From: Arvind Sankar <nivedita@alum.mit.edu>
To: Borislav Petkov <bp@alien8.de>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
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 14:11:25 -0500 [thread overview]
Message-ID: <YAHo3ZEMu+6mESZA@rani.riverdale.lan> (raw)
In-Reply-To: <20210115190729.GE9138@zn.tnic>
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.
next prev parent reply other threads:[~2021-01-15 19:12 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
2021-01-15 19:11 ` Arvind Sankar [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=YAHo3ZEMu+6mESZA@rani.riverdale.lan \
--to=nivedita@alum.mit.edu \
--cc=andy@infradead.org \
--cc=ardb@kernel.org \
--cc=arnd@arndb.de \
--cc=arnd@kernel.org \
--cc=bp@alien8.de \
--cc=clang-built-linux@googlegroups.com \
--cc=dvhart@infradead.org \
--cc=hpa@zytor.com \
--cc=linux-efi@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=natechancellor@gmail.com \
--cc=ndesaulniers@google.com \
--cc=platform-driver-x86@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).