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]

2 files changed, 31 insertions, 6 deletions

diff --git a/boot/Kconfig b/boot/Kconfig
index e1114aea843..e6927d60b7b 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -103,6 +103,19 @@ config FIT_FULL_CHECK
 	  of bugs or omissions in the code. This includes a bad structure,
 	  multiple root nodes and the like.
 
+config CONTROL_DTB_AS_FIT
+	bool "Allow U-Boot's control DTB to act as FIT image"
+	help
+	  Enable this to exempt U-Boot's control DTB from the sanity
+	  checks done to ensure FIT images are valid. This can for
+	  example be used to embed whole scripts in the control DTB,
+	  that can then be invoked using 'source ${fdtcontroladdr}'.
+	  In a secure boot setup, this is safe, as the control DTB is
+	  necessarily covered by any mechanism verifying U-Boot and
+	  can therefore be trusted. This only affects the case where
+	  the image being checked is gd->fdt_blob. See
+	  doc/develop/devicetree/control.rst for details.
+
 config FIT_SIGNATURE
 	bool "Enable signature verification of FIT uImages"
 	depends on DM
diff --git a/boot/image-fit.c b/boot/image-fit.c
index 1cf8e7be957..773eb1857c5 100644
--- a/boot/image-fit.c
+++ b/boot/image-fit.c
@@ -1658,6 +1658,16 @@ static int fdt_check_no_at(const void *fit, int parent)
 	return 0;
 }
 
+static int fit_check_images_node(const void *fit)
+{
+	if (fdt_path_offset(fit, FIT_IMAGES_PATH) < 0) {
+		log_debug("Wrong FIT format: no images parent node\n");
+		return -ENOENT;
+	}
+
+	return 0;
+}
+
 int fit_check_format(const void *fit, ulong size)
 {
 	int ret;
@@ -1670,6 +1680,13 @@ int fit_check_format(const void *fit, ulong size)
 		return -ENOEXEC;
 	}
 
+	/*
+	 * For the control DTB to act as a FIT image, we only require
+	 * an /images node.
+	 */
+	if (CONFIG_IS_ENABLED(CONTROL_DTB_AS_FIT) && fit == gd_fdt_blob())
+		return fit_check_images_node(fit);
+
 	if (CONFIG_IS_ENABLED(FIT_FULL_CHECK)) {
 		/*
 		 * If we are not given the size, make do with calculating it.
@@ -1718,12 +1735,7 @@ int fit_check_format(const void *fit, ulong size)
 	}
 
 	/* mandatory subimages parent '/images' node */
-	if (fdt_path_offset(fit, FIT_IMAGES_PATH) < 0) {
-		log_debug("Wrong FIT format: no images parent node\n");
-		return -ENOENT;
-	}
-
-	return 0;
+	return fit_check_images_node(fit);
 }
 
 int fit_conf_find_compat(const void *fit, const void *fdt)