Suppress setuid and setgid on shebang scripts

[doanbaotrung]

The Linux kernel (fs/exec.c bprm_fill_uid) deliberately ignores setuid/setgid bits on interpreted scripts to prevent privilege escalation via interpreter manipulation. elfuse had no such guard: exec_is_script was never tracked, so the setuid/setgid check was effectively always true for all executables.

Fix by:

• Snapshotting the directly-executed file's stat() before the shebang resolution loop overwrites path_host with the final interpreter path.
• Setting exec_is_script = true on the first confirmed shebang (rc > 0 from elf_read_shebang), so deeply nested chains also trip the flag.
• After elf_load, applying S_ISUID/S_ISGID only when the directly-executed file was a regular non-script ELF.
• Honouring the kernel's S_ISGID-requires-S_IXGRP rule to distinguish mandatory-locking GIDs from setgid execution.

Fix #138
Fix #139

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Summary by cubic

Suppress setuid/setgid on shebang scripts to match Linux and prevent privilege escalation. Switch exec to fd-based loading/mapping with execute-permission checks to avoid TOCTOU.

• Bug Fixes
  • Snapshot the original file’s stat and mark exec_is_script on the first shebang (covers nested chains).
  • Update creds only for non-script regular files; apply sgid only when group-exec is set.
  • Use elf_read_shebang_fd, elf_load_fd, and elf_map_segments_fd to keep stable fds, validate exec perms for both target and interpreter, and commit new creds right before PNR.

Fixes #138.

^{Written for commit e862632. Summary will update on new commits.}

[jserv]

Refined based on the review from @cubic-dev-ai . If you disagree with any of its feedback, discuss it with the reviewer to reach a consensus and consolidate the shared knowledge base.

[doanbaotrung]

Fixed all the comments

[jserv]

Thank @doanbaotrung for contributing!