Hi Paul, > I originally did this [1], and wrote about it in the post-three-dashes notes > for this patch. To quote myself: > > > I originally tried using "objdump -f" to obtain the entry address, which > > works for microMIPS but it always outputs a 32 bit address for a 32 bit > > ELF whilst nm will sign extend to 64 bit. That matters for systems where > > we might want to run a MIPS32 kernel on a MIPS64 CPU & load it with a > > MIPS64 bootloader, which would then jump to a non-canonical > > (non-sign-extended) address. > > > > This works in all cases as it only changes the behaviour for microMIPS > > kernels, but isn't the prettiest solution. A possible alternative would > > be to write a custom tool to just extract, sign extend & print the entry > > point of an ELF executable. I'm open to feedback if that would be > > preferred. > > So if we were to use objdump we'd need to handle sign extending 32 bit > addresses to form a canonical address. Perhaps that'd be cleaner though. Hmm, your reasoning sounds right (and I was blind to miss it entirely, sorry), however reality seems to disagree. As at 5fc9484f5e41^ I get: make -f ./scripts/Makefile.build obj=arch/mips/boot VMLINUX=vmlinux \ VMLINUX_LOAD_ADDRESS=0xffffffff80100000 VMLINUX_ENTRY_ADDRESS=0x804fca20 PLATFORM="generic/" ADDR_BITS=32 arch/mips/boot/vmlinux.srec whereas at 5fc9484f5e41^ I get: make -f ./scripts/Makefile.build obj=arch/mips/boot VMLINUX=vmlinux \ VMLINUX_LOAD_ADDRESS=0xffffffff80100000 VMLINUX_ENTRY_ADDRESS=0x804fca21 PLATFORM="generic/" ADDR_BITS=32 arch/mips/boot/vmlinux.srec so in both cases the entry address is 32-bit, which is why I didn't see any disadvantage from using `objdump -f'. Indeed: $ mips-linux-gnu-nm vmlinux | grep kernel_entry 80100000 T __kernel_entry 804fca20 T kernel_entry $ mips-mti-linux-gnu-nm vmlinux | grep kernel_entry ffffffff80100000 T __kernel_entry ffffffff804fca20 T kernel_entry $ which means you can't rely on `nm' sign-extending addresses to 64 bits with 32-bit binaries. And it looks like a bug to me indeed that some versions of `nm' do such sign-extension, unlike `objdump' and `readelf'. I'll have to bisect it to see when it started happening and take it with upstream binutils. How about this version then? It does the right thing for me: make -f ./scripts/Makefile.build obj=arch/mips/boot VMLINUX=vmlinux \ VMLINUX_LOAD_ADDRESS=0xffffffff80100000 VMLINUX_ENTRY_ADDRESS=0xffffffff804fca21 PLATFORM="generic/" ADDR_BITS=32 arch/mips/boot/vmlinux.srec and given than we need to sign-extend in either case I think retrieving the canonical entry point rather than transforming the entry symbol is simpler and more reliable. Maciej --- arch/mips/Makefile | 19 +++++-------------- 1 file changed, 5 insertions(+), 14 deletions(-) linux-mips-start-address.diff Index: linux-sfr-usead/arch/mips/Makefile =================================================================== --- linux-sfr-usead.orig/arch/mips/Makefile 2017-08-18 22:17:42.962681000 +0100 +++ linux-sfr-usead/arch/mips/Makefile 2017-08-18 23:01:00.997846000 +0100 @@ -244,20 +244,11 @@ ifdef CONFIG_PHYSICAL_START load-y = $(CONFIG_PHYSICAL_START) endif -entry-noisa-y = 0x$(shell $(NM) vmlinux 2>/dev/null \ - | grep "\bkernel_entry\b" | cut -f1 -d \ ) -ifdef CONFIG_CPU_MICROMIPS - # - # Set the ISA bit, since the kernel_entry symbol in the ELF will have it - # clear which would lead to images containing addresses which bootloaders may - # jump to as MIPS32 code. - # - entry-y = $(patsubst %0,%1,$(patsubst %2,%3,$(patsubst %4,%5, \ - $(patsubst %6,%7,$(patsubst %8,%9,$(patsubst %a,%b, \ - $(patsubst %c,%d,$(patsubst %e,%f,$(entry-noisa-y))))))))) -else - entry-y = $(entry-noisa-y) -endif +# Knowing that a 32-bit kernel will be linked at a KSEG address +# sign-extend the entry point to 64 bits if retrieved as a 32-bit +# number by stuffing `ffffffff' after the leading `0x'. +entry-y = $(shell $(OBJDUMP) -f vmlinux 2>/dev/null \ + | sed -n 's/0x\(........\)$$/0xffffffff\1/;s/start address //p') cflags-y += -I$(srctree)/arch/mips/include/asm/mach-generic drivers-$(CONFIG_PCI) += arch/mips/pci/
