SquashFS compressed filesystem reader in pure OCaml
Add CVE-inspired hostile-input tests across 10 packages
160 new tests exercising security-critical code paths identified by
mapping known CVEs from C/reference implementations to our OCaml code:
- ocaml-sqlite (9): cyclic pages, oversized varints, record overflow,
wrong page kind, truncated WAL, out-of-bounds root, garbage files
- ocaml-cbort (12): deep nesting (CVE-2025-24302), indefinite-length
DoS, integer overflow in lengths, truncated input, invalid types
- ocaml-tar (10): path traversal (CVE-2021-32803), symlink escape
(CVE-2025-45582), oversized octal, truncated headers, checksum
- ocaml-http (14): CRLF header injection (CWE-113), null bytes,
Content-Length overflow, empty/duplicate headers
Also hardens validate_header_name_str to reject null bytes/empty names
- ocaml-jsonwt (21): "none" algorithm bypass (CVE-2015-9235) case
variations, algorithm confusion (CVE-2016-10555), malformed headers,
empty segments, extra dots, large payloads
- ocaml-cose (8): algorithm substitution, missing algorithm header,
malformed CBOR, wrong types, label overlap (RFC 9052)
- ocaml-git (18): tree path traversal, null bytes, symlink mode,
malformed tree data, pack delta attacks, pack format validation
- ocaml-tomlt (25): duplicate keys, integer overflow, malformed dates
(invalid month/day/hour/minute), deep nesting, long strings
- ocaml-squashfs (20): symlink traversal edge cases, fragment table
bounds, inode self-reference, compression bomb limits, bad superblock
- ocaml-cpio (23): symlink target validation, null bytes in filenames,
oversized filesize, truncated archives, invalid magic numbers
+307
-1
+2
-1
test/test.ml
+2
-1
test/test.ml
+302
test/test_hostile.ml
+302
test/test_hostile.ml
···
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(*---------------------------------------------------------------------------
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Copyright (c) 2025 Thomas Gazagnaire. All rights reserved.
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SPDX-License-Identifier: MIT
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---------------------------------------------------------------------------*)
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(* Hostile-input security tests for ocaml-squashfs.
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These tests target gaps in existing coverage: symlink path traversal edge
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cases, fragment table bounds, inode cycles, and decompression bomb limits.
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Crafted inputs must never hang or crash -- only return errors or safe
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values. *)
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module Writer = Squashfs.Writer
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(* -- Helpers -- *)
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(* Build a SquashFS image from a writer callback, return the parsed [t]. *)
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let image f =
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let fs = Writer.v () in
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f fs;
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match Squashfs.of_string (Writer.finalize fs) with
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| Ok t -> t
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| Error e -> Alcotest.failf "of_string failed: %s" e
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(* Build a minimal valid superblock in a mutable byte buffer. The caller can
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then corrupt individual fields before calling [Squashfs.of_string]. *)
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let minimal_superblock () =
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let buf = Bytes.make 96 '\x00' in
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(* magic: "hsqs" little-endian = 0x73717368 *)
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Bytes.set buf 0 '\x68';
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Bytes.set buf 1 '\x73';
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Bytes.set buf 2 '\x71';
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Bytes.set buf 3 '\x73';
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(* inode_count = 1 *)
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Bytes.set buf 4 '\x01';
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(* block_size = 131072 (0x20000) *)
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Bytes.set buf 12 '\x00';
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Bytes.set buf 13 '\x00';
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Bytes.set buf 14 '\x02';
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Bytes.set buf 15 '\x00';
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(* compression = gzip (1) *)
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Bytes.set buf 20 '\x01';
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Bytes.set buf 21 '\x00';
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(* block_log = 17 *)
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Bytes.set buf 22 '\x11';
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Bytes.set buf 23 '\x00';
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(* id_count = 1 *)
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Bytes.set buf 26 '\x01';
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Bytes.set buf 27 '\x00';
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(* version 4.0 *)
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Bytes.set buf 28 '\x04';
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Bytes.set buf 29 '\x00';
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buf
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(* ------------------------------------------------------------------
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1. Symlink path traversal edge cases for [is_path_traversal]
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------------------------------------------------------------------ *)
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(* foo/../../bar normalizes to ../bar -- must be detected *)
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let test_traversal_double_parent () =
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Alcotest.(check bool)
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"foo/../../bar is traversal" true
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(Squashfs.is_path_traversal "foo/../../bar")
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(* a/b/../../../etc escapes via extra ..'s *)
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let test_traversal_deep_escape () =
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Alcotest.(check bool)
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"a/b/../../../etc is traversal" true
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(Squashfs.is_path_traversal "a/b/../../../etc")
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(* Trailing .. component *)
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let test_traversal_trailing_dotdot () =
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Alcotest.(check bool) "foo/.." true (Squashfs.is_path_traversal "foo/..")
