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
Date: Fri, 15 Jan 2021 20:07:29 +0100 [thread overview]
Message-ID: <20210115190729.GE9138@zn.tnic> (raw)
In-Reply-To: <20210115183203.GA1991122@ubuntu-m3-large-x86>
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
next prev parent reply other threads:[~2021-01-15 19:08 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 22:34 [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index Arnd Bergmann
2021-01-07 22:42 ` Nathan Chancellor
2021-01-13 17:51 ` Ard Biesheuvel
2021-01-15 18:23 ` Borislav Petkov
2021-01-15 18:32 ` Nathan Chancellor
2021-01-15 19:07 ` Borislav Petkov [this message]
2021-01-15 19:11 ` Arvind Sankar
2021-01-15 19:18 ` Borislav Petkov
2021-01-15 19:54 ` Arnd Bergmann
2021-01-15 20:12 ` Arvind Sankar
2021-01-15 20:32 ` Arvind Sankar
2021-01-15 19:07 ` Arvind Sankar
2021-01-15 20:27 ` Arvind Sankar
2021-01-16 16:34 ` Ard Biesheuvel
2021-01-18 20:24 ` Borislav Petkov
2021-01-18 21:42 ` Arvind Sankar
2021-01-20 9:33 ` Ard Biesheuvel
2021-01-20 11:44 ` Borislav Petkov
2021-02-03 18:51 ` Nathan Chancellor
2021-02-03 20:29 ` Ard Biesheuvel
2021-02-04 10:51 ` Borislav Petkov
2021-02-04 10:59 ` Ard Biesheuvel
2021-02-04 19:16 ` Nathan Chancellor
2021-02-04 21:43 ` Arvind Sankar
2021-02-04 22:13 ` Borislav Petkov
2021-02-05 0:08 ` Arvind Sankar
2021-02-05 11:39 ` [PATCH] x86/efi: Remove EFI PGD build time checks Borislav Petkov
2021-02-05 11:57 ` Ard Biesheuvel
2021-02-05 18:14 ` Nick Desaulniers
2021-02-05 18:56 ` Nathan Chancellor
2021-02-05 10:34 ` [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index Borislav Petkov
2021-02-05 18:27 ` Nick Desaulniers
2021-02-05 18:31 ` Nathan Chancellor
2021-01-20 11:26 ` Kirill A. Shutemov
2021-01-20 11:06 ` Kirill A. Shutemov
2021-02-06 12:56 ` [tip: x86/urgent] x86/efi: Remove EFI PGD build time checks tip-bot2 for Borislav Petkov
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