Re: [Xen-devel] [PATCH v2 3/5] xen/livepatch/ARM32: Don't load and crash on livepatches loaded with wrong alignment.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Sep 12, 2017 at 02:57:04AM -0600, Jan Beulich wrote:
> >>> On 12.09.17 at 02:22, <konrad@xxxxxxxxxx> wrote:
> > On Mon, Sep 11, 2017 at 03:01:15AM -0600, Jan Beulich wrote:
> >> Hmm, as long as the relocation isn't required to be against aligned
> >> fields only (mandated by the processor ABI) I think the code doing
> >> the relocations would instead need to split the access, rather than
> >> calling the section misaligned or increasing alignment beyond what
> >> the ELF section headers say.
> > 
> > Maybe the serial log would explain this better:
> > 
> > xend_config_format     : 4
> > Executing: '(set -e;cd /root/test/livepatch;xen-livepatch load 
> > xen_bye_world.livepatch)' ..(XEN) livepatch.c:413: livepatch: 
> > xen_bye_world: Loaded .note.gnu.build-id at 00a08000
> > (XEN) livepatch.c:413: livepatch: xen_bye_world: Loaded .text at 00a06000
> > (XEN) livepatch.c:413: livepatch: xen_bye_world: Loaded .rodata at 00a08024
> > (XEN) livepatch.c:413: livepatch: xen_bye_world: Loaded .rodata.str1.4 at 
> > 00a08038
> > (XEN) livepatch.c:413: livepatch: xen_bye_world: Loaded .livepatch.depends 
> > at 00a08043
> >[...]
> > Keep in mind that this only happens if I cross-compile ARM32 under x86.
> 
> That would suggest a build environment / build tools issue then:
> Cross builds aren't supposed to produce binaries different from
> native builds.

Hm, the gcc parameters on both native and cross compiler have same args:

konrad@osstest:/srv/cubietruck/source$ diff native.invocation 
/tmp/cross.invocation 
1c1
< gcc -marm -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall 
-Wstrict-prototypes -Wdeclaration-after-statement -Wno-unused-but-set-variable 
-Wno-unused-local-typedefs   -O1 -nostdinc -fno-builtin -fno-common -Werror 
-Wredundant-decls -Wno-pointer-arith -pipe -g -D__XEN__ -include 
/source/xen.orig.git/xen/include/xen/config.h 
'-D__OBJECT_FILE__="xen_bye_world.o"' -Wa,--strip-local-absolute 
-fno-omit-frame-pointer -MMD -MF ./.xen_bye_world.o.d -msoft-float 
-mcpu=cortex-a15 -I/source/xen.orig.git/xen/include -fno-stack-protector 
-fno-exceptions -Wnested-externs -DGCC_HAS_VISIBILITY_ATTRIBUTE -marm 
-DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes 
-Wdeclaration-after-statement -Wno-unused-but-set-variable 
-Wno-unused-local-typedefs   -c xen_bye_world.c -o xen_bye_world.o
---
> arm-linux-gnu-gcc -marm -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall 
> -Wstrict-prototypes -Wdeclaration-after-statement 
> -Wno-unused-but-set-variable -Wno-unused-local-typedefs   -O1 -nostdinc 
> -fno-builtin -fno-common -Werror -Wredundant-decls -Wno-pointer-arith -pipe 
> -g -D__XEN__ -include /home/konrad/A20/xen.git/xen/include/xen/config.h 
> '-D__OBJECT_FILE__="xen_bye_world.o"' -Wa,--strip-local-absolute 
> -fno-omit-frame-pointer -MMD -MF ./.xen_bye_world.o.d -msoft-float 
> -mcpu=cortex-a15 -I/home/konrad/A20/xen.git/xen/include -fno-stack-protector 
> -fno-exceptions -Wnested-externs -DGCC_HAS_VISIBILITY_ATTRIBUTE -marm 
> -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes 
> -Wdeclaration-after-statement -Wno-unused-but-set-variable 
> -Wno-unused-local-typedefs   -c xen_bye_world.c -o xen_bye_world.o


> 
> > If I compile the test-case under ARM32 it works OK (as the
> > .livepatch.depends ends up being aligned to four bytes).
> 
> So why is that? What entity is creating this section (or the
> directive(s) to create it)?

gcc

Looking at the xen_bye_world.o produced by cross-compiler:

xen_bye_world.o:     file format elf32-littlearm

Contents of section .rodata:
 0000 78656e5f 65787472 615f7665 7273696f  xen_extra_versio
 0010 6e00                                 n. 

And native:

xen_bye_world.o:     file format elf32-littlearm

Contents of section .rodata:
 0000 78656e5f 65787472 615f7665 7273696f  xen_extra_versio
 0010 6e000000                             n...      

(The cross compiler is 7.0.1, while native is 4.9.2).


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