On Tue, Mar 27, 2018 at 10:04:10AM +0100, Russell King - ARM Linux wrote:
> On Fri, Mar 23, 2018 at 11:14:53AM -0700, Tony Lindgren wrote:
> > Hi,
> >
> > Looks like commit 5638790dadae ("zboot: fix stack protector in
> > compressed boot phase") breaks booting on arm.
> >
> > This is all I get from the bootloader on omap3:
> >
> > Starting kernel ...
> >
> > data abort
> > pc : [<810002d0>] lr : [<100110a8>]
> > reloc pc : [<9d6002d0>] lr : [<2c6110a8>]
> > sp : 81467c18 ip : 81466bf0 fp : 81466bf0
> > r10: 80fc2c40 r9 : 81000258 r8 : 86fec000
> > r7 : ffffffff r6 : 81466bf8 r5 : 00000000 r4 : 80008000
> > r3 : 81466c14 r2 : 81466c18 r1 : 000a0dff r0 : 00466bf8
> > Flags: nZCv IRQs off FIQs off Mode SVC_32
> > Resetting CPU ...
> >
> > resetting ...
>
> The reason for this is the following code that was introduced by the
> referenced patch:
>
> + ldr r0, =__stack_chk_guard
> + ldr r1, =0x000a0dff
> + str r1, [r0]
>
> This uses the absolute address of __stack_chk_guard in the decompressor,
> which is a self-relocatable image. As with all constructs like the
> above, this absolute address doesn't get fixed up, and so it ends up
> pointing at invalid memory (in this case 0x466bf8) vs RAM at 0x80000000,
> and the decompressor looks to be around 0x81000000.
>
> Such constructs can not be used in the decompressor for exactly this
> reason - they need to use PC-relative addressing instead just like
> everything else does in head.S.
Can someone please answer why this is even needed to begin with? I
don't see any compelling reason __stack_chk_guard needs a particular
value in the decompressor, which is not dealing with any non-constant
input. Just putting __stack_chk_guard in its bss should be fine and
would eliminate all the risks of wrong code to load a value into it.
Alternatively put it in initialized data with the desired value.
Rich
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