Re: gcc-10: kernel stack is corrupted and fails to boot

[Andy Lutomirski]

> On May 13, 2020, at 7:20 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> ïOn Wed, May 13, 2020 at 5:51 PM Nick Desaulniers
> <ndesaulniers@xxxxxxxxxx> wrote:
>>
>> Are you sure LTO treats empty asm statements differently than full
>> memory barriers in regards to preventing tail calls?
>
> It had better.
>
> At link-time, there is nothing left of an empty asm statement. So by
> the time the linker runs, it only sees
>
>    call xyz
>    ret
>
> in the object code. At that point, it's somewhat reasonable for any
> link-time optimizer (or an optimizing assembler, for that matter) to
> say "I'll just turn that sequence into a simple 'jmp xyz' instead".
>

What, what?

LTO isnât a linker taking regular .o files full of regular machine
code and optimizing it. Thatâs nuts.  LTO takes an intermediate
representation and optimizes *that*. This will contain actual
indications that something is inline asm.  If LTO starts rewriting
inline asm, then I bet all kinds of things will go wrong and this is
the least of our worries.  Also, trying to do the kinds of stuff LTO
does by looking at just machine code isn't going to work.

So the difference between:

asm volatile ("nop");

and

asm volatile ("");

will be, literally, the absence of the nop.  (And alignment changes, etc.)