heap: support glibc >= 2.43 (fastbins were removed)

[nkbeast]

Fixes #1225

Problem

glibc 2.43 removed the fastbin infrastructure (upstream commit bb5a4f5),
so struct malloc_state no longer contains fastbinsY[NFASTBINS] nor
have_fastchunks. GEF's GlibcArena.malloc_state_t() was still parsing
the old layout, so on glibc >= 2.43 every heap command reading the arena
showed garbage (heap arenas with wrong top/last_remainder, broken
heap bins, ...).

Fix

Made the arena layout version-aware:

• GlibcArena.has_fastbins(): fastbins exist only on glibc < 2.43
• GlibcArena.malloc_state_t(): no longer defines fastbinsY /
  have_fastchunks on glibc >= 2.43
• GlibcArena.fastbinsY returns an empty array, fastbin() returns
  None, so existing callers behave safely
• heap bins fast exits with a clear message instead of parsing
  corrupted data

Tests

Added HeapCommandNoFastbins (tests/commands/heap.py): it forces
gef.libc._version = (2, 43) regardless of the host glibc and verifies
the 2.43 arena layout has no fastbins and heap bins fast reports them
as unsupported.

The initial CI run failed on fedora-rawhide (glibc 2.44) for two
reasons, both fixed in the follow-up commit:

• HeapCommandFastBins::test_cmd_heap_bins_fast was missing the
  skipif(is_glibc_ge(2, 43)) marker that all the other fastbin tests
  use, and failed on a real no-fastbins glibc
• CanaryCommand::test_cmd_canary asserted that the stack canary equals
  the AT_RANDOM-derived value; on glibc >= 2.44 the canary is no
  longer derived from AT_RANDOM, so the test is skipped there (the
  canary command itself still reads the right value from the TLS slot)

Verified locally against glibc 2.41 (Debian), 2.43 (fedora:44) and
2.44 (fedora:rawhide) containers, plus pre-commit run --all-files.

[github-actions]

🤖 Coverage update for a9be982 🟢

	Old	New
Commit	87f359b	a9be982
Score	70.1989%	70.1989% (0)

[hugsy]

Nicely done ! LGTM if all tests pass

[github-actions]

🤖 Coverage update for 961c7e6 🟢

	Old	New
Commit	87f359b	961c7e6
Score	70.1989%	70.1989% (0)

[Copilot]

Pull request overview

This PR updates GEF’s glibc heap arena parsing to handle glibc ≥ 2.43 where fastbins were removed, preventing incorrect malloc_state decoding that previously caused heap commands (arenas/bins) to display corrupted data on newer glibc versions.

Changes:

• Add a version-aware GlibcArena.has_fastbins() gate and update malloc_state_t() so fastbinsY / have_fastchunks are not parsed on glibc ≥ 2.43.
• Make heap bins fast explicitly unsupported when fastbins are absent, avoiding mis-parsing.
• Adjust/extend tests: skip fastbin tests on glibc ≥ 2.43, add coverage for the no-fastbins layout, and skip the canary derivation assertion on glibc ≥ 2.44.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
gef.py	Makes arena layout parsing conditional on glibc version and blocks heap bins fast on glibc ≥ 2.43.
tests/commands/heap.py	Skips fastbin test on glibc ≥ 2.43 and adds a new test class validating the “no fastbins” behavior/layout.
tests/commands/canary.py	Skips test_cmd_canary on glibc ≥ 2.44 due to changed canary derivation assumptions.

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[hugsy]

All good, merging. Thanks @nkbeast