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I read this in the latest news post:
Lastly, on upgrading filesystem users of kernels prior to linux-3.4 will get a warning about permissions on /sys. This is nothing to worry about, as of linux-3.4 the permissions will be 555, and this upgrade reflects this in the filesystem package.
...huh? I tried Googling this but came up empty-handed. Even the kernel changelog didn't mention sysfs much, and nothing about changing the permissions (unless I just missed it somehow). Is this an Arch-specific change?
While writing this post I realized that my initial alarm was unwarranted: at first I thought that meant I couldn't write to anything in /sys. In a supreme fit of silliness, I even tried
sudo echo -n mem > /sys/power/state
and interpreted the subsequent "Permission denied" as proof that something was horribly wrong. But of course starting with
sudo -i
fixed that, and my fears were alleviated. Turns out sysfs is still mounted rw and all the usual files are still 644.
So now that I think about it having /sys be 555 is kind of obvious. Now I'm rather bewildered as to why the directories inside sysfs are 755! Under what circumstances would userspace create, delete or rename a file inside the sysfs? I'm by no means a Linux programmer (I'm an EE currently working on programming a microcontroller), so I'll bet I'm missing something obvious, but I'm curious what it is.
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The permissions of /sys are set by the kernel. From 3.4 onwards, they are set to 555.
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Thanks for finding the commit. Sounds like they just did it for consistency with /proc, and probably no one should be creating or deleting anything in there anyway. Curiosity sated
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