How to apply a patch to a file in the root file system in yocto recipe

I want to apply a patch to a file in /etc in the rootfs. So I create a Yocto recipes

├── test_1.0.bb
└── files
    └── sshd.patch

and I've started to create the file .bb that is like this:

DESCRIPTION = "Patch files"
SECTION = "Patch"
LICENSE = "MIT"

FILESEXTRAPATHS_prepend := "${THISDIR}/files:"

SRC_URI = " \
    file://sshd.patch \
"

At this point I did not understand, what do I have to use the do_install or do_patch method?
How do I have to specify the path to the file to be patched? (for example the file sshd to be patched is in /etc/ssh)

I tried different "configuration" with do_install(), but they didn't work. (When I do "bitbake core-image-base" there are not problems, but then in the rootfs the patch is not applied)

*English is not my mother tongue; please excuse any errors on my part.

Solution

If you want to patch an existing recipe file, you need to create a openssh_%.bbappend file instead of test_1.0.bb and put every files to replace in SRC_URI (ie file://sshd_config) instead of sshd.patch.

Then, as those files already exist, you don't need to modify do_install from original recipe. Otherwise you need to add

do_install_append(){
    install -Dm 0644 ${S}/myfile ${D}${sysconfdir}/example/myfile 
}

# and modify FILES_${PN} if file is added in unusual place

By the way, a recipe cannot override another recipe file without being in conflict, so another solution is to patch it at rootfs creation, with ROOTFS_POSTPROCESS_COMMAND. (That's why you cannot use test_0.1.bb for that).

Otherwise, patch files are automatically applied so you don't need to modify do_patch nor do_install.

If you add a recipe, don't forget to add it to image with IMAGE_INSTALL_append = " test" in local.conf in your case.