[PATCH v3 2/7] x86/boot/compressed: Force hidden visibility for all symbol references

[Arvind Sankar]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

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.

[Arvind: fixup -include hidden.h to -include $(srctree)/$(src)/hidden.h]

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Acked-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
From: Ard Biesheuvel <ardb@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20200523120021.34996-3-ardb@xxxxxxxxxx
---
 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(+)
 create mode 100644 arch/x86/boot/compressed/hidden.h

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 7619742f91c9..b01c8aed0f23 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -42,6 +42,7 @@ 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 $(srctree)/$(src)/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 b17d218ccdf9..4bcc943842ab 100644
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -81,6 +81,7 @@ SECTIONS
 	DISCARDS
 }
 
+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!")
 #else
-- 
2.26.2