fix: mark wire-format header structs packed, enable UBSan alignment checking (#1104)

[fklassen]

Fixes #1104.

Misaligned struct access was systemic, not the two isolated defects #1100 fixed: parsing a packet here means casting a buffer offset to a header struct, and an Ethernet payload starts 14 bytes in, so any 4-byte field in the header right after it is misaligned by construction - 15 sites across get.c, flows.c and en10mb.c, all the same shape (cast, then dereference).

The fix

Marked the five structs those 15 sites actually cast onto __attribute__((packed)): tcpr_ipv4_hdr, tcpr_ipv6_hdr, tcpr_in6_addr, tcpr_tcp_hdr, tcpr_icmpv4_hdr (src/tcpr.h).

None of them had compiler-inserted padding to begin with - every field already falls on a self-consistent offset - so this is a no-op for sizeof() and wire layout. Verified identical sizeof() before/after for all five:

ipv4_hdr: 20   ipv6_hdr: 40   in6_addr: 16   tcp_hdr: 20   icmpv4_hdr: 28

before and after, unchanged. packed here only removes the compiler's assumption that the struct's start address is naturally aligned - which it never was, since it's overlaid on packet buffers at whatever offset the preceding headers add up to.

Why not the other ~55 wire structs in tcpr.h

Deliberately left alone. They're not implicated by any UBSan finding the corpus has actually produced (BGP4, OSPF, DHCP, SEBEK, and the rest aren't exercised the same way), and packing them without that evidence would trade real, exercised coverage for speculative coverage of paths nothing here tests - same reasoning #1104 itself gave for not folding this into #1100.

CI

-fno-sanitize=alignment dropped from the asan job - alignment checking now runs alongside everything else UBSan already checked (integer overflow, shifts, bad enums, null dereference).

Verification

• Alignment enabled with -fno-sanitize-recover (abort on first violation, not just log it): 78/78, 0 sanitizer reports.
• Same result under the plain non-sanitizer autotools build (matching the tests CI job) and under CMake.
• sudo make tcpreplay (full replay group): 19/19.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NBmWiWg46r8BLbdwozKo6v