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I wrote a script that generates a PDF and an HTML file on RAM-disk:

  • The PDF file can be opened as expected.
  • The HTML file can be opened with an editor, but not with a browser.

This is the error message I get from the browser (Chrome, Firefox) on Ubuntu Linux 24.04:

Access to the file was denied
The file at file:///dev/shm/test.html is not readable. It may have been removed, moved or file permissions may be preventing access.
ERR_ACCESS_DENIED

These are the file details:

/dev/shm> ll
-rw-rw-r-- 1  11K 2024-08-30 11:06 test.html

If I copy the file to the hard disk, it can be opened as expected:

/dev/shm> cp ./test.html ~

If I build Chromium from source as explained here, will I be able to open files stored on RAM-disk like /dev/shm?

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9
  • Can the browser open any html file on /dev/shm? And what OS are you using? Commented Aug 30, 2024 at 10:55
  • 2
    Have you modified the output of ls -l? It usually includes the owner and group. Commented Aug 30, 2024 at 12:04
  • @Kusalananda - Yes, I have. Most of the times I do not need the user and group. Commented Aug 30, 2024 at 13:44
  • 3
    Is the browser a snap? Commented Aug 30, 2024 at 14:15
  • 4
    Then it's probably a snap. Commented Aug 30, 2024 at 14:25

1 Answer 1

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17

It's not that /dev/shm is of filesystem type tmpfs, but directly related to the snap installation. Evidence:

echo '<h1>Header</h1><p>Paragraph of text</p>' >/tmp/file.html
cp /tmp/file.html /dev/shm

firefox file:///tmp/file.html        # Firefox renders the file as expected

firefox file:///dev/shm/file.html    # Firefox fails to render, with permission denied
firefox file:///dev/shm              # Firefox renders the directory as expected

sudo umount /dev/shm                 # Remove the tmpfs filesystem
cp /tmp/file.html /dev/shm

firefox file:///dev/shm/file.html    # Firefox STILL fails to render, with permission denied

Furthermore

sudo mkdir /mnt/shm
sudo mount -t tmpfs /mnt/shm         # Mount a tmpfs filesystem elsewhere
cp /tmp/file.html /mnt/shm

firefox file:///mnt/shm/file.html    # Firefox renders as expected

Ubuntu 22.04 uses a "snap" version of Firefox as part of its standard installation, and such applications are sandboxed and prevented from accessing files in much of the filesystem (including those in /dev/shm but not the directory itself). Furthermore, the set of accessible directories is not configurable.

Relevant references

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  • 5
    Yup. askubuntu.com/questions/1227963/… tells how to disable sandboxing for an app. Strictly speaking this makes a lot of sense; the browser should not need to poke things in /dev normally. Commented Aug 30, 2024 at 14:26
  • @vidarlo thanks. I don't run Ubuntu (or snap) usually so this was an interesting exercise Commented Aug 30, 2024 at 14:30
  • I use the /dev/shm RAM-disk when I have to generate many temporary files, to avoid SSD wearing. I will see how to disable sandboxing for Chrome. Hopefully this will have no side effects... Commented Aug 30, 2024 at 14:43
  • What I would like to do is to add /dev/shm to the list of directories accessible by all snap based applications. Is there a way to do it? Commented Aug 30, 2024 at 15:23
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    @vidarlo "the browser should not need to poke things in /dev normally" /dev/shm is a backing store for shared memory that any process may use. Certainly, chromium on my system uses it, as well as /dev/urandom and /dev/null. So I disagree with your statement there. Commented Aug 31, 2024 at 8:28

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