[Bug sanitizer/114217] -fsanitize=alignment false positive with intended unaligned struct member access

["akihiko.odaki at daynix dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114217

--- Comment #5 from Akihiko Odaki <akihiko.odaki at daynix dot com> ---
(In reply to Andrew Pinski from comment #4)
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/
> include/asm-generic/unaligned.h?h=v6.7
> 
> is correct except it should not expose get_unaligned/put_unaligned since the
> undefined code happens way before.
> 
> The problem is with the btrfs code in btrfs_filldir:
> ```
> static int btrfs_filldir(void *addr, int entries, struct dir_context *ctx)
> {
> 	while (entries--) {
> 		struct dir_entry *entry = addr; /// THIS IS BROKEN and causes the
> -fsanitize=alignment error
> 		char *name = (char *)(entry + 1);
> 
> 		ctx->pos = get_unaligned(&entry->offset);
> 		if (!dir_emit(ctx, name, get_unaligned(&entry->name_len),
> 					 get_unaligned(&entry->ino),
> 					 get_unaligned(&entry->type)))
> 			return 1;
> 		addr += sizeof(struct dir_entry) +
> 			get_unaligned(&entry->name_len);
> 		ctx->pos++;
> 	}
> 	return 0;
> }
> ```
> 
> Added comment on where the error comes from. The get_unaligned macro really
> should not be used here. What should be used here is an unaligned version of
> `struct dir_entry` instead.

I understand the idea. What I don't get is that GCC still emits code for
unaligned memory access in such a case. It is just a waste of performance if
GCC doesn't provide a guarantee that the unaligned access is performed in such
a case and is not optimal.