On Fri, Jan 15, 2021 at 08:07:29PM +0100, Borislav Petkov wrote:

On Fri, Jan 15, 2021 at 11:32:03AM -0700, Nathan Chancellor wrote:

quoted

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...

That's how build-time assertions work: they are _supposed_ to be
optimized away completely when the assertion is true. If they're
_not_ optimized away, the build will fail.