[dlfcn] E: Add dlfcn-weak-c tests for STB_WEAK loader semantics

[esaurez]

Summary

Adds a new src/dlfcn-weak-c/ regression suite covering STB_WEAK undefined-symbol semantics through the public POSIX dlfcn interface (dlopen / dlsym). This is the canonical end-to-end test that pins the Nanvix loader's STB_WEAK handling in place so future loader refactors cannot silently re-break the contract.

Why this matters

The System V gABI ("Symbol Table" chapter) specifies that an unresolved STB_WEAK undefined symbol takes value zero at dynamic link time, and the program is responsible for null-checking before use. Every mainstream ELF loader (glibc elf/dl-lookup.c, musl ldso/dynlink.c, FreeBSD rtld-elf/rtld.c, Android Bionic linker/linker_relocate.cpp) implements this rule. The Nanvix loader honours the contract; until this PR there was no hermetic C-level test driven through the public POSIX dlfcn API to anchor that behaviour.

Test matrix

7 cases, all in main.c. Magic string "ok" is written on full success; assert() aborts on failure (matches the convention of the sibling dlfcn-c, dlfcn-pie-c, dlfcn-global-c, and dlfcn-needed-c suites).

#	Fixture	Relocation	Symbol	Expected behaviour
5	libstrong-missing.so	R_386_JUMP_SLOT	strong_missing	dlopen(RTLD_NOW) fails
1	libweak-func-resolved.so	R_386_GLOB_DAT	main_callback	resolves to main exe; try_callback()==70
2	libweak-func-missing.so	R_386_GLOB_DAT	missing_callback	&fn == NULL; try_callback() == -1
3	libweak-data-resolved.so	R_386_GLOB_DAT	weak_data	resolves to main exe; read_weak_data()==99
4	libweak-data-missing.so	R_386_GLOB_DAT	missing_weak_data	&data == NULL; read_weak_data() == -1
6	libweak-plt-resolved.so	R_386_JUMP_SLOT	main_callback	resolves to main exe; PLT call returns 70
7	libweak-plt-missing.so	R_386_JUMP_SLOT	missing_plt_callback	dlopen(RTLD_NOW) succeeds

Case 5 runs first so that a regression in the strong-undefined path is reported independently of the weak cases. It is also the only case that passes against a loader without STB_WEAK support.

Case 7 deliberately does not call the function pointer it resolves. The JMP_SLOT was zeroed by the loader (weak undefined missing → 0); invoking it would jump to NULL. The case only validates that dlopen(..., RTLD_NOW) accepts the .so despite the weak undefined PLT entry.

Relocation-class coverage

The weak NULL-guarded pattern if (&fn) fn(); in libs/weak_func.c and libs/weak_data.c forces a GOT-indirect call/load and only exercises R_386_GLOB_DAT. To also exercise weak references via the PLT, libs/weak_func_plt.c (an unguarded call) emits R_386_JUMP_SLOT. Together, the two helpers cover both relocation classes the loader must handle for weak undefined symbols in i686 ELF. The strong-undefined regression guard libs/strong_missing.c is unguarded by construction and also emits R_386_JUMP_SLOT.

Build-time fixture verification

The suite Makefile asserts via readelf -rW and readelf -WsD that each produced .so contains both: (1) the intended relocation type AND symbol name (proves the fixture is exercising the symbol we think it is, not some accidental CRT-injected weak symbol); and (2) the intended dynamic-symbol binding (WEAK / GLOBAL) and that the symbol is UND. Helper .so files are compiled with the suite-wide CFLAGS, keeping the test representative of the project's normal -O2 + warning environment.

Linker flags

Main executable: -pie -rdynamic -Wl,--no-dynamic-linker. -pie + -rdynamic export main_callback and weak_data in the main exe's .dynsym so the resolved cases can look them up (matches the existing dlfcn-pie-c pattern). --no-dynamic-linker because Nanvix uses an in-process loader via syscalls, not a separate ld.so.

Suite wiring

The new suite is wired into src/Makefile (SUITES list and the standalone-only comment) and into .nanvix/z.py as an ALL_SUITES entry, a STANDALONE_ONLY_SUITES entry, and a SUITE_RAMFS_LIBS mapping that bundles all 7 fixture .so files into the ramfs under lib/ where main.c opens them.

Validation

End-to-end on a Nanvix microvm guest with the current 0.16.17 release (which already contains the STB_WEAK loader support): all 7 cases pass. The full posix-tests suite (17 binaries, 15 integration runs) also passes. Local z lint (pyright) and z format --check (black) report clean.

[Copilot]

Pull request overview

Adds a new dlfcn-weak-c regression suite that validates ELF STB_WEAK undefined-symbol semantics end-to-end via the public dlopen/dlsym interface, and wires it into the build/test runner so the required .so fixtures are bundled into the ramfs for standalone runs.

Changes:

• Introduces src/dlfcn-weak-c/ with a standalone driver (main.c) plus helper shared-library fixtures covering both R_386_GLOB_DAT and R_386_JUMP_SLOT weak-undefined cases.
• Adds a suite Makefile that builds fixtures and asserts their relocation/symbol-table properties via readelf.
• Wires the new suite into src/Makefile and .nanvix/z.py (suite lists + ramfs library bundling).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Makefile	Adds dlfcn-weak-c to the suite list and updates standalone-only suite comment.
src/dlfcn-weak-c/Makefile	Builds the test binary and 7 .so fixtures; adds readelf-based assertions for relocations and dynsym bindings.
src/dlfcn-weak-c/main.c	Implements the 7-case weak/strong undefined-symbol behavior checks through dlopen/dlsym.
src/dlfcn-weak-c/libs/weak_func.c	Fixture for weak undefined function via GOT (R_386_GLOB_DAT) with NULL-guard.
src/dlfcn-weak-c/libs/weak_func_plt.c	Fixture for weak undefined function via PLT (R_386_JUMP_SLOT) without NULL-guard.
src/dlfcn-weak-c/libs/weak_data.c	Fixture for weak undefined data via GOT (R_386_GLOB_DAT) with NULL-guard.
src/dlfcn-weak-c/libs/strong_missing.c	Fixture for strong undefined function via PLT (R_386_JUMP_SLOT) to ensure dlopen(RTLD_NOW) fails.
.nanvix/z.py	Registers the suite and bundles its .so fixtures into the ramfs for standalone test runs.

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