overlay2 pageSize: Fix an off-by-one bug

[ob]

fixes docker/for-linux#1012 Overlay2 mount fails for larger Dockerfiles when length of data-root is 24

The code was not considering that C strings are NULL-terminated so we need to leave one extra byte.

Without this fix, the testcase in docker/for-linux#1012 fails with

Step 61/1001 : RUN echo 60 > 60
 ---> Running in dde85ac3b1e3
Removing intermediate container dde85ac3b1e3
 ---> 80a12a18a241
Step 62/1001 : RUN echo 61 > 61
error creating overlay mount to /23456789112345678921234/overlay2/d368abcc97d6c6ebcf23fa71225e2011d095295d5d8c9b31d6810bea748bdf07-init/merged: no such file or directory

with the output of dmesg -T as:

[Sat Dec 19 02:35:40 2020] overlayfs: failed to resolve '/23456789112345678921234/overlay2/89e435a1b24583c463abb73e8abfad8bf8a88312ef8253455390c5fa0a765517-init/wor': -2

with this fix, you get the expected:

Step 126/1001 : RUN echo 125 > 125
 ---> Running in 2f2e56da89e0
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[tianon]

Nice catch!

Do you think it's reasonable/possible to add an integration test, or are there "platform" fiddly bits interacting to make that difficult?

[ob]

I'm not sure how that would work. In theory it should be possible to construct a dockerfile that exceeds the maximum number of layers (like the bug report does) but in order to hit exactly the size of a page you'd have to tweak the docker-root path carefully. For most platforms the size of a page is 4k but not all... I'm not familiar with how docker is tested to be able to foresee if this would be a flaky test or fail on some platforms.

[tianon]

Right, that's what I was afraid of (but wasn't 100% understanding all the variables in the issue this fixes).

Thanks for explaining! 👍