On 6/20/22 16:16, Jason Merrill wrote:
> On 6/20/22 07:05, Jakub Jelinek wrote:
>> On Fri, Jun 17, 2022 at 05:20:02PM -0400, Jason Merrill wrote:
>>> Related to PR104642, the current situation where we get less return 
>>> checking
>>> with just -fsanitize=unreachable than no sanitize flags seems 
>>> undesirable; I
>>> propose that we do return checking when -fsanitize=unreachable.
>>
>> __builtin_unreachable itself (unless turned into trap or
>> __ubsan_handle_builtin_unreachable) is not any kind of return 
>> checking, it
>> is just an optimization.
> 
> Yes, but I'm talking about "when -fsanitize=unreachable".
> 
>>> Looks like clang just traps on missing return if not 
>>> -fsanitize=return, but
>>> the approach in this patch seems more helpful to me if we're already
>>> sanitizing other should-be-unreachable code.
>>>
>>> I'm assuming that the difference in treatment of SANITIZE_UNREACHABLE 
>>> and
>>> SANITIZE_RETURN with regard to loop optimization is deliberate.
>>
>> return and unreachable are separate sanitizers and such silent one way
>> implication can have quite unexpected consequences, especially with
>> -fsanitize-trap=.
>> Say with -fsanitize=unreachable -fsanitize-trap=unreachable, both current
>> trunk and clang will link without -lubsan, because the only enabled UBSan
>> sanitizers use __builtin_trap () which doesn't need library.
>> With -fsanitize=unreachable silently meaning 
>> -fsanitize=unreachable,return
>> the above would link in -lubsan, because while SANITIZE_UNREACHABLE uses
>> __builtin_trap, SANITIZE_RETURN doesn't.
>> Similarly, one has no_sanitize attribute, one could in certain function
>> __attribute__((no_sanitize ("unreachable"))) and because on the command
>> line using -fsanitize=unreachable assume other sanitizers aren't enabled,
>> but the silent addition of return sanitizer would break that.
> 
> Ah, true.  How about this approach instead?

Or, this approach relies on the PR104642 patch, and just fixes the line 
number issue.  This is less clear about the problem than using the 
return ubsan library function, but avoids using one entry point to 
implement the other sanitizer, if that's important.