Support vmlinux struct casting, field access, and struct-typed HashMaps

[Copilot]

Adds machinery to use kernel structures from vmlinux in Python BPF programs, enabling struct casting, field access on vmlinux structs, and using structs as HashMap key/value types.

Changes

Vmlinux struct handling

• Pointer casting to vmlinux structs via constructor calls (e.g., struct_request(ctx.di))
• Field access on vmlinux structs including register fields (e.g., req.__data_len, ctx.r15)
• CO-RE (Compile Once, Run Everywhere) support via BTF probe_read helpers

Allocation & type system

• Enhanced allocation pass to handle vmlinux struct pointer casts
• Map lookup return value allocation based on value type for struct-typed maps
• Improved debug info generation for struct types

HashMap extensions

• Structs as HashMap key/value types with proper type handling

New helpers

• bpf_probe_read_kernel for BTF-based reads
• Helper wrappers: prandom, smp_processor_id, uid_gid

Example

from vmlinux import struct_request, struct_pt_regs
from pythonbpf import bpf, section

@bpf
@section("kprobe/blk_mq_start_request")
def trace_request(ctx: struct_pt_regs) -> c_int64:
    req = struct_request(ctx.di)  # Cast pointer to struct
    data_len = req.__data_len      # Access struct fields
    timeout = req.timeout
    print(f"request: len={data_len}, timeout={timeout}")
    return c_int64(0)

Moves tests/failing_tests/vmlinux/requests.py → tests/passing_tests/vmlinux/requests.py. All existing passing tests unchanged.

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