linux-kernel - [PATCH 1/2] docs: filesystems: add info about efivars content

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Date:   Tue, 5 May 2020 12:17:15 +0200
From:   Flavio Suligoi <f.suligoi@...m.it>
To:     Jonathan Corbet <corbet@....net>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
CC:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Flavio Suligoi <f.suligoi@...m.it>
Subject: [PATCH 1/2] docs: filesystems: add info about efivars content

When an EFI variable is reading from:

/sys/firmware/efi/efivars

(for example using "hexdump"), the first 4 bytes of the
output are not the real EFI variable data, but the variable
attributes (in little-endian format).

Signed-off-by: Flavio Suligoi <f.suligoi@...m.it>
---
 Documentation/filesystems/efivarfs.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/filesystems/efivarfs.rst b/Documentation/filesystems/efivarfs.rst
index 90ac656..6380336 100644
--- a/Documentation/filesystems/efivarfs.rst
+++ b/Documentation/filesystems/efivarfs.rst
@@ -24,3 +24,20 @@ files that are not well-known standardized variables are created
 as immutable files.  This doesn't prevent removal - "chattr -i" will work -
 but it does prevent this kind of failure from being accomplished
 accidentally.
+
+.. warning ::
+      When a content of an UEFI variable in ``/sys/firmware/efi/efivars`` is
+      displayed, for example using ``hexdump``, pay attention that the first
+      4 bytes of the output represent the UEFI variable attributes,
+      in little-endian format.
+
+      Practically the output of each ``efivar`` is composed of:
+
+          +-----------------------------------+
+          |4_bytes_of_attributes + efivar_data|
+          +-----------------------------------+
+
+*See also:*
+
+- ``Documentation/admin-guide/acpi/ssdt-overlays.rst``
+- ``Documentation/ABI/stable/sysfs-firmware-efi-vars``
-- 
2.7.4