[POWERPC] Discourage people from fiddling with kernel data from prom_init

As BenH said the other day, it is an "accident" that prom_init.o is
linked with the rest of the kernel.  The truth is a little more
subtle, prom_init isn't truly bootloader, it does access kernel data
in a few places.

What we can do is discourage people from adding new code that accesses
data outside of prom_init.  And hence this patch; from the script:

 # This script checks prom_init.o to see what external symbols it
 # is using, if it finds symbols not in the whitelist it returns
 # an error. The point of this is to discourage people from
 # intentionally or accidentally adding new code to prom_init.c
 # which has side effects on other parts of the kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index ce1e8d24e747c3ee56c19793700a5721df64c8b0..9177b21b1a954a17b531fc1884e6b8bc61510cb1 100644 (file)
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -106,4 +106,13 @@ PHONY += systbl_chk
 systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
        $(call cmd,systbl_chk)
 
+$(obj)/built-in.o:             prom_init_check
+
+quiet_cmd_prom_init_check = CALL    $<
+      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" "$(obj)/prom_init.o"
+
+PHONY += prom_init_check
+prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
+       $(call cmd,prom_init_check)
+
 clean-files := vmlinux.lds

diff --git a/arch/powerpc/kernel/prom_init_check.sh b/arch/powerpc/kernel/prom_init_check.sh
new file mode 100644 (file)
index 0000000..8e24fc1
--- /dev/null
+++ b/arch/powerpc/kernel/prom_init_check.sh
@@ -0,0 +1,58 @@
+#!/bin/sh
+#
+# Copyright © 2008 IBM Corporation
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version
+# 2 of the License, or (at your option) any later version.
+
+# This script checks prom_init.o to see what external symbols it
+# is using, if it finds symbols not in the whitelist it returns
+# an error. The point of this is to discourage people from
+# intentionally or accidentally adding new code to prom_init.c
+# which has side effects on other parts of the kernel.
+
+# If you really need to reference something from prom_init.o add
+# it to the list below:
+
+WHITELIST="add_reloc_offset __bss_start __bss_stop copy_and_flush
+_end enter_prom memcpy memset reloc_offset __secondary_hold
+__secondary_hold_acknowledge __secondary_hold_spinloop __start
+strcmp strcpy strlcpy strlen strncmp strstr logo_linux_clut224
+reloc_got2"
+
+NM="$1"
+OBJ="$2"
+
+ERROR=0
+
+for UNDEF in $($NM -u $OBJ | awk '{print $2}')
+do
+       # On 64-bit nm gives us the function descriptors, which have
+       # a leading . on the name, so strip it off here.
+       UNDEF="${UNDEF#.}"
+
+       if [ $KBUILD_VERBOSE ]; then
+               if [ $KBUILD_VERBOSE -ne 0 ]; then
+                       echo "Checking prom_init.o symbol '$UNDEF'"
+               fi
+       fi
+
+       OK=0
+       for WHITE in $WHITELIST
+       do
+               if [ "$UNDEF" = "$WHITE" ]; then
+                       OK=1
+                       break
+               fi
+       done
+
+       if [ $OK -eq 0 ]; then
+               ERROR=1
+               echo "Error: External symbol '$UNDEF' referenced" \
+                    "from prom_init.c" >&2
+       fi
+done
+
+exit $ERROR