Re: [PATCH] MIPS: Set ISA bit in entry-y for microMIPS kernels
From: Paul Burton <hidden>
Date: 2017-08-18 17:19:30
Hi Maciej, On Friday, 18 August 2017 09:46:30 PDT Maciej W. Rozycki wrote:
quoted hunk ↗ jump to hunk
On Fri, 18 Aug 2017, Maciej W. Rozycki wrote:quoted
quoted
When building a kernel for the microMIPS ISA, ensure that the ISA bit (ie. bit 0) in the entry address is set. Otherwise we may include an entry address in images which bootloaders will jump to as MIPS32 code.Hmm, what's going on here? The ISA bit is set by the linker according to the mode the code at the entry symbol has been assembled for, e.g.: $ readelf -h vmlinux | grep Entry Entry point address: 0x804355e1 $ readelf -s vmlinux | grep kernel_entry 156535: 80100400 0 FUNC GLOBAL DEFAULT [MICROMIPS] 1 __kernel_entry 156742: 804355e0 146 FUNC GLOBAL DEFAULT [MICROMIPS] 1 kernel_entry $ or no microMIPS (or MIPS16) executable could work. Is your build process or toolchain used broken by any chance?It is indeed the build process. You've come up with a valid, however a complicated solution. How about the change below, on top of yours -- does it work for you? If so, then I'll wrap it up and submit as an update. Maciej --- arch/mips/Makefile | 17 ++--------------- 1 file changed, 2 insertions(+), 15 deletions(-) linux-mips-start-address.diff Index: linux-sfr-usead/arch/mips/Makefile ===================================================================--- linux-sfr-usead.orig/arch/mips/Makefile 2017-08-18 15:25:58.196676000 +0100 +++ linux-sfr-usead/arch/mips/Makefile 2017-08-18 15:27:55.309653000 +0100 @@ -242,21 +242,8 @@ include arch/mips/Kbuild.platforms 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 bootloadersmay - # 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 +entry-y = $(shell $(OBJDUMP) -f vmlinux 2>/dev/null \ + | sed -n 's/start address //p') cflags-y += -I$(srctree)/arch/mips/include/asm/mach-generic drivers-$(CONFIG_PCI) += arch/mips/pci/
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.
Thanks,
Paul
[1] https://patchwork.linux-mips.org/patch/14020/ Attachments
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