| Date | Tue, 19 May 2020 14:15:57 +0200 | From | Pavel Machek <> | Subject | Re: [PATCH 4.19 63/80] x86: Fix early boot crash on gcc-10, third try |
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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 [unhandled content-type:application/pgp-signature] |