[RFT PATCH] arm64: relocatable: build the kernel as a proper shared library

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

readelf complains about the section layout of vmlinux when building
with CONFIG_RELOCATABLE=y (for KASLR):

  readelf: Warning: [21]: Link field (0) should index a symtab section.
  readelf: Warning: [21]: Info field (0) should index a relocatable section.

Also, it seems that our use of '-pie -shared' is contradictory, and
thus ambiguous. In general, the way KASLR is wired up at the moment
is highly tailored to how ld.bfd happens to implement (and conflate)
PIE executables and shared libraries, so given the current effort to
support other toolchains, let's fix some of these issues as well.

- Drop the -pie linker argument and just leave -shared. In ld.bfd,
  the differences between them are unclear (except for the ELF type
  of the produced image [0]) but lld chokes on seeing both at the
  same time.

- Rename the .rela output section to .rela.dyn, as is customary for
  shared libraries and PIE executables.

- Don't discard the .dynamic, .dynsym, .dynstr and .hash sections.
  Instead, make sure that they are [mostly] empty by marking all
  symbols as local, and emit them into the .init segment.

These changes only affect the ELF image, and produce the same binary
image, with the exception of a couple of bytes of .init data for the
empty .dynsym and .dynstr sections.

[0] b9dce7f1ba01 ("arm64: kernel: force ET_DYN ELF type for ...")

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Smith <peter.smith@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/Makefile             |  2 +-
 arch/arm64/kernel/vmlinux.lds.S | 30 ++++++++++++++++++++++++------
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 6cb9fc7e9382..7221494bcf60 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -18,7 +18,7 @@ ifeq ($(CONFIG_RELOCATABLE), y)
 # Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour
 # for relative relocs, since this leads to better Image compression
 # with the relocation offsets always being zero.
-LDFLAGS_vmlinux		+= -pie -shared -Bsymbolic \
+LDFLAGS_vmlinux		+= -shared -Bsymbolic \
 			$(call ld-option, --no-apply-dynamic-relocs)
 endif
 
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 03b00007553d..9ba4016090b1 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -98,8 +98,6 @@ SECTIONS
 		EXIT_CALL
 		*(.discard)
 		*(.discard.*)
-		*(.interp .dynamic)
-		*(.dynsym .dynstr .hash)
 	}
 
 	. = KIMAGE_VADDR + TEXT_OFFSET;
@@ -192,12 +190,25 @@ SECTIONS
 
 	PERCPU_SECTION(L1_CACHE_BYTES)
 
-	.rela : ALIGN(8) {
+#ifdef CONFIG_RELOCATABLE
+	.rela.dyn : ALIGN(8) {
 		*(.rela .rela*)
 	}
-
-	__rela_offset	= ABSOLUTE(ADDR(.rela) - KIMAGE_VADDR);
-	__rela_size	= SIZEOF(.rela);
+	.dynamic : {
+		*(.dynamic)
+	}
+	.dynsym : ALIGN(8) {
+		*(.dynsym)
+	}
+	.dynstr : {
+		*(.dynstr)
+	}
+	.hash : {
+		*(.hash)
+	}
+	__rela_offset	= ABSOLUTE(ADDR(.rela.dyn) - KIMAGE_VADDR);
+	__rela_size	= SIZEOF(.rela.dyn);
+#endif
 
 	. = ALIGN(SEGMENT_ALIGN);
 	__initdata_end = .;
@@ -244,6 +255,13 @@ SECTIONS
 	HEAD_SYMBOLS
 }
 
+#ifdef CONFIG_RELOCATABLE
+VERSION {
+	/* mark all symbols as local so they are not listed in .dynsym */
+	{ local: *; };
+}
+#endif
+
 /*
  * The HYP init code and ID map text can't be longer than a page each,
  * and should not cross a page boundary.
-- 
2.19.2


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