Consider putting a tmpfs at /dev/shm, to support shm_open and friends.

[zenhack]

background: yesterday @orblivion hit an issue where postgres wouldn't start because it was trying to open something in /dev/shm, which doesn't currently exist in the sandbox. It may be possible to work around this, but it seems sensible to make a tmpfs available to grains at /dev/shm to generally improve compatibility with software that expects it.

[ocdtrekkie]

This sounds plausibly harmless, after I read what /dev/shm is. Are there any downsides you can think of? Postgres support is pretty important, I think, there's several interesting apps that expect it as their sole supported database.

[orblivion]

It's possible to turn off the need for shared memory in Postgres. I'm far off from getting my app to work, so I don't know yet how that affects performance. I think Ian did mention one possible downside: it might be smaller than Postgres (or other programs) expects it to be. I suppose that's still better than it not being there at all though? So long as the error messages are useful.

…

On Fri, Jan 31, 2020 at 3:08 PM Jacob Weisz ***@***.***> wrote: This sounds plausibly harmless, after I read what /dev/shm is. Are there any downsides you can think of? Postgres support is pretty important, I think, there's several interesting apps that expect it as their sole supported database. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#3196?email_source=notifications&email_token=AAAKH6DSKSNLLLVHZVU75ALRASALDA5CNFSM4KOMFO4KYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEKP3GIY#issuecomment-580891427>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAKH6CM6U3HNLS5AX2HCX3RASALDANCNFSM4KOMFO4A> .

[abliss]

refs: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/posix/shm_open.c;h=eeb6cb558901681958fd082dfe3c6c8f1233296b;hb=eeb6cb558901681958fd082dfe3c6c8f1233296b
https://code.woboq.org/userspace/glibc/sysdeps/posix/shm-directory.h.html
https://code.woboq.org/userspace/glibc/sysdeps/posix/shm-directory.c.html#_M/SHMDIR
# define SHMDIR (_PATH_DEV "shm/")

[kentonv]

Yeah this would be fine, and should be easy. I believe /dev in the sandbox is already a tmpfs, so probably we just need one extra mkdir() in there to create shm.

[zenhack]

Quoting Kenton Varda (2020-02-02 21:02:37)

Yeah this would be fine, and should be easy. I believe /dev in the sandbox is already a tmpfs, so probably we just need one extra mkdir() in there to create shm.

It is, but it's mounted read-only, so that would presumably require more than a mkdir() for apps to actually use it.

[kentonv]

@zenhack I mean we should add a mkdir() to the supervisor to create this directory at the same time as it creates the rest of dev's contents.