Fix install time error of glb-redirect on ubuntu noble

[mpenny-github]

The glb-redirect-iptables-dkms package failed to build on Ubuntu Noble (kernel 6.8+) because two things in the source are incompatible with newer kernels and toolchains.

1. The kernel module passed a third argument to __cookie_v[46]_check(), which dropped that argument in the 6.x kernel API.
2. The userspace xtables target defined a _init() function that collides with crti.o's _init on Noble's gcc-13/binutils.

# Kernel module (DKMS build, kernel 6.8):
ipt_GLBREDIRECT.c: In function 'glbredirect_tg4':
ipt_GLBREDIRECT.c:614:51: error: too many arguments to function '__cookie_v4_check'
  614 |     int mss = __cookie_v4_check(iph_v4, th, ntohl(th->ack_seq) - 1);
      |                                                   ^~~~~~~~~~~~~
In file included from ipt_GLBREDIRECT.c:NN:
.../include/net/tcp.h:NNN:5: note: declared here
  NNN | int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th);
      |     ^~~~~~~~~~~~~~~~~
ipt_GLBREDIRECT.c:654:51: error: too many arguments to function '__cookie_v6_check'
  654 |     int mss = __cookie_v6_check(iph_v6, th, ntohl(th->ack_seq) - 1);
      |                                                   ^~~~~~~~~~~~~

# Userspace xtables target (link stage, gcc-13/binutils on noble):
/usr/bin/ld: libxt_GLBREDIRECT.c:(.text+0xNN): multiple definition of `_init';
  /usr/lib/x86_64-linux-gnu/crti.o:(.init+0x0): first defined here
collect2: error: ld returned 1 exit status
make: *** [Makefile:35: libxt_GLBREDIRECT.so] Error 1

Because the package ships as a source-only DKMS deb, these errors only surfaced at install time and were never caught in CI. The fix version-guards the cookie calls and replaces _init() with a constructor function, plus adds a compile check to the test suite.

[Copilot]

Pull request overview

Fixes Ubuntu Noble (kernel 6.8+/newer toolchains) install-time build failures for the glb-redirect-iptables-dkms source-only DKMS package by updating both the kernel module and the userspace xtables plugin, and adding a compile-time build check to catch regressions earlier in CI.

Changes:

• Version-guards __cookie_v[46]_check() calls to match newer kernel APIs.
• Replaces the userspace _init() entry point with a constructor to avoid linker symbol collisions on newer toolchains.
• Adds a compile-check step to the glb-redirect test script to build the kernel module and xtables target in CI.

Show a summary per file

File	Description
src/glb-redirect/script/test	Adds a compile-check that builds ipt_GLBREDIRECT.ko and libxt_GLBREDIRECT.so during tests.
src/glb-redirect/libxt_GLBREDIRECT.c	Switches from legacy _init to a constructor-based registration function for the xtables target.
src/glb-redirect/ipt_GLBREDIRECT.c	Adds kernel-version guards around syncookie __cookie_v4_check/__cookie_v6_check calls.

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• Files reviewed: 3/3 changed files
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• Files reviewed: 4/4 changed files
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• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Medium