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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@asem.it>
Link: https://lore.kernel.org/r/20200519084128.12756-1-f.suligoi@asem.it
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Flavio Suligoi 2020-05-19 10:41:27 +02:00 committed by Jonathan Corbet
parent e2d467de34
commit 2dcc51b3fe
1 changed files with 17 additions and 0 deletions
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17
Documentation/filesystems/efivarfs.rst
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@ -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