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s390: Compile kernel with -fPIC and link with -no-pie
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When the kernel is built with CONFIG_PIE_BUILD option enabled it
uses dynamic symbols, for which the linker does not allow more
than 64K number of entries. This can break features like kpatch.

Hence, whenever possible the kernel is built with CONFIG_PIE_BUILD
option disabled. For that support of unaligned symbols generated by
linker scripts in the compiler is necessary.

However, older compilers might lack such support. In that case the
build process resorts to CONFIG_PIE_BUILD option-enabled build.

Compile object files with -fPIC option and then link the kernel
binary with -no-pie linker option.

As result, the dynamic symbols are not generated and not only kpatch
feature succeeds, but also the whole CONFIG_PIE_BUILD option-enabled
code could be dropped.

[ agordeev: Reworded the commit message ]

Suggested-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
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Sumanth Korikkar authored and Alexander Gordeev committed Apr 29, 2024
1 parent 5f90003 commit 00cda11
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Showing 7 changed files with 3 additions and 108 deletions.
12 changes: 0 additions & 12 deletions arch/s390/Kconfig
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Original file line number Diff line number Diff line change
Expand Up @@ -593,18 +593,6 @@ config RELOCATABLE
Note: this option exists only for documentation purposes, please do
not remove it.

config PIE_BUILD
def_bool CC_IS_CLANG && !$(cc-option,-munaligned-symbols)
help
If the compiler is unable to generate code that can manage unaligned
symbols, the kernel is linked as a position-independent executable
(PIE) and includes dynamic relocations that are processed early
during bootup.

For kpatch functionality, it is recommended to build the kernel
without the PIE_BUILD option. PIE_BUILD is only enabled when the
compiler lacks proper support for handling unaligned symbols.

config RANDOMIZE_BASE
bool "Randomize the address of the kernel image (KASLR)"
default y
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9 changes: 2 additions & 7 deletions arch/s390/Makefile
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Expand Up @@ -14,14 +14,9 @@ KBUILD_AFLAGS_MODULE += -fPIC
KBUILD_CFLAGS_MODULE += -fPIC
KBUILD_AFLAGS += -m64
KBUILD_CFLAGS += -m64
ifdef CONFIG_PIE_BUILD
KBUILD_CFLAGS += -fPIE
LDFLAGS_vmlinux := -pie -z notext
else
KBUILD_CFLAGS += $(call cc-option,-munaligned-symbols,)
LDFLAGS_vmlinux := --emit-relocs --discard-none
KBUILD_CFLAGS += -fPIC
LDFLAGS_vmlinux := -no-pie --emit-relocs --discard-none
extra_tools := relocs
endif
aflags_dwarf := -Wa,-gdwarf-2
KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
ifndef CONFIG_AS_IS_LLVM
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7 changes: 1 addition & 6 deletions arch/s390/boot/Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -37,8 +37,7 @@ CFLAGS_sclp_early_core.o += -I$(srctree)/drivers/s390/char

obj-y := head.o als.o startup.o physmem_info.o ipl_parm.o ipl_report.o vmem.o
obj-y += string.o ebcdic.o sclp_early_core.o mem.o ipl_vmparm.o cmdline.o
obj-y += version.o pgm_check_info.o ctype.o ipl_data.o
obj-y += $(if $(CONFIG_PIE_BUILD),machine_kexec_reloc.o,relocs.o)
obj-y += version.o pgm_check_info.o ctype.o ipl_data.o relocs.o
obj-$(findstring y, $(CONFIG_PROTECTED_VIRTUALIZATION_GUEST) $(CONFIG_PGSTE)) += uv.o
obj-$(CONFIG_RANDOMIZE_BASE) += kaslr.o
obj-y += $(if $(CONFIG_KERNEL_UNCOMPRESSED),,decompressor.o) info.o
Expand All @@ -49,9 +48,7 @@ targets := bzImage section_cmp.boot.data section_cmp.boot.preserved.data $(obj-y
targets += vmlinux.lds vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2
targets += vmlinux.bin.xz vmlinux.bin.lzma vmlinux.bin.lzo vmlinux.bin.lz4
targets += vmlinux.bin.zst info.bin syms.bin vmlinux.syms $(obj-all)
ifndef CONFIG_PIE_BUILD
targets += relocs.S
endif

OBJECTS := $(addprefix $(obj)/,$(obj-y))
OBJECTS_ALL := $(addprefix $(obj)/,$(obj-all))
Expand Down Expand Up @@ -110,13 +107,11 @@ OBJCOPYFLAGS_vmlinux.bin := -O binary --remove-section=.comment --remove-section
$(obj)/vmlinux.bin: vmlinux FORCE
$(call if_changed,objcopy)

ifndef CONFIG_PIE_BUILD
CMD_RELOCS=arch/s390/tools/relocs
quiet_cmd_relocs = RELOCS $@
cmd_relocs = $(CMD_RELOCS) $< > $@
$(obj)/relocs.S: vmlinux FORCE
$(call if_changed,relocs)
endif

suffix-$(CONFIG_KERNEL_GZIP) := .gz
suffix-$(CONFIG_KERNEL_BZIP2) := .bz2
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6 changes: 0 additions & 6 deletions arch/s390/boot/boot.h
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Expand Up @@ -24,14 +24,8 @@ struct vmlinux_info {
unsigned long bootdata_size;
unsigned long bootdata_preserved_off;
unsigned long bootdata_preserved_size;
#ifdef CONFIG_PIE_BUILD
unsigned long dynsym_start;
unsigned long rela_dyn_start;
unsigned long rela_dyn_end;
#else
unsigned long got_start;
unsigned long got_end;
#endif
unsigned long amode31_size;
unsigned long init_mm_off;
unsigned long swapper_pg_dir_off;
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42 changes: 0 additions & 42 deletions arch/s390/boot/startup.c
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Expand Up @@ -151,41 +151,6 @@ static void copy_bootdata(void)
memcpy((void *)vmlinux.bootdata_preserved_off, __boot_data_preserved_start, vmlinux.bootdata_preserved_size);
}

