Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

[Arvind Sankar]

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:
> > 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.