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Date:   Fri, 15 Jan 2021 20:07:29 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Arnd Bergmann <arnd@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-efi@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index
Message-ID: <20210115190729.GE9138@zn.tnic>
References: <20210107223424.4135538-1-arnd@kernel.org>
 <20210115182300.GD9138@zn.tnic>
 <20210115183203.GA1991122@ubuntu-m3-large-x86>
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On Fri, Jan 15, 2021 at 11:32:03AM -0700, Nathan Chancellor wrote:
> I triggered it with CONFIG_UBSAN=y + CONFIG_UBSAN_UNSIGNED_OVERFLOW=y
> (it can be exposed with an allyesconfig/allmodconfig on mainline
> currently).

Yah, I can trigger with that, thanks.

But I'll be damned, check this out:

clang preprocesses to this:

 do { extern void __compiletime_assert_332(void) ; if (!(!(p4d_index((-68 * ((1UL) << 30))) != p4d_index((0xffffffffff000000UL))))) __compiletime_assert_332(); } while (0);

The resulting asm is:

.LBB1_32:
        movabsq $-73014444032, %r13     # imm = 0xFFFFFFEF00000000
        testb   $1, %al
        jne     .LBB1_33
.LBB1_34:
        xorl    %r14d, %ebx
        testl   $33554431, %ebx         # imm = 0x1FFFFFF
        je      .LBB1_36
# %bb.35:
        callq   __compiletime_assert_332

so the undefined symbol is there, leading to:

ld: arch/x86/platform/efi/efi_64.o: in function `efi_sync_low_kernel_mappings':
/home/boris/kernel/linux/arch/x86/platform/efi/efi_64.c:140: undefined reference to `__compiletime_assert_332'

Now look at gcc:

It preprocesses to:

 do { extern void __compiletime_assert_332(void) __attribute__((__error__("BUILD_BUG_ON failed: " "p4d_index(EFI_VA_END) != p4d_index(MODULES_END)"))); if (!(!(p4d_index((-68 * ((1UL) << 30))) != p4d_index((0xffffffffff000000UL))))) __compiletime_assert_332(); } while (0);


Resulting asm:

$ grep __compiletime_assert_332  arch/x86/platform/efi/efi_64.s
$

That thing has been optimized away!

Which means, those build assertions are gone on gcc and they don't catch
diddly squat. I sure hope I'm missing something here...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette