Re: [Xen-devel] livepatch-build: What does getting no output from "readelf -wi xen-syms" usually mean?

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 12/27/19 5:06 PM, Andrew Cooper wrote:
> On 02/12/2019 08:22, Andy Smith wrote:
>> Hi,
>>
>> I've been looking into live patching for the first time.
> 
> CC'ing livepatch maintainers.
> 
>>
>> Starting with a 4.12.1 build:
>>
>> $ cd ~/dev
>> $ ls -l
>> total 8
>> drwxr-xr-x 3 andy andy 4096 Oct 25 16:11 xen
>> drwxr-xr-x 6 andy andy 4096 Dec  2 01:16 livepatch-build-tools
>>
>> (there is already a 4.12.1 hypervisor built in /xen and is what's
>> running on this host with build_id
>> b18af774b56b0c98cfa6940a725ba2ba26066929)
>>
>> $ cp -a xen xen-lptest
>> $ cd livepatch-build-tools
>> $ ./livepatch-build -j 1 -s /home/andy/dev/xen-lptest/xen-4.12.1 -c 
>> /home/andy/dev/xen-lptest/xen-4.12.1/xen/.config -p ./lptest.patch -o lptest 
>> -d --depends b18af774b56b0c98cfa6940a725ba2ba26066929
>> Building LivePatch patch: lptest
>>
>> Xen directory: /home/andy/dev/xen-lptest/xen-4.12.1
>> Patch file: /home/andy/dev/livepatch-build-tools/lptest.patch
>> .config file: /home/andy/dev/xen-lptest/xen-4.12.1/xen/.config
>> Output directory: /home/andy/dev/livepatch-build-tools/xsa310
> 
> As a note, XSA-310 isn't safe to livepatch.
> 
> While (in general) you can build a livepatch, applying it is not safe
> because the before/after logic is not symmetric with how references are
> taken.
> 
> Depending on the exact interaction of applying the livepatch with
> pagetable deferral operations (VM start,migrate,shutdown, or large
> quantities of pagetable changes, e.g. a fork() system call), you can end
> up with too few or many references dropped.  Too few generally results
> in an unkillable zombie domain, while too many will hit various BUG()s
> in the MM code and take the system down.
> 
>> ================================================
>>
>> Perform full initial build with 1 CPU(s)...
>> Reading special section data
>> ERROR: can't find special struct size.
>>
>> So it seems it completed the initial build without error but looking
>> at the livepatch-build script it runs readelf like this:
>>
>> $ readelf -wi lptest/xen-syms
>> $
>>
>> For me this produces no output. I've probably done something simple
>> wrong. Does that indicate some simple mistake in my process?
> 

Your process looks correct. You need to use readelf from binutils. Are you
perhaps using some other readelf (e.g. from busybox) which gives unexpected
results? Even if lptest/xen-syms is invalid or empty, readelf should still
give some error output.

-- 
Ross Lagerwall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel