test: add overlay copy-up tests for FIFOs and sockets

[wpan36]

Description

Add CopyUpTest coverage for non-copy-upable lower-layer FIFOs and Unix domain sockets under overlayfs.

The tests verify that:

• access(..., W_OK) fails with EACCES for lower-layer FIFOs and sockets.
• open(O_WRONLY) fails with EPERM instead of proceeding through an invalid copy-up path.

The host FIFO and UDS test fixtures are also adjusted for overlay variants. Individual special-file bind mounts are omitted because a FIFO or socket cannot itself be used as an overlay lower root, while the directory-backed mounts remain available to expose these files as lower-layer dentries.

This covers the non-copy-upable FIFO/socket cases from #13998. The remaining copy-up trigger, permission, and read-only mount cases can be covered separately.

Updates #13998

Testing

• make syscall-tests OPTIONS="--nocache_test_results" TARGETS="//test/syscalls:copy_up_test_runsc_systrap_overlay"
• make syscall-tests OPTIONS="--nocache_test_results" TARGETS="//test/syscalls:copy_up_test_runsc_systrap_directfs"
• make syscall-tests OPTIONS="--nocache_test_results" TARGETS="//test/syscalls:pipe_external_test_runsc_systrap_directfs"
• make syscall-tests OPTIONS="--nocache_test_results" TARGETS="//test/syscalls:connect_external_test_runsc_systrap_directfs"
• make test TARGETS="//test/runner:runner_nogo"

Also verified the regression coverage by locally restoring the writable-open ordering that previously bypassed the non-copy-upable lower-layer check: LowerFIFOOpenForWriteFails failed with EROFS instead of the expected EPERM. Restoring the current implementation makes the test pass.

[konstantin-s-bogom]

Why "allow_native = False" ? The whole point of these tests is to test compatibility against Linux...

[wpan36]

Thank you! I originally set allow_native = False because the generated native syscall-test variant does not get the runner-provided overlay setup. In test/runner/defs.bzl, allow_native = True generates a native variant with overlay = False. As a result, my current native tests would just hit the IsOverlayfs() skip rather than actually exercise Linux overlayfs.

I also tested the equivalent setup manually on native Linux overlayfs on Ubuntu 22.04.5 LTS with kernel 6.8.0-136-generic. I observed:

• access(path, W_OK) on lower FIFO/socket succeeds.
• open(FIFO, O_WRONLY | O_NONBLOCK) returns ENXIO when there is no reader.
• This differs from the current gVisor expectations of EACCES / EPERM.

Would you prefer that I update the expected behavior accordingly?

Also, for the native variant, would you recommend having the test create its own Linux overlayfs setup (lower/upper/work/merged), or extending the runner so that the native variant gets an overlay-backed test tree as well? I don't see an existing syscall_test option that generates a native variant with the runner's overlay setup, so I'd appreciate your guidance on which approach fits the test infrastructure better.

[konstantin-s-bogom]

Hmm I see. IMO it makes sense to do it right and extend the runner, and yeah for those places where we have a divergence with Linux we should fix it unless it's there for a specific reason.

@milantracy WDYT?