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Revision 99f857db8857aff691c51302f93648263ed07eb1 authored by David Woodhouse on 10 January 2013, 14:31:59 UTC, committed by H. Peter Anvin on 28 January 2013, 04:19:37 UTC 
x86, build: Dynamically find entry points in compressed startup code
 
We have historically hard-coded entry points in head.S just so it's easy
to build the executable/bzImage headers with references to them.

Unfortunately, this leads to boot loaders abusing these "known" addresses
even when they are *explicitly* told that they "should look at the ELF
header to find this address, as it may change in the future". And even
when the address in question *has* actually been changed in the past,
without fanfare or thought to compatibility.

Thus we have bootloaders doing stunningly broken things like jumping
to offset 0x200 in the kernel startup code in 64-bit mode, *hoping*
that startup_64 is still there (it has moved at least once
before). And hoping that it's actually a 64-bit kernel despite the
fact that we don't give them any indication of that fact.

This patch should hopefully remove the temptation to abuse internal
addresses in future, where sternly worded comments have not sufficed.
Instead of having hard-coded addresses and saying "please don't abuse
these", we actually pull the addresses out of the ELF payload into
zoffset.h, and make build.c shove them back into the right places in
the bzImage header.

Rather than including zoffset.h into build.c and thus having to rebuild
the tool for every kernel build, we parse it instead. The parsing code
is small and simple.

This patch doesn't actually move any of the interesting entry points, so
any offending bootloader will still continue to "work" after this patch
is applied. For some version of "work" which includes jumping into the
compressed payload and crashing, if the bzImage it's given is a 32-bit
kernel. No change there then.

[ hpa: some of the issues in the description are addressed or
  retconned by the 2.12 boot protocol.  This patch has been edited to
  only remove fixed addresses that were *not* thus retconned. ]

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Link: http://lkml.kernel.org/r/1358513837.2397.247.camel@shinybook.infradead.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Matt Fleming <matt.fleming@intel.com>
1 parent b607e21
Raw File
Kbuild 
#
# Kbuild for top-level directory of the kernel
# This file takes care of the following:
# 1) Generate bounds.h
# 2) Generate asm-offsets.h (may need bounds.h)
# 3) Check for missing system calls

#####
# 1) Generate bounds.h

bounds-file := include/generated/bounds.h

always  := $(bounds-file)
targets := $(bounds-file) kernel/bounds.s

quiet_cmd_bounds = GEN     $@
define cmd_bounds
	(set -e; \
	 echo "#ifndef __LINUX_BOUNDS_H__"; \
	 echo "#define __LINUX_BOUNDS_H__"; \
	 echo "/*"; \
	 echo " * DO NOT MODIFY."; \
	 echo " *"; \
	 echo " * This file was generated by Kbuild"; \
	 echo " *"; \
	 echo " */"; \
	 echo ""; \
	 sed -ne $(sed-y) $<; \
	 echo ""; \
	 echo "#endif" ) > $@
endef

# We use internal kbuild rules to avoid the "is up to date" message from make
kernel/bounds.s: kernel/bounds.c FORCE
	$(Q)mkdir -p $(dir $@)
	$(call if_changed_dep,cc_s_c)

$(obj)/$(bounds-file): kernel/bounds.s Kbuild
	$(Q)mkdir -p $(dir $@)
	$(call cmd,bounds)

#####
# 2) Generate asm-offsets.h
#

offsets-file := include/generated/asm-offsets.h

always  += $(offsets-file)
targets += $(offsets-file)
targets += arch/$(SRCARCH)/kernel/asm-offsets.s


# Default sed regexp - multiline due to syntax constraints
define sed-y
	"/^->/{s:->#\(.*\):/* \1 */:; \
	s:^->\([^ ]*\) [\$$#]*\([-0-9]*\) \(.*\):#define \1 \2 /* \3 */:; \
	s:^->\([^ ]*\) [\$$#]*\([^ ]*\) \(.*\):#define \1 \2 /* \3 */:; \
	s:->::; p;}"
endef

quiet_cmd_offsets = GEN     $@
define cmd_offsets
	(set -e; \
	 echo "#ifndef __ASM_OFFSETS_H__"; \
	 echo "#define __ASM_OFFSETS_H__"; \
	 echo "/*"; \
	 echo " * DO NOT MODIFY."; \
	 echo " *"; \
	 echo " * This file was generated by Kbuild"; \
	 echo " *"; \
	 echo " */"; \
	 echo ""; \
	 sed -ne $(sed-y) $<; \
	 echo ""; \
	 echo "#endif" ) > $@
endef

# We use internal kbuild rules to avoid the "is up to date" message from make
arch/$(SRCARCH)/kernel/asm-offsets.s: arch/$(SRCARCH)/kernel/asm-offsets.c \
                                      $(obj)/$(bounds-file) FORCE
	$(Q)mkdir -p $(dir $@)
	$(call if_changed_dep,cc_s_c)

$(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s Kbuild
	$(call cmd,offsets)

#####
# 3) Check for missing system calls
#

always += missing-syscalls
targets += missing-syscalls

quiet_cmd_syscalls = CALL    $<
      cmd_syscalls = $(CONFIG_SHELL) $< $(CC) $(c_flags) $(missing_syscalls_flags)

missing-syscalls: scripts/checksyscalls.sh $(offsets-file) FORCE
	$(call cmd,syscalls)

# Keep these two files during make clean
no-clean-files := $(bounds-file) $(offsets-file)
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