Re: Regression with arm in next with stack protector

[Rich Felker <dalias@libc.org>]

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