[PATCH v3 11/21] arm64: avoid R_AARCH64_ABS64 relocations for Image header fields

On 13 January 2016 at 19:48, Ard Biesheuvel [off-list ref] wrote:

On 13 January 2016 at 19:12, Mark Rutland [off-list ref] wrote:

quoted

On Mon, Jan 11, 2016 at 02:19:04PM +0100, Ard Biesheuvel wrote:

quoted

Unfortunately, the current way of using the linker to emit build time
constants into the Image header will no longer work once we switch to
the use of PIE executables. The reason is that such constants are emitted
into the binary using R_AARCH64_ABS64 relocations, which we will resolve
at runtime, not at build time, and the places targeted by those
relocations will contain zeroes before that.

So move back to assembly time constants or R_AARCH64_ABS32 relocations
(which, interestingly enough, do get resolved at build time)

To me it seems very odd that ABS64 and ABS32 are treated differently,
and it makes me somewhat uncomfortable becuase it feels like a bug.

Do we know whether the inconsistency between ABS64 and ABS32 was
deliberate?

I couldn't spot anything declaring a difference in the AArch64 ELF
spec, and I'm not sure where else to look.

My assumption is that PIE only defers resolving R_AARCH64_ABS64
relocations since those are the only ones that can be used to refer to
memory addresses

OK, digging into the binutils source code, it turns out that indeed,
ABSnn relocations where nn equals the ELFnn memory size are treated
differently, but only if they have default visibility. This is simply
a result of the fact the code path is shared between shared libraries
and PIE executables, since PIE executable are fully linked. It also
means that we can simply work around it by emitting the linker symbols
as hidden.

quoted

quoted

Signed-off-by: Ard Biesheuvel <redacted>
---
 arch/arm64/include/asm/assembler.h | 15 ++++++++
 arch/arm64/kernel/head.S           | 17 +++++++--
 arch/arm64/kernel/image.h          | 37 ++++++--------------
 3 files changed, 40 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index d8bfcc1ce923..e211af783a3d 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h

@@ -222,4 +222,19 @@ lr       .req    x30             // link register
      .size   __pi_##x, . - x;        \
      ENDPROC(x)

+     .macro  le16, val
+     .byte   \val & 0xff
+     .byte   (\val >> 8) & 0xff
+     .endm
+
+     .macro  le32, val
+     le16    \val
+     le16    (\val >> 16)
+     .endm
+
+     .macro  le64, val
+     le32    \val
+     le32    (\val >> 32)
+     .endm
+
 #endif       /* __ASM_ASSEMBLER_H */

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 350515276541..211f75e673f4 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S

@@ -51,6 +51,17 @@
 #define KERNEL_START _text
 #define KERNEL_END   _end

+#ifdef CONFIG_CPU_BIG_ENDIAN
+#define __HEAD_FLAG_BE       1
+#else
+#define __HEAD_FLAG_BE       0
+#endif
+
+#define __HEAD_FLAG_PAGE_SIZE ((PAGE_SHIFT - 10) / 2)
+
+#define __HEAD_FLAGS ((__HEAD_FLAG_BE << 0) |        \
+                      (__HEAD_FLAG_PAGE_SIZE << 1))
+
 /*
  * Kernel startup entry point.
  * ---------------------------

@@ -83,9 +94,9 @@ efi_head:
      b       stext                           // branch to kernel start, magic
      .long   0                               // reserved
 #endif
-     .quad   _kernel_offset_le               // Image load offset from start of RAM, little-endian
-     .quad   _kernel_size_le                 // Effective size of kernel image, little-endian
-     .quad   _kernel_flags_le                // Informative flags, little-endian
+     le64    TEXT_OFFSET                     // Image load offset from start of RAM, little-endian
+     .long   _kernel_size_le, 0              // Effective size of kernel image, little-endian
+     le64    __HEAD_FLAGS                    // Informative flags, little-endian
      .quad   0                               // reserved
      .quad   0                               // reserved
      .quad   0                               // reserved

diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
index bc2abb8b1599..bb6b0e69d0a4 100644
--- a/arch/arm64/kernel/image.h
+++ b/arch/arm64/kernel/image.h

@@ -26,41 +26,26 @@
  * There aren't any ELF relocations we can use to endian-swap values known only
  * at link time (e.g. the subtraction of two symbol addresses), so we must get
  * the linker to endian-swap certain values before emitting them.
+ * Note that this will not work for 64-bit values: these are resolved using
+ * R_AARCH64_ABS64 relocations, which are fixed up at runtime rather than at
+ * build time when building the PIE executable (for KASLR).
  */
 #ifdef CONFIG_CPU_BIG_ENDIAN
-#define DATA_LE64(data)                                      \
-     ((((data) & 0x00000000000000ff) << 56) |        \
-      (((data) & 0x000000000000ff00) << 40) |        \
-      (((data) & 0x0000000000ff0000) << 24) |        \
-      (((data) & 0x00000000ff000000) << 8)  |        \
-      (((data) & 0x000000ff00000000) >> 8)  |        \
-      (((data) & 0x0000ff0000000000) >> 24) |        \
-      (((data) & 0x00ff000000000000) >> 40) |        \
-      (((data) & 0xff00000000000000) >> 56))
+#define DATA_LE32(data)                              \
+     ((((data) & 0x000000ff) << 24) |        \
+      (((data) & 0x0000ff00) << 8)  |        \
+      (((data) & 0x00ff0000) >> 8)  |        \
+      (((data) & 0xff000000) >> 24))
 #else
-#define DATA_LE64(data) ((data) & 0xffffffffffffffff)
+#define DATA_LE32(data) ((data) & 0xffffffff)
 #endif

-#ifdef CONFIG_CPU_BIG_ENDIAN
-#define __HEAD_FLAG_BE       1
-#else
-#define __HEAD_FLAG_BE       0
-#endif
-
-#define __HEAD_FLAG_PAGE_SIZE ((PAGE_SHIFT - 10) / 2)
-
-#define __HEAD_FLAGS ((__HEAD_FLAG_BE << 0) |        \
-                      (__HEAD_FLAG_PAGE_SIZE << 1))
-
 /*
  * These will output as part of the Image header, which should be little-endian
- * regardless of the endianness of the kernel. While constant values could be
- * endian swapped in head.S, all are done here for consistency.
+ * regardless of the endianness of the kernel.
  */
 #define HEAD_SYMBOLS                                         \
-     _kernel_size_le         = DATA_LE64(_end - _text);      \
-     _kernel_offset_le       = DATA_LE64(TEXT_OFFSET);       \
-     _kernel_flags_le        = DATA_LE64(__HEAD_FLAGS);
+     _kernel_size_le         = DATA_LE32(_end - _text);

 #ifdef CONFIG_EFI

--

2.5.0