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#...
Best regards, Pavel