Hi!


> To fix that, the initial attempt was to mark the one function which
> generates the stack canary with:
> 
>   __attribute__((optimize("-fno-stack-protector"))) ... start_secondary(void *unused)
> 
> however, using the optimize attribute doesn't work cumulatively
> as the attribute does not add to but rather replaces previously
> supplied optimization options - roughly all -fxxx options.
> 
> The key one among them being -fno-omit-frame-pointer and thus leading to
> not present frame pointer - frame pointer which the kernel needs.
> 
> The next attempt to prevent compilers from tail-call optimizing
> the last function call cpu_startup_entry(), shy of carving out
> start_secondary() into a separate compilation unit and building it with
> -fno-stack-protector, was to add an empty asm("").
> 
> This current solution was short and sweet, and reportedly, is supported
> by both compilers but we didn't get very far this time: future (LTO?)
> optimization passes could potentially eliminate this, which leads us
> to the third attempt: having an actual memory barrier there which the
> compiler cannot ignore or move around etc.
> 
> That should hold for a long time, but hey we said that about the other
> two solutions too so...

You need compiler barrier, but mb() compiles down to

asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "mfence", \
    				     		          X86_FEATURE_XMM2)
    				     		          :::
    				     		          "memory",
    				     		          "cc")

I believe that is a bit of overkill.

I see that empty asm("") is not effective. asm volatile("", :::
"memory") should be effective, AFAICT. You should be able to use
existing barrier() macro.

https://elixir.bootlin.com/linux/latest/source/include/linux/compiler-gcc.h#L20

Best regards,
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html