ASan `stack-buffer-overflow` in SLIRP networking due to struct mismatches with LKL

[ChuWeiChang]

Problem

Running kbox with SLIRP networking enabled (./kbox image -S alpine.ext4 --net) triggers a stack-buffer-overflow detected by AddressSanitizer (ASan).

The root cause stems from mismatches between the custom-defined network structures (ifr and rt) in net-slirp.c and the actual structures expected by the LKL interface. This manifests in two distinct ways:

1. ifr struct (Architecture Mismatch): The current custom struct hardcodes a size of 32 bytes (16 for name + 16 for the union/address). While this matches the 32-bit Linux struct ifreq(https://man7.org/linux/man-pages/man7/netdevice.7.html), the standard size on 64-bit architectures is 40 bytes. Running this on a 64-bit host results in an 8-byte discrepancy, causing the overflow.
2. rt struct (Incomplete Definition): The custom rt struct, corresponding to Linux struct rtentry , is structurally incomplete. It is missing two standard fields: unsigned long rt_window and unsigned short rt_irtt, causing the overflow.

Proposed Changes

Modify net-slirp.c to improve memory safety and compatibility:
That is, replacing the custom, hardcoded ifr and rt struct definitions with the standard Linux structs (struct ifreq and struct rtentry) .

Considerations

• Architecture Portability: Using the standard headers natively resolves the 64-bit pointer and padding differences for ifreq across different host architectures (x86_64, aarch64).
• Current Workarounds: For debugging purposes, the ASan checks can currently be bypassed by manually padding the ifr struct with 8 bytes and the rt struct with 16 bytes on 64-bit system. However, this is brittle and should be discarded in favor of standard headers.
• Scope of Impact: The necessary modifications are strictly localized to net-slirp.c.

[jserv]

Proposed Changes
Refactor net-slirp.c to improve memory safety and compatibility: That is, replacing the custom, hardcoded ifr and rt struct definitions with the standard Linux structs (struct ifreq and struct rtentry) .

Quote from Wikipedia:

code refactoring is the process of restructuring existing source code—changing the factoring—without changing its external behavior. Refactoring is intended to improve the design, structure, and/or implementation of the software (its non-functional attributes), while preserving its functionality.

However, the action you are taking is to change the behavior. Don't use wrong verb. Eliminate all known memory errors and weakness.