ARM: zImage: make sure appended DTB doesn't get overwritten by kernel .bss

The appended DTB gets relocated with the decompressor code to get out
of the way of the decompressed kernel.  However the kernel's .bss section
may be larger than the relocated code and data, and then the DTB gets
overwritten.  Let's make sure the relocation takes care of moving zImage
far enough so no such conflict with .bss occurs.

Thanks to Tony Lindgren <tony@atomide.com> for figuring out this issue.

While at it, let's clean up the code a bit so that the wont_overwrite
symbol is used while determining if a conflict exists, making the above
change more precise as well as eliminating some ARM/THUMB alternates.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dave Martin <dave.martin@linaro.org>
Tested-by: Thomas Abraham <thomas.abraham@linaro.org>

2 files changed, 18 insertions, 4 deletions

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 0c74a6fab952..4867647b9796 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -104,6 +104,9 @@ endif
 ccflags-y := -fpic -fno-builtin
 asflags-y := -Wa,-march=all
 
+# Supply kernel BSS size to the decompressor via a linker symbol.
+KBSS_SZ = $(shell size $(obj)/../../../../vmlinux | awk 'END{print $$3}')
+LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)
 # Supply ZRELADDR to the decompressor via a linker symbol.
 ifneq ($(CONFIG_AUTO_ZRELADDR),y)
 LDFLAGS_vmlinux += --defsym zreladdr=$(ZRELADDR)
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 3ce5738ddb98..ba5c552f8c69 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -248,6 +248,18 @@ restart:	adr	r0, LC0
 
                 mov     r8, r6                  @ use the appended device tree
 
+                /*
+                 * Make sure that the DTB doesn't end up in the final
+                 * kernel's .bss area. To do so, we adjust the decompressed
+                 * kernel size to compensate if that .bss size is larger
+                 * than the relocated code.
+                 */
+                ldr     r5, =_kernel_bss_size
+                adr     r1, wont_overwrite
+                sub     r1, r6, r1
+                subs    r1, r5, r1
+                addhi   r9, r9, r1
+
                 /* Get the dtb's size */
                 ldr     r5, [r6, #4]
 #ifndef __ARMEB__
@@ -276,15 +288,14 @@ dtb_check_done:
  *   r10 = end of this image, including  bss/stack/malloc space if non XIP
  * We basically want:
  *   r4 - 16k page directory >= r10 -> OK
- *   r4 + image length <= current position (pc) -> OK
+ *   r4 + image length <= address of wont_overwrite -> OK
  */
                 add     r10, r10, #16384
                 cmp     r4, r10
                 bhs     wont_overwrite
                 add     r10, r4, r9
-   ARM(         cmp     r10, pc         )
- THUMB(         mov     lr, pc          )
- THUMB(         cmp     r10, lr         )
+                adr     r9, wont_overwrite
+                cmp     r10, r9
                 bls     wont_overwrite
 
 /*