[Bug sanitizer/84210] __ubsan_handle_builtin_unreachable shoun't be const

[ryabinin.a.a at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84210

--- Comment #2 from Andrey Ryabinin <ryabinin.a.a at gmail dot com> ---
(In reply to Richard Biener from comment #1)
> Confirmed.  Note that I'm not sure it makes no sense - it just means the
> function has no side-effect besides not returning ;)
> 

Well, GCC docs say that const functions "have no effects except to return a
value" and "It does not make sense for a const function to return void."
I suppose that noreturn functions fells into the same category here as return
void functions.

Besides, __ubsan_handle_builtin_unreachable() does have side-effect, it prints
error.


> lib/ubsan.c is from the kernel I guess where you "copy" the attributes from
> sanitizer.def?
> 

Yes, lib/ubsan.c is kernel's implementation of libubsan. We don't exactly
'copy' attributes, but __ubsan_handle_builtin_unreachable() does have noreturn.
I've tried to add const to be consistent with GCC, but it become only worse:

../lib/ubsan.c:432:1: warning: ignoring attribute ‘noreturn’ in declaration of
a built-in function ‘__ubsan_handle_builtin_unreachable’ because it conflicts
with attribute ‘const’ [-Wattributes]
 {
 ^
<built-in>: note: previous declaration here
../lib/ubsan.c:432:1: warning: ‘const’ attribute on function returning ‘void’
[-Wattributes]
../lib/ubsan.c:432:1: warning: ignoring attribute ‘const’ in declaration of a
built-in function ‘__ubsan_handle_builtin_unreachable’ because it conflicts
with attribute ‘noreturn’ [-Wattributes]
<built-in>: note: previous declaration here


> GCC internally could also use "no vops" but you couldn't replicate that.
> 

We probably don't have to replicate all attributes. At least userspace libubsan
doesn't do that at all.

In fact we could workaround this warning simply by dropping noreturn:
http://lkml.kernel.org/r/20180202154813.1625742-1-arnd@arndb.de

But, I'm thinking that there is something wrong in GCC, hence filed this bug.

> Not sure if it is/was important to have the function 'const'.