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Old 01-25-2023, 12:07 AM   #1
Cranegsh
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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
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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
 
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Old 01-25-2023, 01:43 AM   #3
pan64
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why do you want to do that at all?
 
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Old 01-25-2023, 11:22 PM   #4
Cranegsh
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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
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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
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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
 
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Old 01-26-2023, 10:12 PM   #7
Cranegsh
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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
 
  


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