RFC: Add ELF note for description with JSON data

RFC: Add ELF note for description with JSON data

[Sebastian Huber]

Hello,

I would like to add some metadata to executable ELF files for the RTEMS 
real-time operating system. It seems that ELF notes are a good way to do 
this. Would it be possible to add a standard or GNU-specific note type 
for descriptions which contain data in JSON format? This would help to 
use standard tools such as readelf to get the descriptions of such 
notes. For example:

diff --git a/include/elf/common.h b/include/elf/common.h
index 6f64f05890c..95d6068178f 100644
--- a/include/elf/common.h
+++ b/include/elf/common.h
@@ -824,6 +824,7 @@
  #define NT_GNU_BUILD_ID                3       /* Generated by ld 
--build-id.  */
  #define NT_GNU_GOLD_VERSION    4       /* Generated by gold.  */
  #define NT_GNU_PROPERTY_TYPE_0  5      /* Generated by gcc.  */
+#define NT_GNU_JSON            6       /* Contains arbitrary JSON data.  */

  #define NT_GNU_BUILD_ATTRIBUTE_OPEN    0x100
  #define NT_GNU_BUILD_ATTRIBUTE_FUNC    0x101


$ readelf -n --format=json file.elf
{"note-sections": [
   {"section": ".note.gnu.json", "notes": [
     {"name": "GNU",
      "type": 6,
      "type-info": "NT_GNU_JSON",
      "description": {"key": "value"}}]}

The note was produced by:

.pushsection .note.gnu.json , "", %note
.balign 4
.long 2f - 1f
.long 4f - 3f
.long 6
1: .asciz "GNU"
2: .balign 4
3: .asciz "{\"key\": \"value\"}"
4: .balign 4
.popsection

A basic support would require the allocation of a node type number and a 
new type-specific output.

Optionally, we could add JSON output to readelf. One option would be to 
do this using libxo:

https://juniper.github.io/libxo/libxo-manual.html

Would such a feature acceptable in general?

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Re: RFC: Add ELF note for description with JSON data

[Nick Clifton]

Hi Sebastian,

> I would like to add some metadata to executable ELF files for the RTEMS real-time operating system.

Have you considered using the --package-data linker option ?

   '--package-metadata=JSON'
      Request the creation of a '.note.package' ELF note section.  The
      contents of the note are in JSON format, as per the package
      metadata specification.  For more information see:
      https://systemd.io/ELF_PACKAGE_METADATA/ If the JSON argument is
      missing/empty then this will disable the creation of the metadata
      note, if one had been enabled by an earlier occurrence of the
      -package-metdata option.  If the linker has been built with
      libjansson, then the JSON string will be validated.

Cheers
   Nick

Re: RFC: Add ELF note for description with JSON data

[Sebastian Huber]

Hello Nick,

On 29.03.23 16:27, Nick Clifton wrote:
> Hi Sebastian,
> 
>> I would like to add some metadata to executable ELF files for the 
>> RTEMS real-time operating system.
> 
> Have you considered using the --package-data linker option ?
> 
>    '--package-metadata=JSON'
>       Request the creation of a '.note.package' ELF note section.  The
>       contents of the note are in JSON format, as per the package
>       metadata specification.  For more information see:
> https://systemd.io/ELF_PACKAGE_METADATA/ If the JSON argument is
>       missing/empty then this will disable the creation of the metadata
>       note, if one had been enabled by an earlier occurrence of the
>       -package-metdata option.  If the linker has been built with
>       libjansson, then the JSON string will be validated.

thanks, this looks like the thing we need.

Independent of this, selecting the output format of readelf (and other 
tools) would be nice.

-- 
embedded brains GmbH
Herr Sebastian HUBER
Dornierstr. 4
82178 Puchheim
Germany
email: sebastian.huber@embedded-brains.de
phone: +49-89-18 94 741 - 16
fax:   +49-89-18 94 741 - 08

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Registernummer: HRB 157899
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Unsere Datenschutzerklärung finden Sie hier:
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Re: RFC: Add ELF note for description with JSON data

[Nick Clifton]

Hi Sebastian,

> Independent of this, selecting the output format of readelf (and other tools) would be nice.

Yes, sorry.  I did not read your original email properly.


 > +#define NT_GNU_JSON            6       /* Contains arbitrary JSON data.  */

I would have no objection to adding this particular note type.
It would be best if if this was agreed upon by the ELF ABI folks though.


 > Optionally, we could add JSON output to readelf. One option would be to do this using libxo:

Personally I would prefer not to complicate readelf by adding another
dependency.  Are there tools that can read JSON output from a pipe ?

I was thinking that it might be reasonable to add an option to readelf
along the lines of --note-body-parser=<program> and have it pass the
contents of any note it is displaying to <program>.  This would allow
for any kind of formatting inside notes, and would not add any dependencies
to readelf.

Cheers
   Nick