readelf: add support for ELF package metadata note

[emaste]

Summary

References:

• https://fedoraproject.org/wiki/Changes/Package_information_on_ELF_objects
• https://systemd.io/ELF_PACKAGE_METADATA/

[fuz]

contrib/elftoolchain/readelf/readelf.c

3838

Can we get a symbolic constant for this type?

3841–3842

Cast to avoid sign-extension, clearer code. Is the charset of these ASCII? Or is it UTF-8?

[brooks]

The general idea seems fine.

Selfishly, it's convenient to see someone else driving a need for JSON handling in ELF files since we're going to want something like that for CHERI compartmentalization. CC @jhb

contrib/elftoolchain/readelf/readelf.c

3841–3842

From the systemd.io link "The format is a single JSON object, encoded as a zero-terminated UTF-8 string". We should probably act accordingly.

[emaste]

contrib/elftoolchain/readelf/readelf.c

3838

D57525

3841–3842

Yes, although it seems we need some generalized safe UTF-8 printing function. GNU readelf just passes it to printf %s.

[fuz]

contrib/elftoolchain/readelf/readelf.c

3841–3842

Scratch that, my code is still wrong. buf is a const char *, so buf[i] can be negative. The isprint call should be isprint((unsigned char)buf[i]).

For more comprehensive support, we could call iconv to transcode the buffer contents into the current locale.

[emaste]

use strvisx to handle possibly bogus UTF-8

[fuz]

Good iea using strvisx.