On 18 August 2017 23:13:39 BST, "Maciej W. Rozycki" <macro@xxxxxxxxxx> wrote: >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 thats not true with CONFIG_KVM_GUEST kernels, which use a separate set of emulated guest kernel segments in useg, i.e. at 0x40000000. I've also seen EVA kernels linked at low addresses like around 0x20000000, though entry gets a bit fiddly for EVA depending on whether bootloader already has the chosen segment configuration set up. Cheers James >+# 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/ -- James Hogan
