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docs/ELF_DLOPEN_METADATA: add detailed example

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Zbigniew Jędrzejewski-Szmek 2024-09-13 14:51:25 +02:00
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docs/ELF_DLOPEN_METADATA.md
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@ -87,3 +87,90 @@ of the libraries they specify in order to be enabled.
| required | Core functionality needs the dependency, the binary will not work if it cannot be found |
| recommended | Important functionality needs the dependency, the binary will work but in most cases the dependency should be provided |
| suggested | Secondary functionality needs the dependency, the binary will work and the dependency is only needed for full-featured installations |
### Displaying `dlopen()` notes
The raw ELF section can be extracted using `objdump`:
```console
$ objdump -j .note.dlopen -s /usr/lib64/systemd/libsystemd-shared-257.so
/usr/lib64/systemd/libsystemd-shared-257.so: file format elf64-x86-64
Contents of section .note.dlopen:
0334 04000000 8e000000 0a0c7c40 46444f00 ..........|@FDO.
0344 5b7b2266 65617475 7265223a 22627066 [{"feature":"bpf
0354 222c2264 65736372 69707469 6f6e223a ","description":
0364 22537570 706f7274 20666972 6577616c "Support firewal
0374 6c696e67 20616e64 2073616e 64626f78 ling and sandbox
0384 696e6720 77697468 20425046 222c2270 ing with BPF","p
0394 72696f72 69747922 3a227375 67676573 riority":"sugges
03a4 74656422 2c22736f 6e616d65 223a5b22 ted","soname":["
03b4 6c696262 70662e73 6f2e3122 2c226c69 libbpf.so.1","li
03c4 62627066 2e736f2e 30225d7d 5d000000 bbpf.so.0"]}]...
03d4 04000000 9e000000 0a0c7c40 46444f00 ..........|@FDO.
...
```
It is more convenient to use a higher level tool:
```console
$ dlopen-notes /usr/lib64/systemd/libsystemd-shared-257.so
# /usr/lib64/systemd/libsystemd-shared-257.so
[
{
"feature": "archive",
"description": "Support for decompressing archive files",
"priority": "suggested",
"soname": [
"libarchive.so.13"
]
},
{
"feature": "bpf",
"description": "Support firewalling and sandboxing with BPF",
"priority": "suggested",
"soname": [
"libbpf.so.1",
"libbpf.so.0"
]
},
...
```
`dlopen-notes` can display the notes grouped in a few different ways.
One option is to filter the libraries by "feature". This answers the
question "what libraries are needed to provide specified features":
```console
$ dlopen-notes.py -f archive,bpf /usr/lib64/systemd/libsystemd-shared-257.so
# grouped by feature
{
"bpf": {
"description": "Support firewalling and sandboxing with BPF",
"sonames": {
"libbpf.so.1": "suggested",
"libbpf.so.0": "suggested"
}
},
"archive": {
"description": "Support for decompressing archive files",
"sonames": {
"libarchive.so.13": "suggested"
}
}
}
The format that is used when building `deb` packages:
```console
$ dlopen-notes -s /usr/lib64/systemd/libsystemd-shared-257.so
libarchive.so.13 suggested
libbpf.so.0 suggested
libbpf.so.1 suggested
...
```
The format that can be useful when building `rpm` packages:
```console
$ dlopen-notes --rpm-requires archive --rpm-recommends bpf /usr/lib64/systemd/libsystemd-shared-257.so
Requires: libarchive.so.13()(64bit)
Recommends: libbpf.so.1()(64bit)
```