linux: fix regression in libcrun_configure_network

[eriksjolund]

commit ab64a5c introduced the regression.

Closes: #1828

Summary by Sourcery

Fix regression in libcrun_configure_network by properly handling netlink NLMSG_ERROR messages using struct nlmsgerr and ignoring success acknowledgements.

Bug Fixes:

• Use struct nlmsgerr to parse netlink error codes instead of manually reading bytes
• Avoid treating successful NLM_F_ACK messages as errors

[sourcery-ai]

Reviewer's Guide

This patch corrects a regression in libcrun_configure_network by replacing manual parsing of netlink error messages with use of the standard nlmsgerr structure and properly distinguishing between error and acknowledgment responses.

Sequence diagram for revised netlink error handling in libcrun_configure_network

sequenceDiagram
    participant libcrun_configure_network
    participant kernel
    libcrun_configure_network->>kernel: send netlink request
    kernel-->>libcrun_configure_network: receive netlink response
    alt NLMSG_ERROR
        libcrun_configure_network->>libcrun_configure_network: parse nlmsgerr from response
        alt err_data->error < 0
            libcrun_configure_network->>libcrun_configure_network: call crun_make_error
        else err_data->error == 0
            libcrun_configure_network->>libcrun_configure_network: treat as success (ack)
        end
    end

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Class diagram for updated error handling in libcrun_configure_network

classDiagram
    class libcrun_configure_network {
        +int libcrun_configure_network(libcrun_container_t *container, libcrun_error_t *err)
    }
    class nlmsghdr {
        +int nlmsg_type
    }
    class nlmsgerr {
        +int error
    }
    libcrun_configure_network --> nlmsghdr : uses
    libcrun_configure_network --> nlmsgerr : uses

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File-Level Changes

Change	Details	Files
Refactor netlink error handling to use struct nlmsgerr	Replace manual buffer cast and errno assignment with NLMSG_DATA macro Introduce err_data->error < 0 check to detect real errors Omit error return when err_data->error == 0 (success ack) Remove deprecated int32_t casting logic	src/libcrun/linux.c

Possibly linked issues

• #26662: The PR fixes the exact regression in crun's network configuration, specifically an error handling bug with PF_NETLINK, that caused the issue.
• #26662: The PR fixes a crun network config error, resolving the device access regression on WSL2.

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[sourcery-ai]

Hey @eriksjolund - I've reviewed your changes - here's some feedback:

• Before casting NLMSG_DATA to struct nlmsgerr, verify that the netlink message payload length is at least sizeof(struct nlmsgerr) to avoid potential invalid memory access.
• Make the zero-error acknowledgment path (err_data->error == 0) explicit—either return success immediately or clearly continue—so the intent is obvious and we don’t fall through accidentally.

Prompt for AI Agents

Please address the comments from this code review:
## Overall Comments
- Before casting NLMSG_DATA to struct nlmsgerr, verify that the netlink message payload length is at least sizeof(struct nlmsgerr) to avoid potential invalid memory access.
- Make the zero-error acknowledgment path (err_data->error == 0) explicit—either return success immediately or clearly continue—so the intent is obvious and we don’t fall through accidentally.

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[eriksjolund]

Please take an extra look when doing the review. I haven't used netlink before.