Losing the magic

> I ask because some day, a very smart compiler might see that dead write of `fs->magic = 0;` given the immediate free afterwards and optimize it out as UB to observe.

That day was at least 8 years ago. GCC 4.9 with -O1 will optimise away the writes, defeating this attempt at memory protection, because ext2fs_free_mem is an inline function so the compiler knows the object is passed to free() and can no longer be observed. See e.g. https://godbolt.org/z/nWYEa34a6

I guess the cheapest way to prevent that is to insert a compiler barrier (`asm volatile ("" ::: "memory")`) just after writing to fs->magic, to prevent the compiler making assumptions about observability of memory.