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

[Peter Zijlstra]

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.