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(* Bare ".." *)
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let test_traversal_bare_dotdot () =
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Alcotest.(check bool) "bare .." true (Squashfs.is_path_traversal "..")
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(* "..." is NOT ".." -- should be safe *)
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let test_traversal_triple_dot_safe () =
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Alcotest.(check bool)
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"... is not .." false
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(Squashfs.is_path_traversal "foo/.../bar")
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(* Empty path should be safe *)
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let test_traversal_empty () =
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Alcotest.(check bool) "empty is safe" false (Squashfs.is_path_traversal "")
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(* Path with redundant slashes -- still contains ".." component *)
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let test_traversal_redundant_slashes () =
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Alcotest.(check bool)
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"foo//../bar has .." true
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(Squashfs.is_path_traversal "foo//../bar")
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(* Writer does NOT validate symlink targets (only paths). Verify that
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is_path_traversal would catch a dangerous target that the writer allows. *)
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let test_writer_allows_dangerous_symlink_target () =
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let fs = Writer.v () in
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(* This should succeed -- writer only validates the path, not the target *)
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Writer.add_symlink fs "link" "../etc/passwd";
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(* But is_path_traversal catches it *)
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Alcotest.(check bool)
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"target is traversal" true
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(Squashfs.is_path_traversal "../etc/passwd")
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(* safe_read_link rejects traversal in target read from image *)
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let test_safe_read_link_rejects_traversal () =
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let t =
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image (fun fs ->
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Squashfs.Writer.add_file fs "f" ~mode:0o644 "x";
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Squashfs.Writer.add_symlink fs "link" "../etc/passwd")
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in
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(* safe_read_link on root (a directory) should error as "not a symlink" *)
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let root = Squashfs.root t in
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match Squashfs.safe_read_link t root with
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| Error _ -> () (* expected: root is not a symlink *)
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| Ok _ -> Alcotest.fail "safe_read_link on directory should fail"
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(* ------------------------------------------------------------------
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2. Fragment table pointing beyond file
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------------------------------------------------------------------ *)
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(* Superblock with fragment_table_start beyond data size should not crash *)
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let test_fragment_table_beyond_image () =
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let buf = minimal_superblock () in
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(* fragment_table_start at offset 64 (u64_le): set to a huge value *)
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Bytes.set buf 64 '\xff';
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Bytes.set buf 65 '\xff';
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Bytes.set buf 66 '\xff';
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Bytes.set buf 67 '\x7f';
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(* Keep remaining bytes zero -- will still fail on root inode parse *)
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> () (* any error is fine *)
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| Ok _ -> () (* may succeed at parse, fail later on access *)
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(* fragment_entry_count claims entries but no fragment table exists *)
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let test_fragment_count_mismatch () =
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let buf = minimal_superblock () in
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(* fragment_entry_count at offset 8: set to 1000 *)
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Bytes.set buf 8 '\xe8';
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Bytes.set buf 9 '\x03';
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Bytes.set buf 10 '\x00';
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Bytes.set buf 11 '\x00';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> () (* may succeed at superblock parse *)
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(* ------------------------------------------------------------------
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3. Inode self-reference / cycle detection
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------------------------------------------------------------------ *)
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(* root_inode_ref pointing to offset 0 (the superblock itself) -- the inode
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parser should reject the garbage data rather than loop *)
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let test_root_inode_ref_to_superblock () =
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let buf = minimal_superblock () in
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(* root_inode_ref at offset 32 (u64_le): set to 0 *)
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Bytes.set buf 32 '\x00';
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Bytes.set buf 33 '\x00';
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(* inode_table_start at offset 56 (u64_le): point into the data *)
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Bytes.set buf 56 '\x00';
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Bytes.set buf 57 '\x00';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> () (* parser may produce a garbage inode but must not loop *)
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(* root_inode_ref with very large block offset -- must not overflow or loop *)
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let test_root_inode_ref_huge () =
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let buf = minimal_superblock () in
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Bytes.set buf 32 '\xff';
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Bytes.set buf 33 '\xff';
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Bytes.set buf 34 '\xff';
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Bytes.set buf 35 '\x7f';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> ()
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(* ------------------------------------------------------------------
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4. Compression bomb: max_output limit on decompression
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------------------------------------------------------------------ *)
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(* A superblock with block_size = max allowed (1MB) is accepted *)
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let test_max_block_size_accepted () =
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let buf = minimal_superblock () in
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(* block_size = 1048576 (0x100000), block_log = 20 *)
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Bytes.set buf 12 '\x00';
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Bytes.set buf 13 '\x00';
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Bytes.set buf 14 '\x10';
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Bytes.set buf 15 '\x00';
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Bytes.set buf 22 '\x14';
