renameat2 syscall failure on Ubuntu 20.04 causes capnproto/kj/filesystem-disk-test failures

[Troy Farrell]

Hi everyone,

I tried compiling Sandstorm on an Ubuntu 20.04 system today.  I was unable to complete the build because of the filesystem-disk-test, compiled as part of capnproto, failed.  The failures started with the "DiskDirectory symlinks" test.  With gdb, I was able to track the cause down to renameat2 syscalls failing (called at line 583 of filesystem-disk-test.c++.)  Oddly, the errno value seemed to be zero.

https://github.com/capnproto/capnproto/blob/master/c%2B%2B/src/kj/filesystem-disk-test.c%2B%2B#L583

I don't have the logs on this laptop, but I'll send those tonight if you're interested.

I don't know whether this is a problem specific to this laptop or something more widespread.  If no one knows, I'll try to test it on a cloud server in the next few days.

If anyone knows what I might be missing, please clue me in.  Otherwise, I'll send a follow-up message if I make any progress.

Thanks,

Troy

[Jacob Weisz]

You may want to open this as an issue on the capnproto repo. There's a number of people who support and participate in CapnProto which aren't involved with Sandstorm, so you may get a faster response.

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  Jacob Weisz

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[Troy Farrell]

On Wednesday, December 9, 2020 at 1:48:24 PM UTC-5 Jacob Weisz wrote:

You may want to open this as an issue on the capnproto repo. There's a number of people who support and participate in CapnProto which aren't involved with Sandstorm, so you may get a faster response.

Thanks!  I was not able to replicate this failure on a different Ubuntu 20.04 system, so I wiped and reinstalled the laptop with the failure.  Sandstorm compiled correctly after the reinstall.  (I didn't have time to dive deeper into the problem.)

[Kenton Varda]

This could have been related to the filesystem you were using... Was it something unusual, by any chance?

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[Troy Farrell]

On Friday, December 11, 2020 at 11:13:00 AM UTC-5 kenton wrote:

This could have been related to the filesystem you were using... Was it something unusual, by any chance?

I believe it was ext4, but it might have been ext4 inside whatever container Ubuntu makes for an encrypted home directory.

[Kenton Varda]

Ah yes, I believe "encrypted home directory" on Ubuntu means ecryptfs, which is really its own filesystem implementation (using some other filesystem as a backing store). I could imagine it has a buggy renameat2 implementation.

Apparently it is deprecated these days so probably not worth chasing down the issue here.

-Kenton

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