Intercept fstatfs and synthesize procfs magic for all /proc files

[doanbaotrung]

When fstatfs is called on open file descriptors pointing to /proc files (e.g., /proc/self/maps, /proc/uptime, /proc/loadavg), it returned the host filesystem magic (e.g. APFS) instead of LINUX_PROC_SUPER_MAGIC (0x9fa0).

Fix this by:

• Modifying resolve_virtual_path in fs.c to resolve and normalize all paths under /proc (not just virtual directories/stateful files) and record them in the fd table's proc_path field.
• Introducing a fill_proc_statfs helper in fs-stat.c that populates a static linux_statfs_t layout with PROC_SUPER_MAGIC (0x9fa0) and zero blocks/free space.
• Intercepting both sys_statfs and sys_fstatfs when the target path or fd's proc_path points to /proc, and directly writing the synthesized procfs metadata back to the guest.

Fix #142

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Summary by cubic

Return the correct procfs magic for /proc files by intercepting statfs/fstatfs and normalizing /proc paths, while skipping procfs symlinks so only real proc entries report procfs. Fixes #142.

• Bug Fixes
  • Normalize and record /proc paths in resolve_virtual_path, mapping /proc/<pid> (for the current pid) to /proc/self, and exclude procfs symlinks (/proc/self/{exe,cwd,root}, /proc/self/fd/<n>, and task equivalents) so they report the target filesystem.
  • Add fill_proc_statfs to synthesize a linux_statfs_t with PROC_SUPER_MAGIC (0x9fa0), 4K block size, and zeroed counts.
  • Intercept sys_statfs and sys_fstatfs for /proc targets and return the synthesized procfs stats.

^{Written for commit 92599f3. Summary will update on new commits.}

[jserv]

statfs on a nonexistent /proc path now returns success instead of ENOENT. statfs_path_is_proc() matches any /proc-prefixed path, so this block fills PROC_SUPER_MAGIC and returns 0 without confirming the entry exists. The previous code fell through proc_intercept_statfs -> proc_intercept_open -> statfs(host_path), so a miss propagated ENOENT.

Trigger: statfs("/proc/bogus", &buf) or statfs("/proc/self/not_a_file", &buf) -- real Linux and pre-PR return -ENOENT; this returns 0 with f_type=0x9fa0. Programs that use statfs as an existence/feature probe are misled.

Fix: gate synthesis on a positive existence check (only synthesize when proc_intercept_stat(tx.intercept_path, ...) recognizes the path, otherwise fall through to the host statfs that yields ENOENT). Only f_type needed to change; existence semantics should be preserved.

[jserv]

Stamping proc_path for every non-symlink /proc/* entry -- including regular files like /proc/cpuinfo and /proc/self/maps -- leaks dirfd semantics into consumers that key off proc_path. dirfd_guest_base_path (path.c:883) accepts any fd with a non-empty proc_path as a resolution base and returns before the snap.type != FD_DIR -> ENOTDIR guard at path.c:909 -- unlike resolve_proc_dirfd_path (path.c:522), which requires FD_DIR.

Trigger: open /proc/cpuinfo as fd 5, then openat(5, "x", ...). Real Linux returns ENOTDIR; this builds /proc/cpuinfo/x and resolves it (wrong errno / wrong target). It also reroutes plain fstat on such fds through proc_intercept_stat.

Fix: only stamp proc_path for /proc entries that are directories (the cases where the fd is a legitimate dirfd), or record a separate proc-mount flag used solely by the statfs path rather than overloading proc_path, which carries dirfd-resolution meaning.

[jserv]

The linux_statfs_t layout here is complete and correct (all fields covered by memset(0); f_type/f_bsize/f_namelen/f_frsize consistent with a real procfs). No regression test asserts the magic, though -- the existing tests only compare statfs vs fstatfs, not the value 0x9fa0. Worth adding one assertion for f_type == 0x9fa0 on both statfs("/proc/...") and fstatfs(open("/proc/...")).