Add syscall coverage audit and direct-syscall smoke suite

[jserv]

scripts/check-syscall-coverage.py parses src/syscall/dispatch.tbl and scans tests/ for direct or aliased references to every entry. C sources are matched on call-shape (name followed by an opening paren) or the SYS_/_NR macro forms; non-C sources require the explicit macro form so that coreutils applet invocations in the shell suites (run sync 0, run kill ..., run chroot ...) cannot falsely cover the like-named syscalls. ALIASES maps the pread64/pwrite64 dispatch names to the pread/pwrite libc wrappers; INDIRECT_COVERAGE documents the xattr family, rt_sig{return,suspend,pending}, ptrace, chroot, truncate, exit_group, and get_robust_list, which are exercised only through structural or out-of-band paths. mk/tests.mk runs the script as a check-syscall-coverage target that make check depends on.

src/syscall/syscall.c::sc_memfd_create rejects a NULL name pointer with EFAULT, probes the first byte of the guest pointer to surface EFAULT before the temp file is created, and reads the flag bits through named LINUX_MFD_CLOEXEC and LINUX_MFD_ALLOW_SEALING constants in src/syscall/abi.h instead of the prior raw bitmask arithmetic. Unknown MFD_* bits continue to be accepted silently to match the behavior the existing fd-lifecycle tests expect.

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Summary by cubic

Adds a syscall coverage audit and a direct-syscall smoke suite, and wires both into the test workflow. Also tightens memfd_create error handling and flags, and streamlines the coreutils tests.

• New Features

  • scripts/check-syscall-coverage.py audits src/syscall/dispatch.tbl against tests and runs via make check (check-syscall-coverage).
  • New tests/test-syscall-smoke.c covers less-common syscalls (e.g., pwrite64/preadv2, splice/vmsplice, close_range, execveat, waitid, accept4, SysV semaphores) plus expected stub errnos.
  • Low-base ET_EXEC memory regression tests (test-lowbase-mem-200000, -300000); Makefile builds non-PIE binaries to exercise old guard windows.
  • Coreutils suite modularized into tests/lib/coreutils-suite.sh with smoke/full profiles; unified static and dynamic runners; removed tests/test-coreutils-smoke.sh.
  • Test driver now auto-detects when to fail on missing binaries (default in-repo) or skip (external bins), and adds host-side assertion helpers; new coreutils_* asserts for file checks.
  • fd-lifecycle suite extended to verify memfd_create accepts valid Linux flags (e.g., MFD_HUGETLB).

• Bug Fixes

  • sc_memfd_create rejects NULL name with EFAULT, validates the first byte of the guest pointer before creating the temp file, and reads flags via LINUX_MFD_CLOEXEC/LINUX_MFD_ALLOW_SEALING (unknown bits remain accepted to preserve existing test expectations).

^{Written for commit 684654f. Summary will update on new commits.}