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Using a computer with Ubuntu 16.04 (no update suggestions please) I mounted /proc/ at my Raspberry Pi 3B + (linux kernel version 4.19.88) via SSHFS:

sshfs [email protected]:/proc ~/procAtPi
ls -la ~/procAtPi

Here ls shows me all files in the Pi's /proc without any problems, just as I had connected directly via ssh. But when I try to read the files with cat, nothing is displayed here. The file content is only output to me when I connect directly via

user@remote:~$ ssh [email protected]
[email protected]:~$ cat /proc/stat

How can I read out a file from procfs that I access via SSHFS?

Some more observations I made:

  • Using sudo cat ~/procAtPi/stat leads to Access denied (So root seems to have fewer access rights than the normal user?) but the files owner is root:

    user@remote:~$ ls -lai ~/procAtPi/stat
      16 -r--r--r--   1 root             root                0 Dez 24 00:00 stat

  • The displayed owner differs depending on the access:

    user@remote:~$ ls -lai ~/procAtPi/
     ...
     171 dr-xr-xr-x   1 user             user                0 Jan 20 09:18 11045
     ...
[email protected]:~$ ls -lai /proc/
     ...
2035700 dr-xr-xr-x   8 pi               pi                  0 Jan 20 09:18 11045
     ...

  • Remote access to files in the Pi's sysfs (/sys/) seems to work fine.

A current workaround is to run a server on the Pi that reads the data from /proc/stat and makes it available via TCP. To do so, however, I have to start the server manually every time (I don't want an autostart because I don't need it all the time). I need access to proc/stat for a htop-like program that is supposed to monitor my Pi cluster.

fcdt
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    Regarding the root access denied, [FUSE file systems are inaccessible to users other than their owners, including root](https://unix.stackexchange.com/a/607691/86440). – Stephen Kitt Jan 20 '21 at 09:20

1 Answers1

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ls shows me all files in the Pi's /proc without any problems, just as I had connected directly via ssh. But when I try to read the files with cat, nothing is displayed here.

As a workaround, use the -o direct_io option of sshfs:

# sshfs localhost:/proc /mnt/tmp
root@localhost's password:
# cat /mnt/tmp/self/stat
<nothing!>
# umount /mnt/tmp

# sshfs -o direct_io localhost:/proc /mnt/tmp
root@localhost's password:
# cat /mnt/tmp/self/stat
8242 (sftp-server) R 8236 8242 8242 0 -1 4194560 335 0 0 0 0 0 0 0 20 0 1 0 1846105 2506752 441 18446744073709551615 94732486082560 94732486157085 140730261312560 0 0 0 0 0 0 0 0 0 17 3 0 0 0 0 0 94732486190800 94732486194248 94732486483968 140730261319328 140730261319357 140730261319357 140730261319643 0

See here for a description of what that option is doing.

The problem with those files under /proc is that they appear as regular files of size = 0, yet they're not empty and contain data when read.

Notice that this problem is NOT caused by the SFTP protocol, and is not specific to sshfs.