Re: ELF Notes for kernel image

[Carl Patenaude Poulin <carl.patenaudepoulin@mail.mcgill.ca>]

Hi Andrew,

Thank you so much for your help! In particular, I find your
xen-test-framework code to be very instructive.

I have a question about that code, though. I can't find any place
where you're specifying that .note should be loaded as a PT_NOTE ELF
header. Do you do that anywhere? Or is it done implicitly somehow?

On Sun, Oct 25, 2015 at 2:45 PM, Andrew Cooper
<andrew.cooper3@citrix.com> wrote:
> On 25/10/2015 00:07, Carl Patenaude Poulin wrote:
>> Hi all,
>>
>> For my undergraduate project, I am developing a simple
>> paravirtualized x64 Xen unikernel. I've been hitting some issues
>> that I hope you can help me with!
>>
>> I'm trying to figure out how to equip an ELF binary with an ELF note
>> that marks it as a valid Xen image. I've found the following
>> resources:
>>
>> http://wiki.xenproject.org/wiki/X86_Paravirtualised_Memory_Management#Start_Of_Day
>> http://www.netbsd.org/docs/kernel/elf-notes.html#note-creation
>> http://xenbits.xen.org/docs/4.3-testing/hypercall/include,public,elfnote.h.html
>>
>> Unfortunately, they are quite hard to decipher!
>
> I have to concur.  The Xen elf note documentation is in a poor state.
>
> I have recently made a set of unikernels from scratch for a different
> project, and I had to reverse engineer some bits of the elfnote layout
> from the domain building code.
>
>>  I've tried to
>> summarize what little information I've extracted. Can you please
>> verify that the following is accurate, and maybe help me fill in the
>> blanks or point me to additional resources?
>>
>> * A "PT_NOTE section" is any ELF header section which conforms to
>> the PT_NOTE specification. It is typically named ".note.*" and
>> marked allocatable.
>> * A PT_NOTE section is a series of "notes", or key-value pairs, each
>> formatted as `key_size: int32, value_size: int32, value_type: int32,
>> key: (int32|int64|asciz), value: (int32|int64|asciz)`.
>
> This is not the PT_NOTE specification.
>
> Notes are:
> name size, [4 bytes]
> desc size, [4 bytes]
> type [4 bytes]
> name [variable length, typically NUL terminated ASCII string, padded to
> 4 byte]
> desc [variable length, padded to 4 bytes]
>
> For Xen notes, 'name' is always "Xen" to indicate the specifying entity
> (i.e. us).
> 'type' is the integer index from elfnote.h, and is effectively the key.
> 'desc' is the value associated with 'type'.  The type of data in 'desc'
> is specified by 'type'
>
>> * The Xen PT_NOTE section is named ".note.Xen", and its keys are
>> listen (in order) in $XEN_ROOT/xen/include/public/elfnote.h.
>
> There is no ordering requirements about notes, not an expectation that
> all will be filled in.  Very few are actually required to get a Paravirt
> kernel started.
>
>>
>> My questions:
>> * Every PT_NOTE entry contains a type field. What does it do, and
>> how do I decide what goes in it?
>
> I hope these are now clear, given the explanation above.
>
>> * In particular, what are the types of the various entries in the
>> Xen PT_NOTES (as seen in elfnote.h)?
>
> An interpretation of the text describing the 'type'.  Mostly NUL
> terminated ASCII strings, but not always.
>
>> * After all of this is done, how can I verify that my compiled
>> binary indeed has a valid PT_NOTE entry for Xen? Does `readelf` have
>> a switch that's particularly appropriate for this?
>
> There is a utility called readnotes (tools/xcutils/readnotes.c in the
> Xen source tree) which can parse and print the Xen elf notes, similar to
> `readelf`
>
> To get you started,
> http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen-test-framework.git;a=blob;f=arch/x86/boot/head_pv.S;h=078d6b85af704840a0abb700a4d5e40253c21fdf;hb=HEAD
> are the minimal set of elfnotes to get a PV guest booted.  (The
> hypercall page note isn't strictly needed, but you would silly not to
> include it).
>
> Alternatively,
> http://lxr.free-electrons.com/source/arch/x86/xen/xen-head.S is the set
> of elfnotes used by Linux.
>
> ~Andrew