Re: [patch V6 00/37] x86/entry: Rework leftovers and merge plan

[Peter Zijlstra]

On Sat, May 23, 2020 at 03:08:36PM +0200, Peter Zijlstra wrote:
> On Sat, May 23, 2020 at 10:52:24AM +0800, Lai Jiangshan wrote:
> 
> > Hello,
> > 
> > I, who don't know how does the objtool handle it, am just curious.
> > _begin() and _end() are symmetrical, which means if _end() (without nop)
> > can escape, so can _begin() in a reverse way. For example:
> > 
> > noinstr void foo()
> > {
> >     instrumentation_begin();
> >     do {
> >             instrumentation_begin();
> >             ...
> >             instrumentation_end();
> >     } while (cond);
> >     bar();
> >     instrumentation_end();
> > }
> > 
> > Here, the first _begin() can be "dragged" into the do-while block.
> > Expectedly, objtool validation should not complain here.
> > 
> > But objtool validation's not complaining means it can handle it
> > magically correctly (by distinguishing how many _begin()s should
> > be taken around the jmp target when jmp in a specific path), or
> > handle it by not checking if all paths have the same count onto
> > a jmp target (a little nervous to me), or other possible ways.
> 
> No, I tihnk you're right. It could be we never hit this particular
> problem. Even the one described, where end leaks out, is quite rare. For
> instance, the last one I debgged (that led to this patch) only showed
> itself with gcc-9, but not with gcc-8 for example.
> 
> Anyway, if we ever find the above, I'll add the NOP to begin too.

FYI, I just found one, I'll be making instrumentation_begin() a NOP
too.