On Tue, Feb 13, 2024 at 11:47:06AM +0100, Sumanth Korikkar wrote: > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > > On s390, currently kernel uses the '-fPIE' compiler flag for compiling > vmlinux. This has a few problems: > > - It uses dynamic symbols (.dynsym), for which the linker refuses to > allow more than 64k sections. This can break features which use > '-ffunction-sections' and '-fdata-sections', including kpatch-build > [1] and Function Granular KASLR. > > - It unnecessarily uses GOT relocations, adding an extra layer of > indirection for many memory accesses. > > Instead of using '-fPIE', resolve all the relocations at link time and > then manually adjust any absolute relocations (R_390_64) during boot. > > This is done by first telling the linker to preserve all relocations > during the vmlinux link. (Note this is harmless: they are later > stripped in the vmlinux.bin link.) > > Then use the 'relocs' tool to find all absolute relocations (R_390_64) > which apply to allocatable sections. The offsets of those relocations > are saved in a special section which is then used to adjust the > relocations during boot. > > (Note: For some reason, Clang occasionally creates a GOT reference, even > without '-fPIE'. So Clang-compiled kernels have a GOT, which needs to > be adjusted.) > > On my mostly-defconfig kernel, this reduces kernel text size by ~1.3%. > > [1] https://github.com/dynup/kpatch/issues/1284 > [2] https://gcc.gnu.org/pipermail/gcc-patches/2023-June/622872.html > [3] https://gcc.gnu.org/pipermail/gcc-patches/2023-August/625986.html > > Compiler consideration: > > Gcc recently implemented an optimization [2] for loading symbols without > explicit alignment, aligning with the IBM Z ELF ABI. This ABI mandates > symbols to reside on a 2-byte boundary, enabling the use of the larl > instruction. However, kernel linker scripts may still generate unaligned > symbols. To address this, a new -munaligned-symbols option has been > introduced [3] in recent gcc versions. This option has to be used with > future gcc versions. > > Older Clang lacks support for handling unaligned symbols generated > by kernel linker scripts when the kernel is built without -fPIE. However, > future versions of Clang will include support for the -munaligned-symbols > option. When the support is unavailable, compile the kernel with -fPIE > to maintain the existing behavior. > > In addition to it: > move vmlinux.relocs to safe relocation > > When the kernel is built with CONFIG_KERNEL_UNCOMPRESSED, the entire > uncompressed vmlinux.bin is positioned in the bzImage decompressor > image at the default kernel LMA of 0x100000, enabling it to be executed > in-place. However, the size of .vmlinux.relocs could be large enough to > cause an overlap with the uncompressed kernel at the address 0x100000. > To address this issue, .vmlinux.relocs is positioned after the > .rodata.compressed in the bzImage. Nevertheless, in this configuration, > vmlinux.relocs will overlap with the .bss section of vmlinux.bin. To > overcome that, move vmlinux.relocs to a safe location before clearing > .bss and handling relocs. > > [Rebased Josh Poimboeuf patches and move vmlinux.relocs > to safe location: Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx>] > Tested-by: Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > --- > arch/s390/Kconfig | 15 ++++-- > arch/s390/Makefile | 8 ++- > arch/s390/boot/.gitignore | 1 + > arch/s390/boot/Makefile | 14 ++++- > arch/s390/boot/boot.h | 6 +++ > arch/s390/boot/startup.c | 80 +++++++++++++++++++++++++--- > arch/s390/boot/vmlinux.lds.S | 18 +++++++ > arch/s390/include/asm/physmem_info.h | 1 + > arch/s390/kernel/vmlinux.lds.S | 9 ++++ > 9 files changed, 139 insertions(+), 13 deletions(-) Acked-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
