Merge patch series "allow control DTB to double as "FIT image""

Rasmus Villemoes <[email protected]> says:

The commit message for patch 1 explains what it is I'd like to be able
to do, but here's some more background:

For a long time, we've embedded the boot script in the U-Boot binary
by building a bootscript.itb, and using a .dtsi like

  / {
          config {
                 bootscript = /incbin/("/path/to/bootscript.itb");
          };
  };

which in turn is mentioned in CONFIG_DEVICE_TREE_INCLUDES, that
bootscript.itb FIT image has been embedded in U-Boot's control
dtb. Running that was then a matter of doing

  fdt addr ${fdtcontroladdr} && fdt get addr bsaddr /config bootscript && source ${bsaddr}

There are a couple of advantage of having the bootscript (and other
script logic) embedded in the U-Boot binary. First, there's no need to
figure out some separate partition to store the script in, and making
sure that gets updated whenever the bootloader itself does. Second,
one doesn't need to worry about verifying the script; whatever steps
one needs to take to implement secure boot for U-Boot itself will by
necessity also cover the control dtb (if nothing else then because
that's where the public key for the kernel verification lives). And
third, the boot script is automatically updated together with U-Boot
itself; and if U-Boot is stored in an eMMC boot partition, that update
is guaranteed to be atomic.

Now with the stricter requirements of libfdt starting from v2026.04,
the above command no longer worked, or only half the time, because the
embedded FIT image may not land on an 8-byte aligned address. So that
line had to be changed a little (line breaks added)

  fdt addr ${fdtcontroladdr}
    && fdt get addr bsaddr /config bootscript
    && fdt get size bssize /config bootscript
    && cp.b ${bsaddr} ${loadaddr} ${bssize}
    && source ${loadaddr}

which is getting quite unwieldy.

Then it struck me that one could perhaps simplify all of this quite a
lot: Cut out the intermediate bootscript.itb, just create a .dtsi
which directly puts a /images node inside the control dtb

/ {
  	images {
		default = "bootscript";
		bootscript {
			description = "Boot script";
			data = /incbin/("/path/to/bootscript.sh");
			type = "script";
			compression = "none";
		};
	};
};

and treat the control dtb itself as a FIT image; so the command to put
in $bootcmd becomes simply

  source ${fdtcontroladdr}:bootscript

and embedding other pieces of callable scripts is quite trivial.

And that almost works out-of-the-box, except for the fit_check_format() sanity check.

Introduce a CONFIG_ knob that allows one to opt out of those sanity
checks, for the special case of the address being checked being
identical to gd->fdt_blob.

Link: https://lore.kernel.org/r/[email protected]

1 files changed, 58 insertions, 0 deletions

diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
index 634114af59a..7d6117d5c4b 100644
--- a/doc/develop/devicetree/control.rst
+++ b/doc/develop/devicetree/control.rst
@@ -232,6 +232,64 @@ outside the U-Boot repository. You can use `DEVICE_TREE_INCLUDES` Kconfig
 option to specify a list of .dtsi files that will also be included when
 building .dtb files.
 
+Scripts embedded in control DTB
+-------------------------------
+
+The `DEVICE_TREE_INCLUDES` option can also be used to make the control
+DTB serve double duty as a FIT image. By including a `scripts.dtsi`
+file containing something like::
+
+  / {
+	images {
+		default = "boot";
+		boot {
+			description = "Bootscript";
+			data = /incbin/("boot.sh");
+			type = "script";
+			compression = "none";
+		};
+		factory-reset {
+			description = "Script for performing factory reset";
+			data = /incbin/("factory-reset.sh");
+			type = "script";
+			compression = "none";
+		};
+	};
+  };
+
+one can call those scripts using the `source` command in the U-Boot shell::
+
+  source ${fdtcontroladdr}:boot
+
+or just ``source ${fdtcontroladdr}`` for invoking the default.
+
+Since one does not need to separately build a "real" FIT image
+containing those scripts, this simplifies both the build process and
+the boot logic, as the latter does not need to first load the FIT
+image from storage.
+
+Another advantage is that when the bootloader and boot script must be
+updated together, it is easier to achieve a guaranteed atomic update
+when the boot script is embedded inside the U-Boot binary, instead of
+stored separately.
+
+For the above to work, one must enable the `CONTROL_DTB_AS_FIT` config
+option, which will (when the address passed to the `source` command is
+the address of U-Boot's control DTB) elide certain sanity checks that
+are normally done: With the above `.dtsi` snippet, the control DTB
+does not quite become a "real" FIT image - it lacks `timestamp` and
+`description` properties, but more importantly, FIT images cannot
+contain nodes with `@` in their names (unit addresses) anywhere, and
+the control DTB obviously does have such nodes.
+
+This is not a security problem, as the control DTB is necessarily
+trusted. In any secure boot setup where the bootloader is verified,
+that mechanism must also include verification of the control DTB. So
+in fact, since the scripts embedded this way are then also
+automatically verified, it simplifies implementation of secure
+boot. When using a separate FIT image, one must build it with
+appropriate signatures, just as when building a FIT image containing a
+kernel/dtb/initramfs.
 
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