Re: [Xen-devel] [PATCH v4 07/11] livepatch/x86/arm[32, 64]: Force .livepatch.depends section to be uint32_t aligned.

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 09/20/2017 11:31 PM, Konrad Rzeszutek Wilk wrote:
> By default when using objcopy we lose the alignment when we copy it from 
> xen-syms -
> with the result that alignment (on ARM32 for example) can be 1:
>
>    [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf 
> Al
> ..
>    [ 6] .livepatch.depend PROGBITS        00000000 000093 000024 00   A  0   0  
> 1
>
> That, combined with wacky offset means it will be loaded in
> memory with the wrong alignment:
>
> (XEN) livepatch.c:425: livepatch: xen_bye_world: Loaded .livepatch.depends at 
> 000a08043
>
> And later we crash as the .livepatch.depends is not aligned to four bytes, while
> the xen_build_id_check expects the code to be four byte aligned and we
> get an hypervisor crash (on ARM32):
>
> (XEN) CPU0: Unexpected Trap: Data Abort
> (XEN) ----[ Xen-4.10Hello World  arm32  debug=y   Not tainted ]----
> (XEN) CPU:    0
> (XEN) PC:     002400a0 xen_build_id_check+0x8/0xe8
> ..snip..
> (XEN) Xen call trace:
> (XEN)    [<002400a0>] xen_build_id_check+0x8/0xe8 (PC)
> (XEN)    [<0021a9c0>] livepatch_op+0x768/0x1610 (LR)
> (XEN)    [<0023bbe4>] do_sysctl+0x9c8/0xa9c
> (XEN)    [<002673c4>] do_trap_guest_sync+0x11e0/0x177c
> (XEN)    [<0026b6a0>] entry.o#return_from_trap+0/0x4
> (XEN)
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 0:
> (XEN) CPU0: Unexpected Trap: Data Abort
>
> This fix forces all the test-cases to be built with a
> .livepatch.depends structure containing the build-id extracted from
> the hypervisor (except the xen_bye_world test-case).
>
> We use the 'mkhex' tool instead of 'xxd' as the end result is an 'unsigned'
> instead of 'char' type array - which naturally forces the alignment to be of 
> four.
> Also the 'mkhex' tools allows us to pass the section name as parameter.
>
> The end result is much better alignment:
>
>    [ 7] .livepatch.depend PROGBITS        00000000 000094 000024 00   A  0   0  
> 4
>
> Note that thanks to 'unsigned int .. __note_depends' the symbol becomes
> global:
>
> $ readelf --symbols *.livepatch | grep depen
>      23: 0000000000000000    36 OBJECT  GLOBAL HIDDEN     6 note_depends
>      49: 0000000000000000    36 OBJECT  GLOBAL HIDDEN    17 note_depends
>      16: 0000000000000000    36 OBJECT  GLOBAL HIDDEN     3 note_depends
>      21: 0000000000000000    36 OBJECT  GLOBAL HIDDEN     6 note_depends
>
> See patch titled: "livepatch/arm/x86: Rename note_depends symbol from 
> test-cases."
> which fixes this.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>
> ---
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Acked-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

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