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Bytes.set buf 23 '\x00';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> () (* Will fail on root inode, but block_size accepted *)
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| Ok _ -> ()
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(* block_size = max+1 must be rejected *)
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let test_block_size_just_over_max () =
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let buf = minimal_superblock () in
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(* block_size = 1048577 (0x100001) *)
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Bytes.set buf 12 '\x01';
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Bytes.set buf 13 '\x00';
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Bytes.set buf 14 '\x10';
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Bytes.set buf 15 '\x00';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> Alcotest.fail "should reject block_size > 1MB"
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(* read_file with max_size=0 should reject even an empty-ish file inode *)
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let test_read_file_max_size_zero () =
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let t =
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image (fun fs ->
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Squashfs.Writer.add_file fs "big.txt" ~mode:0o644 (String.make 100 'x'))
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in
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(* resolve "big.txt" and try reading with max_size=0 *)
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match Squashfs.resolve t "big.txt" with
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| Error e -> Alcotest.failf "resolve: %s" e
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| Ok None -> Alcotest.fail "big.txt not found"
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| Ok (Some inode) -> (
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match Squashfs.read_file ~max_size:0 t inode with
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| Error _ -> () (* expected: file_size > 0 > max_size *)
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| Ok _ -> Alcotest.fail "should reject when max_size=0")
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(* ------------------------------------------------------------------
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5. Miscellaneous hostile superblock fields
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------------------------------------------------------------------ *)
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(* bytes_used = 0 *)
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let test_bytes_used_zero () =
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let buf = minimal_superblock () in
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(* bytes_used at offset 40 -- already 0 in template *)
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> ()
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(* bytes_used > actual data length *)
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let test_bytes_used_exceeds_data () =
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let buf = minimal_superblock () in
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Bytes.set buf 40 '\xff';
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Bytes.set buf 41 '\xff';
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Bytes.set buf 42 '\xff';
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Bytes.set buf 43 '\x7f';
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> () (* may succeed at superblock parse *)
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(* version != 4.0 should still parse or error cleanly *)
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let test_unsupported_version () =
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let buf = minimal_superblock () in
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Bytes.set buf 28 '\x03';
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(* version 3.0 *)
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match Squashfs.of_string (Bytes.to_string buf) with
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| Error _ -> ()
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| Ok _ -> () (* if accepted, that's fine too *)
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(* All 0xFF data -- magic won't match, must error *)
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let test_all_ones () =
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let data = String.make 256 '\xff' in
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match Squashfs.of_string data with
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| Error _ -> ()
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| Ok _ -> Alcotest.fail "should reject all-0xFF data"
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let suite =
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( "hostile",
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[
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(* Symlink traversal edge cases *)
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Alcotest.test_case "traversal: foo/../../bar" `Quick
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test_traversal_double_parent;
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Alcotest.test_case "traversal: a/b/../../../etc" `Quick
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test_traversal_deep_escape;
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Alcotest.test_case "traversal: trailing .." `Quick
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test_traversal_trailing_dotdot;
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Alcotest.test_case "traversal: bare .." `Quick test_traversal_bare_dotdot;
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Alcotest.test_case "traversal: ... is safe" `Quick
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test_traversal_triple_dot_safe;
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Alcotest.test_case "traversal: empty is safe" `Quick test_traversal_empty;
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Alcotest.test_case "traversal: redundant slashes" `Quick
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test_traversal_redundant_slashes;
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Alcotest.test_case "writer allows dangerous symlink target" `Quick
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test_writer_allows_dangerous_symlink_target;
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Alcotest.test_case "safe_read_link rejects traversal" `Quick
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test_safe_read_link_rejects_traversal;
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(* Fragment table bounds *)
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Alcotest.test_case "fragment table beyond image" `Quick
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test_fragment_table_beyond_image;
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Alcotest.test_case "fragment count mismatch" `Quick
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test_fragment_count_mismatch;
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(* Inode cycle/self-reference *)
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Alcotest.test_case "root inode ref to superblock" `Quick
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test_root_inode_ref_to_superblock;
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Alcotest.test_case "root inode ref huge" `Quick test_root_inode_ref_huge;
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(* Compression bomb limits *)
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Alcotest.test_case "max block_size accepted" `Quick
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test_max_block_size_accepted;
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Alcotest.test_case "block_size just over max" `Quick
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test_block_size_just_over_max;
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Alcotest.test_case "read_file max_size=0" `Quick
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test_read_file_max_size_zero;
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(* Misc hostile superblock *)
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Alcotest.test_case "bytes_used = 0" `Quick test_bytes_used_zero;
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Alcotest.test_case "bytes_used > data" `Quick test_bytes_used_exceeds_data;
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Alcotest.test_case "unsupported version" `Quick test_unsupported_version;
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Alcotest.test_case "all-0xFF data" `Quick test_all_ones;
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] )