Re: [Xen-devel] [livepatch-build-tools 4/4] livepatch-build: Handle newly created object files

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 4/8/19 9:32 AM, Pawel Wieczorkiewicz wrote:
> Up to now the livepatch-build ignores newly created object files.
> When patch applies new .c file and augments its Makefile to build it
> the resulting object file is not taken into account for final linking
> step.
>
> Such newly created object files can be detected by comparing patched/
> and original/ directories and copied over to the output directory for
> the final linking step.
>
> Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
> Reviewed-by: Andra-Irina Paraschiv <andraprs@xxxxxxxxxx>
> Reviewed-by: Bjoern Doebel <doebel@xxxxxxxxx>
> Reviewed-by: Norbert Manthey <nmanthey@xxxxxxxxx>
> ---
>   livepatch-build | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/livepatch-build b/livepatch-build
> index 796838c..b43730c 100755
> --- a/livepatch-build
> +++ b/livepatch-build
> @@ -146,6 +146,12 @@ function create_patch()
>           fi
>       done
>   
> +    NEW_FILES=$(comm -23 <(find patched -type f -name '*.o' | cut -f2- -d'/' | sort -u) <(find original -type f -name '*.o' | cut -f2- -d'/' | sort -u))
The paths should be `patched/xen` and `original/xen` so that only 
hypervisor changes are processed. It is done this way earlier (see 
FILES="$(find xen ...)").
Since process substitution creates a subshell, it might be simpler to do 
<(cd patched/xen && find . ...) and then drop the `cut`. Also, the `-u` 
argument to sort seems unnecessary.
> +    for i in $NEW_FILES; do
> +        cp "patched/$i" "output/$i"
> +        CHANGED=1
> +    done
If the live patch for whatever reason has no "patched" object files and 
only "new" object files, then it is not going to do anything useful 
since nothing will use the new functions. This should fail to build with 
an error. E.g. set NEW=1 above and then later check for CHANGED=0 && NEW=1.
Previously all object files that were linked into the livepatch were 
from the output of create-diff-object. This change just includes the 
newly built object files directly. I wonder if there are any issues or 
subtle differences when doing this? A couple of differences off the top 
of my head:
1) The object file will include _everything_ (e.g. potentially unused 
functions) while create-diff-object generally only includes the minimum 
that is needed.
2) Hooks and ignored functions/sections in the new object file would not 
be processed at all.
The was previously a patch on xen-devel which took a different approach 
(https://lists.xenproject.org/archives/html/xen-devel/2017-06/msg03532.html). 
Perhaps you could look at it and see which approach would be better?
Thanks,
--
Ross Lagerwall

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