[RESOLVED] /dev/mem and even /dev/port can't open even as root!

[TheRandomGuyNamedJoe12]

I made a program that tries to do mmaping on /dev/mem and it fails with:

Failed to open /dev/mem: Operation not permitted

And I tried using od as a test for my code being bad too:

od: /dev/mem: Operation not permitted

od: /dev/port: Operation not permitted

How do I make these files open-able?

[Schwarzer Kater]

Welcome!

You could read those with elevated privileges as you can see with ls -lh /dev/ .

What did you try to do and how did you try it - exactly?

[TheRandomGuyNamedJoe12]

Write to memory-mapped vga controller using a C program I made

[jlittle]

FWIW,

sudo od /dev/mem | more
sudo od /dev/port | more

have no trouble for me.

[TheRandomGuyNamedJoe12]

And neither does my VM running the same version of Kubuntu as the host, Well technically the /dev/mem fails with the same error but it's after it has read at least some data.

[TheRandomGuyNamedJoe12]

There's no point of /dev/mem or /dev/port if you can't open them

[Schwarzer Kater]

To cite from https://bakhi.github.io/devmem/ :

" To use /dev/mem, your kernel must be configured with “CONFIG_STRICT_DEVMEM=n”, or it prevent access from even privileged user.

“CONFIG_STRICT_DEVMEM=y”, the default kernel configuration in general, disallows to access RAM area via /dev/mem or only allows first 1MB size of RAM

FYI: “CONFIG_IO_STRICT_DEVMEM=y”, disallows to access register via dev/mem "


This seems to be a safety precaution and as far as I have read on other internet sites CONFIG_STRICT_DEVMEM has to be configured before kernel compilation…

[TheRandomGuyNamedJoe12]

I thought it limited reads and writes to 1 MiB, not disabling them and my VM shows that is true and that dosen't even apply to mmaped memory and CONFIG_STRICT_DEVMEM as a fact dosen't affect /dev/port.

[TheRandomGuyNamedJoe12]

This seems to be specific to my machine, my VM has no problems with this.

[jlittle]

Do you (Joe) have any PPAs that may have installed a kernel? What does uname -rv say?

$ uname -rv
5.15.0-71-generic #78-Ubuntu SMP Tue Apr 18 09:00:29 UTC 2023​

[TheRandomGuyNamedJoe12]

Here is the output:

5.15.0-71-generic #78-Ubuntu SMP Tue Apr 18 09:00:29 UTC 2023

And no PPAs that could have installed a new kernel that I know of.

[Schwarzer Kater]

[…] and CONFIG_STRICT_DEVMEM as a fact dosen't affect /dev/port.

I did not claim that.


Kubuntu 22.04:

uname -rv
5.19.0-41-generic #42~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Apr 18 17:40:00 UTC 2​

My (partial) output of LC_ALL=C sudo od /dev/port in Kubuntu 22.04:

…
0170120 000020 134317 010540 000403 010421 000021 000000 000000
…​

and no error at all.


My output of LC_ALL=C sudo od /dev/mem | echo $((($(wc -l)-2)/8)) in Kubuntu 22.04:

od: /dev/mem: Operation not permitted
1024​

= 1 MiB limitation.


Similar outputs of od in Debian and openSUSE.
And in your Kubuntus you get the error message only from reading /dev/port and /dev/mem and no other output at all?

[TheRandomGuyNamedJoe12]

My point is that opening is supposed to be allowed.

[TheRandomGuyNamedJoe12]

My output of

LC_ALL=C sudo od /dev/mem | echo $((($(wc -l)-2)/8))

od: /dev/mem: Operation not permitted
0

[TheRandomGuyNamedJoe12]

It is a security update, marking as wontfix.