ARM: zImage: make sure no GOTOFF relocs are used with .bss symbols

To be able to relocate the .bss section at run time independently from
the rest of the code, we must make sure that no GOTOFF relocations are
used with .bss symbols.  This usually means that no global variables can
be marked static unless they're also const.

To enforce this, suffice to fail the build whenever a private symbol
is allocated to .bss and list those symbols for convenience.

The user_stack and user_stack_end labels in head.S were converted into
non exported symbols to remove false positives.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Tony Lindgren <tony@atomide.com>

2 files changed, 17 insertions, 4 deletions

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 79b5c62746df..23aad0722303 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -120,10 +120,23 @@ lib1funcs = $(obj)/lib1funcs.o
 $(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S FORCE
         $(call cmd,shipped)
 
+# We need to prevent any GOTOFF relocs being used with references
+# to symbols in the .bss section since we cannot relocate them
+# independently from the rest at run time.  This can be achieved by
+# ensuring that no private .bss symbols exist, as global symbols
+# always have a GOT entry which is what we need.
+# The .data section is already discarded by the linker script so no need
+# to bother about it here.
+check_for_bad_syms = \
+bad_syms=$$($(CROSS_COMPILE)nm $@ | sed -n 's/^.\{8\} [bc] \(.*\)/\1/p') && \
+[ -z "$$bad_syms" ] || \
+  ( echo "following symbols must have non local/private scope:" >&2; \
+    echo "$$bad_syms" >&2; rm -f $@; false )
+
 $(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
                 $(addprefix $(obj)/, $(OBJS)) $(lib1funcs) FORCE
         $(call if_changed,ld)
-        @:
+        @$(check_for_bad_syms)
 
 $(obj)/piggy.$(suffix_y): $(obj)/../Image FORCE
         $(call if_changed,$(suffix_y))
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index b541217bd040..8d5d91aefbb9 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -372,7 +372,7 @@ LC0:		.word	LC0			@ r1
                 .word   input_data_end - 4      @ r10 (inflated size location)
                 .word   _got_start              @ r11
                 .word   _got_end                @ ip
-                .word   user_stack_end          @ sp
+                .word   .L_user_stack_end       @ sp
                 .size   LC0, . - LC0
 
 #ifdef CONFIG_ARCH_RPC
@@ -1100,5 +1100,5 @@ reloc_code_end:
 
                 .align
                 .section ".stack", "aw", %nobits
-user_stack:     .space  4096
-user_stack_end:
+.L_user_stack:  .space  4096
+.L_user_stack_end: