From: Ingo Molnar <mingo@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org,
Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Arvind Sankar <nivedita@alum.mit.edu>
Subject: Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references
Date: Sun, 24 May 2020 17:12:14 +0200 [thread overview]
Message-ID: <20200524151214.GB11617@gmail.com> (raw)
In-Reply-To: <20200523120021.34996-3-ardb@kernel.org>
* Ard Biesheuvel <ardb@kernel.org> wrote:
> Eliminate all GOT entries in the decompressor binary, by forcing hidden
> visibility for all symbol references, which informs the compiler that
> such references will be resolved at link time without the need for
> allocating GOT entries.
>
> To ensure that no GOT entries will creep back in, add an assertion to
> the decompressor linker script that will fire if the .got section has
> a non-zero size.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/boot/compressed/Makefile | 1 +
> arch/x86/boot/compressed/hidden.h | 19 +++++++++++++++++++
> arch/x86/boot/compressed/vmlinux.lds.S | 1 +
> 3 files changed, 21 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 5f7c262bcc99..aa9ed814e5fa 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -40,6 +40,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> KBUILD_CFLAGS += -Wno-pointer-sign
> KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> +KBUILD_CFLAGS += -include hidden.h
> + * When building position independent code with GCC using the -fPIC option,
> + * (or even the -fPIE one on older versions), it will assume that we are
> + * building a dynamic object (either a shared library or an executable) that
> + * may have symbol references that can only be resolved at load time. For a
> + * variety of reasons (ELF symbol preemption, the CoW footprint of the section
> + * that is modified by the loader), this results in all references to symbols
> + * with external linkage to go via entries in the Global Offset Table (GOT),
> + * which carries absolute addresses which need to be fixed up when the
> + * executable image is loaded at an offset which is different from its link
> + * time offset.
> + *
> + * Fortunately, there is a way to inform the compiler that such symbol
> + * references will be satisfied at link time rather than at load time, by
> + * giving them 'hidden' visibility.
> + */
> +
> +#pragma GCC visibility push(hidden)
BTW., how many such GOT entries did we have before this change, on a typical kernel?
> +ASSERT (SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
s/ASSERT (
/ASSERT(
Thanks,
Ingo
next prev parent reply other threads:[~2020-05-24 15:12 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-23 12:00 [PATCH v2 0/3] x86/boot: get rid of GOT entries and associated fixup code Ard Biesheuvel
2020-05-23 12:00 ` [PATCH v2 1/3] x86/boot/compressed: move .got.plt entries out of the .got section Ard Biesheuvel
2020-05-24 15:08 ` Ingo Molnar
2020-05-24 15:11 ` Ard Biesheuvel
2020-05-24 15:14 ` Ingo Molnar
2020-05-23 12:00 ` [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references Ard Biesheuvel
2020-05-24 15:12 ` Ingo Molnar [this message]
2020-05-24 15:15 ` Ard Biesheuvel
2020-05-27 14:36 ` Arvind Sankar
2020-05-27 18:29 ` Brian Gerst
2020-05-27 18:30 ` Ard Biesheuvel
2020-05-28 7:46 ` Ard Biesheuvel
2020-05-23 12:00 ` [PATCH v2 3/3] x86/boot/compressed: get rid of GOT fixup code Ard Biesheuvel
2020-05-23 15:17 ` Arvind Sankar
2020-05-24 8:42 ` Ard Biesheuvel
2020-05-23 15:16 ` [PATCH v2 0/3] x86/boot: get rid of GOT entries and associated " Arvind Sankar
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=20200524151214.GB11617@gmail.com \
--to=mingo@kernel.org \
--cc=ardb@kernel.org \
--cc=linux-efi@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=maarten.lankhorst@linux.intel.com \
--cc=nivedita@alum.mit.edu \
--cc=torvalds@linux-foundation.org \
--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).