#ifdef CONFIG_PIE_BUILD
static void kaslr_adjust_relocs(unsigned long min_addr, unsigned long max_addr,
unsigned long offset, unsigned long phys_offset)
{
Elf64_Rela *rela_start, *rela_end, *rela;
int r_type, r_sym, rc;
Elf64_Addr loc, val;
Elf64_Sym *dynsym;

rela_start = (Elf64_Rela *) vmlinux.rela_dyn_start;
rela_end = (Elf64_Rela *) vmlinux.rela_dyn_end;
dynsym = (Elf64_Sym *) vmlinux.dynsym_start;
for (rela = rela_start; rela < rela_end; rela++) {
loc = rela->r_offset + phys_offset - __START_KERNEL;
val = rela->r_addend;
r_sym = ELF64_R_SYM(rela->r_info);
if (r_sym) {
if (dynsym[r_sym].st_shndx != SHN_UNDEF)
val += dynsym[r_sym].st_value + offset - __START_KERNEL;
} else {
/*
* 0 == undefined symbol table index (SHN_UNDEF),
* used for R_390_RELATIVE, only add KASLR offset
*/
val += offset - __START_KERNEL;
}
r_type = ELF64_R_TYPE(rela->r_info);
rc = arch_kexec_do_relocs(r_type, (void *) loc, val, 0);
if (rc)
error("Unknown relocation type");
}
}

static void kaslr_adjust_got(unsigned long offset) {}
#else
static void kaslr_adjust_relocs(unsigned long min_addr, unsigned long max_addr,
unsigned long offset, unsigned long phys_offset)
{
Expand All @@ -212,7 +177,6 @@ static void kaslr_adjust_got(unsigned long offset)
for (entry = (u64 *)vmlinux.got_start; entry < (u64 *)vmlinux.got_end; entry++)
*entry += offset - __START_KERNEL;
}
#endif

/*
* Merge information from several sources into a single ident_map_size value.
Expand Down Expand Up @@ -398,14 +362,8 @@ static void kaslr_adjust_vmlinux_info(long offset)
{
vmlinux.bootdata_off += offset;
vmlinux.bootdata_preserved_off += offset;
#ifdef CONFIG_PIE_BUILD
vmlinux.rela_dyn_start += offset;
vmlinux.rela_dyn_end += offset;
vmlinux.dynsym_start += offset;
#else
vmlinux.got_start += offset;
vmlinux.got_end += offset;
#endif
vmlinux.init_mm_off += offset;
vmlinux.swapper_pg_dir_off += offset;
vmlinux.invalid_pg_dir_off += offset;
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2 changes: 0 additions & 2 deletions arch/s390/boot/vmlinux.lds.S
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Original file line number Diff line number Diff line change
Expand Up @@ -99,14 +99,12 @@ SECTIONS

_decompressor_end = .;

#ifndef CONFIG_PIE_BUILD
. = ALIGN(4);
.vmlinux.relocs : {
__vmlinux_relocs_64_start = .;
*(.vmlinux.relocs_64)
__vmlinux_relocs_64_end = .;
}
#endif

#ifdef CONFIG_KERNEL_UNCOMPRESSED
. = ALIGN(PAGE_SIZE);
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33 changes: 0 additions & 33 deletions arch/s390/kernel/vmlinux.lds.S
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Original file line number Diff line number Diff line change
Expand Up @@ -192,31 +192,6 @@ SECTIONS

PERCPU_SECTION(0x100)

#ifdef CONFIG_PIE_BUILD
.dynsym ALIGN(8) : {
__dynsym_start = .;
*(.dynsym)
__dynsym_end = .;
}
.rela.dyn ALIGN(8) : {
__rela_dyn_start = .;
*(.rela*)
__rela_dyn_end = .;
}
.dynamic ALIGN(8) : {
*(.dynamic)
}
.dynstr ALIGN(8) : {
*(.dynstr)
}
.hash ALIGN(8) : {
*(.hash)
}
.gnu.hash ALIGN(8) : {
*(.gnu.hash)
}
#endif

. = ALIGN(PAGE_SIZE);
__init_end = .; /* freed after init ends here */

Expand All @@ -238,14 +213,8 @@ SECTIONS
QUAD(__boot_data_preserved_start) /* bootdata_preserved_off */
QUAD(__boot_data_preserved_end -
__boot_data_preserved_start) /* bootdata_preserved_size */
#ifdef CONFIG_PIE_BUILD
QUAD(__dynsym_start) /* dynsym_start */
QUAD(__rela_dyn_start) /* rela_dyn_start */
QUAD(__rela_dyn_end) /* rela_dyn_end */
#else
QUAD(__got_start) /* got_start */
QUAD(__got_end) /* got_end */
#endif
QUAD(_eamode31 - _samode31) /* amode31_size */
QUAD(init_mm)
QUAD(swapper_pg_dir)
Expand Down Expand Up @@ -281,12 +250,10 @@ SECTIONS
*(.plt) *(.plt.*) *(.iplt) *(.igot .igot.plt)
}
ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure linkages detected!")
#ifndef CONFIG_PIE_BUILD
.rela.dyn : {
*(.rela.*) *(.rela_*)
}
ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
#endif

/* Sections to be discarded */
DISCARDS
Expand Down

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