This didn't look right to me. That's outside the build tree, yes?
It’s inside the build tree. “../../../../usr/include“ may look a little confusing, it’s actually linux/usr/include (linux/ is top directory of the repo we git clone), i.e. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/usr/... The file tools/testing/sefltests/Makefile can install kernel headers in default path “usr/include”. “../../../../usr/include“ is also used in other Makefile of selftests, like https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tool...
I thought there was a proper "updated kernel headers" dependency that should be used for this?
Updating kernel headers could be one solution. Adding “../../../../usr/include“ in the header file search path could build the tests inside the sandbox.
Sherry