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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/114217] -fsanitize=alignment false positive with
 intended unaligned struct member access
Date: Sun, 03 Mar 2024 19:01:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114217

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC actually doesn't diagnose on mere pointer assignment, but what triggers=
 the
alignment check is
&entry->offset
even when the code later on just takes its address, entry must be sufficien=
tly
aligned, otherwise entry->offset is invalid.
Under standard C rules, already forming the pointer would be UB, so somewhe=
re
in the caller when you prepare what to pass to the f function.

If you want something that will still be invalid C,
but will not trigger UBSAN errors, then e.g.
unsigned long long h(struct dir_entry *entry)
{
    return get_unaligned((unsigned long long *) (((char *) entry) + offsetof
(struct dir_entry, offset)));
}
will do.
If you want something that will be valid even in C, don't pass struct dir_e=
ntry
*entry
argument, but void *entry instead, and use e.g.
__get_unaligned_t(__typeof(((struct dir_entry *)0)->offset), ((char
*)entry)+offsetof(struct dir_entry, offset)))
You can surely hide that all under some macro.=