ubsan errors

[Mindavi]

Describe the bug

I've built Nix with UBSAN on my system (added NIX_CFLAGS_COMPILE = -fstack-protector-all -fsanitize=undefined -fsanitize-recover=all -fno-common -fno-omit-frame-pointer -O1 -fno-optimize-sibling-calls to be precise). Whenever I start a build I get these errors:

src/libexpr/eval.cc:2579:23: runtime error: member call on address 0x7f2108816340 which does not point to an object of type 'BaseSetting'
0x7f2108816340: note: object has invalid vptr
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              invalid vptr
src/libexpr/eval.cc:2579:53: runtime error: member call on address 0x7f21088163c0 which does not point to an object of type 'BaseSetting'
0x7f21088163c0: note: object has invalid vptr
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              invalid vptr
src/libexpr/primops.cc:2468:36: runtime error: variable length array bound evaluates to non-positive value 0
src/libexpr/primops.cc:2797:35: runtime error: variable length array bound evaluates to non-positive value 0
src/libexpr/primops.cc:3174:24: runtime error: variable length array bound evaluates to non-positive value 0

Steps To Reproduce

1. Rebuild with listed flags (main thing is -fsanitize=undefined).
2. Start a build with nix-build or nix build (shouldn't matter).
3. Observe listed errors.

Expected behavior

I expect that nix doesn't rely on undefined behavior to work correctly.

nix-env --version output

src/libexpr/eval.cc:2579:23: runtime error: member call on address 0x7f6b8cc16340 which does not point to an object of type 'BaseSetting'
0x7f6b8cc16340: note: object has invalid vptr
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              invalid vptr
src/libexpr/eval.cc:2579:53: runtime error: member call on address 0x7f6b8cc163c0 which does not point to an object of type 'BaseSetting'
0x7f6b8cc163c0: note: object has invalid vptr
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              invalid vptr
nix-env (Nix) 2.11.0

Additional context

As far as I know, VLAs may not be length zero, that's why UBSAN is complaining about it. I think the other issue about the member call is due to object initialization order.

It seems to me that the VLA issues could be relatively easily resolved (however, I am not 100% sure how to resolve that without resorting to malloc, which I assume is not preferred due to performance), I am not so sure about the issue in eval.cc.