Re: V2 [PATCH] i386: Insert ENDBR for NOTE_INSN_DELETED_LABEL only if needed

[Jeff Law <law@redhat.com>]

On 5/28/19 9:48 AM, Jakub Jelinek wrote:
> On Tue, May 28, 2019 at 08:30:59AM -0700, H.J. Lu wrote:
>>> We shouldn't generate ENDBR in that case, nothing can goto to bar (otherwise
>>> it would remain a normal label, not a deleted label).
>>>
>>
>> But return value of func () may be used with indirect jump.
> 
> No, it may be used say to print that address, but computed goto can't be
> used to jump from one function to a different function, see
> https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
> "You may not use this mechanism to jump to code in a different function.
> If you do that, totally unpredictable things happen."
Right.  We disallowed this case some time ago IIRC.  It's essentially
undefined behavior.  I would even claim that in a CET world that we
*want* a CET fault if something tried to use the deleted label as a jump
target and thus an ENDBR is undesirable.


> 
> NOTE_INSN_DELETED_LABEL is not guaranteed to be followed by any sensible
> code, the only reason it is kept is that there is or might be something
> referencing the label and so you want to emit the label somewhere in the
> function, but don't care much where in the function.
Exactly.

Jeff