[PATCH 0/3] kallsyms: remove special handling for CONFIG_ARM

On 3 February 2016 at 14:33, Chris Brandt [off-list ref] wrote:

On 2 Feb 2016, Chris Brandt wrote:

quoted

I then applied the 3 patches and tried again and this time it booted
up....almost.
It looks like it makes it all the way up to when it is going to mount
my rootfs, but then dies.

I did some debugging, and here's why I'm crashing.


If you look at my System.map file, you'll see that I've got 2 "__stubs_start" symbols.

One at 0x bf353edc, the other at 0x bf353ee0.

bf353edc T __stubs_start

This is a global symbol

bf353edc T _etext
bf353ee0 t __stubs_start

This is a local symbol

bf353ee4 t vector_rst
bf353f00 t vector_irq
bf353f80 t vector_dabt
bf354000 t vector_pabt
bf354080 t vector_und
bf354100 t vector_addrexcptn
bf354120 T vector_fiq
bf3541a0 T __stubs_end
bf3541a0 T __vectors_start
bf3541c0 t __mmap_switched
bf3541c0 T __vectors_end


So, when you get to early_trap_init(), it grabs the first __stubs_start for the memcpy:

        memcpy((void *)vectors + 0x1000, __stubs_start, __stubs_end - __stubs_start);

Indeed. It grabs the first one, which is the one defined by the linker script.

And when a printed out the destination, I get:

   Stubs at c09ff000 :00000000 BF006F20 EF9F0000 EA000064 etc...

Notice the first entry is 0....that's not right...and we know it's using that first __stubs_start symbol.

The first quantity after __stubs_start should be the address of
vector_swi. Can you check if the second value coincides with that?

So, I hacked the code and put an offset of 4 like this:

        memcpy((void *)vectors + 0x1000, __stubs_start+4, __stubs_end - (__stubs_start+4));


And sure enough it booted fine.


So, why are there 2 __stubs_start symbols???
I haven't figured that one out yet.

One defined by entry-armv.S and one defined by the linker script

And, I'm assuming you guys don't have 2 __stubs_start symbols either, correct?

Yes, we do.

This is caused by the fact that __stubs_start is not aligned correctly on XIP.

Could you try this, please?

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 7160658fd5d4..a5b8e7b80d17 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S

@@ -164,8 +164,8 @@ SECTIONS
         * The vectors and stubs are relocatable code, and the
         * only thing that matters is their relative offsets
         */
-       __stubs_start = .;
        .stubs : {
+               __stubs_start = .;
                *(.stubs)
        }
        __stubs_end = .;