View source for Blog/Of the Linux kernel, GNU Emacs, clangd and eglot

This is a short post, containing some quick notes on reading the Linux kernel sources with GNU Emacs.

In my previous post [[Blog/More random notes about network programming (Episode 2)]] in the section about [https://santoro.tk/w/index.php/Blog/More_random_notes_about_network_programming_(Episode_2)#The_cmsghdr_structure_and_its_usage the cmsg structure and its usage] I wondered what other data can I find in there.

Tonight I decided to go and take a look. I haven't found all the answers so far, this still an ongoing investigation.

As that feature is most likely implementation dependant (except for the SOL_SOCKET/SCM_RIGHT combo) I decided to look into the Linux kernel sources.

I cloned the sources from https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ and opened that into Emacs.

Intuition suggested me to open <code>net/ipv4/udp.c</code> and look for the recvmsg function.

As I would like to get better at reading and writing code with Emacs, I enabled eglot to get lsp-based superpowers, and it kinda worked.

== clangd and the Linux kernel ==
It kinda worked out of the box because it could look up function and structure declaration, but only within the same file.

The first intuition was to run a basic build of the kernel. I did a very quick <code>make alldefconfig</code> followed by <code>make -j4</code> and then <code>make headers</code>.

But it still didn't work.

[https://stackoverflow.com/a/71543187 This] stackoverflow reply contained the right suggestion:

in the scripts/clang-tools from the kernel top-level-directory, the gen_compile_commands.py must be ran:<syntaxhighlight lang="text">
./scripts/clang-tools/gen_compile_commands.py
</syntaxhighlight>Then a <code>compile_commands.json</code> file will appear, and it'll contain all the necessary directions for clangd to work properly.

Once that was done, a shutdown of the lsp server and a restart was sufficient.

== emacs and eglot-based jump to definition ==
(More a note to myself than an important piece of content)

When using eglot and an lsp server GNU Emacs reuses the Xref features to browse source code.

Hence:

* <code>M-.</code> will jump to the definition at point
* <code>M-,</code> will go back to whence we came

More on [https://www.gnu.org/software/emacs/manual/html_mono/emacs.html#Xref Xref here].

== cmsg: what I've found  so far ==
I haven't done a proper dive-deep yet, however:

In <code>net/ipv4/udp.c</code> at line 1860 i see the following:<syntaxhighlight lang="c" line="1" start="1860">
	sock_recv_cmsgs(msg, sk, skb);

</syntaxhighlight>In <code>net/ipv4/udp.c</code> at line 1876 i see the following:<syntaxhighlight lang="c" line="1" start="1876">
	if (udp_test_bit(GRO_ENABLED, sk))
		udp_cmsg_recv(msg, sk, skb);

	if (inet_cmsg_flags(inet))
		ip_cmsg_recv_offset(msg, sk, skb, sizeof(struct udphdr), off);

</syntaxhighlight>So the next logical step would be to look into these three functions:

* sock_recv_cmsgs
* udp_cmsg_recv
* ip_cmsg_recv_offset

My guesses are that:

* they're respectively generic socket control message, udp-level control messages, ip-level control messages
* there wil be an equivalent tcp_cmsg_recv (and possibly other for other protocols)


To the next post, I guess :)


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