Linux-EFI Archive on lore.kernel.org
 help / color / Atom feed
From: Ard Biesheuvel <ardb@kernel.org>
To: linux-efi@vger.kernel.org
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Arvind Sankar <nivedita@alum.mit.edu>
Subject: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references
Date: Sat, 23 May 2020 14:00:20 +0200
Message-ID: <20200523120021.34996-3-ardb@kernel.org> (raw)
In-Reply-To: <20200523120021.34996-1-ardb@kernel.org>

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
 
 KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
 GCOV_PROFILE := n
diff --git a/arch/x86/boot/compressed/hidden.h b/arch/x86/boot/compressed/hidden.h
new file mode 100644
index 000000000000..49a17b6b5962
--- /dev/null
+++ b/arch/x86/boot/compressed/hidden.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * 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)
diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
index ce3fdfb93b57..60a99dfb9d72 100644
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -79,6 +79,7 @@ SECTIONS
 	_end = .;
 }
 
+ASSERT (SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
 #ifdef CONFIG_X86_64
 ASSERT (SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18,
 	"Unexpected GOT/PLT entries detected!")
-- 
2.20.1


  parent reply index

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 ` Ard Biesheuvel [this message]
2020-05-24 15:12   ` [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references Ingo Molnar
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=20200523120021.34996-3-ardb@kernel.org \
    --to=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

Linux-EFI Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-efi/0 linux-efi/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-efi linux-efi/ https://lore.kernel.org/linux-efi \
		linux-efi@vger.kernel.org
	public-inbox-index linux-efi

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-efi


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git