Build failure with lib-lwip when switching to Memory Pools mode: "token '{' is not valid in preprocessor expressions"

[ThestralWarrior]

Description

When switching lib-lwip's memory allocation mode from Heap to Memory Pools via menuconfig, the build for the nginx application in catalog-core fails due to invalid use of GNU statement expressions in preprocessor expressions.

Steps to reproduce

1. Clone catalog-core and setup according to the instructions
2. Change into nginx directory and enter menuconfig

make menuconfig

3. Navigate to

Library Configuration -> lwip -> Memory allocation mode -> Memory pools

4. Save and exit menuconfig
5. Build with

make -j $(nproc)

6. Observe the error

Observed Error

/home/unikraft/nginx/nginx-pan/workdir/unikraft/include/uk/essentials.h:105:10: error: token "{" is not valid in preprocessor expressions
   105 |         ({ \
       |          ^
/home/unikraft/nginx/nginx-pan/workdir/libs/lib-lwip/include/lwipopts.h:293:24: note: in expansion of macro ‘MAX’
   293 | #define PBUF_POOL_SIZE MAX(((TCP_WND + TCP_MSS - 1) / TCP_MSS), 2 * IP_REASS_MAX_PBUFS)
       |                        ^~~
...

Root Cause

The macro MAX(...) from uk/essentials.h is defined as

#define MAX(a, b)                                                              \
	({                                                                     \
		__typeof__(a) __a = (a);                                       \
		__typeof__(b) __b = (b);                                       \
		__a > __b ? __a : __b;                                         \
	})

This is a GNU C Statement Expression which appears to fail when used inside preprocessor directives.

Environment

• Unikraft version: 0.19.0~9603a4ab
• Host OS: Ubuntu 24.04
• Architecture: x86_64