Hi Maciej,
On Friday, 18 August 2017 09:46:30 PDT Maciej W. Rozycki wrote:
> On Fri, 18 Aug 2017, Maciej W. Rozycki wrote:
> > > 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 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
> +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/Attachment:
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