elf.5: Document .init_array and .fini_array

[emaste]

Summary

And note that .init and .fini are legacy interfaces.

[emaste]

Good blog on these sections: https://maskray.me/blog/2021-11-07-init-ctors-init-array

[kib]

You might want to mention that for dynamic binaries rtld selects which init model to use based on the presence of the FreeBSD note NOINIT.

[emaste]

mention that for dynamic binaries rtld selects which init model to use

We do describe NT_FREEBSD_NOINIT_TAG in here already. If we add more documentation should it be here or in rtld(1) with an explicit cross reference?

Old binaries: .init and .ctors, called by csu
New binaries with NT_FREEBSD_NOINIT_TAG: .init, .ctors, and .init_array, called by rtld

[kib]

In D50380#1150046, @emaste wrote:

mention that for dynamic binaries rtld selects which init model to use

We do describe NT_FREEBSD_NOINIT_TAG in here already. If we add more documentation should it be here or in rtld(1) with an explicit cross reference?

Old binaries: .init and .ctors, called by csu
New binaries with NT_FREEBSD_NOINIT_TAG: .init, .ctors, and .init_array, called by rtld

rtld(1) main subject is the rtld use, e.g. env vars affecting the runtime linker, defaults and other user-tweakable stuff. There we describe the binaries format, and IMO there is the right place to add a sentence to point to the NOINIT note.

[emaste]

OK, we already describe NT_FREEBSD_NOINIT_TAG in this man page (elf.5) so perhaps just a mention there?

 .It Dv NT_FREEBSD_NOINIT_TAG Pq Value: 2
 Indicates that the C startup does not call initialization routines, and thus
 .Xr rtld 1
 must do so.
 .Va desc
 is ignored.
+This note must be present for the .init_array and .fini_array sections to be
+processed.

[kib]

In D50380#1150495, @emaste wrote:

OK, we already describe NT_FREEBSD_NOINIT_TAG in this man page (elf.5) so perhaps just a mention there?

 .It Dv NT_FREEBSD_NOINIT_TAG Pq Value: 2
 Indicates that the C startup does not call initialization routines, and thus
 .Xr rtld 1
 must do so.
 .Va desc
 is ignored.
+This note must be present for the .init_array and .fini_array sections to be
+processed.

IMO it would be redundant. I suggested to mention the note near description of the section (really it should be dyn tags, but ok) to provide a cross-reference.
Otherwise whole man page needs to be memoized to make the right conclusion.

[emaste]

So just a reference to the description of NT_FREEBSD_NOINIT_TAG later in this page?

[kib]

In D50380#1150661, @emaste wrote:

So just a reference to the description of NT_FREEBSD_NOINIT_TAG later in this page?

Yes.

[emaste]

add rtld and elf note mention

[kib]

share/man/man5/elf.5

967

971

[emaste]

rework NOINIT_TAG reference

[emaste]

I didn't want to have to repeat the description of rtld/NOINIT_TAG twice for .fini_array and .init_array so just put a reference to the existing NOINIT_TAG description.

[kib]

share/man/man5/elf.5

970

I would put the sentence last in the paragraph.