replies Last Post
16.04 - Permission denied though I'm sudo and have permission to file insomniacno1 Ubuntu 5 10-22-2020 04:30 AM
[SOLVED] Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied) Ztcoracat Debian 5 02-23-2019 11:02 AM
E: Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied) Grimm666 Linux - Newbie 3 07-16-2015 03:16 AM
open lock file /var/lib/postfix/master.lock: cannot open file: Permission denied gabsik Linux - Server 6 08-30-2012 10:39 PM
Could not open lock file /var/lib/dpkg/lock - open (13 Permission denied) basina Linux - Laptop and Netbook 3 09-05-2011 06:18 PM

LinuxQuestions.org > Forums > Non-*NIX Forums > Programming

All times are GMT -5. The time now is 11:57 PM.

Main Menu
Advertisement
My LQ
Write for LQ
LinuxQuestions.org is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
Syndicate
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Open Source Consulting | Domain Registration