From 2dcc51b3fe1708d3cda1b3fe9a30c09550791387 Mon Sep 17 00:00:00 2001 From: Flavio Suligoi Date: Tue, 19 May 2020 10:41:27 +0200 Subject: [PATCH] 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 Link: https://lore.kernel.org/r/20200519084128.12756-1-f.suligoi@asem.it Signed-off-by: Jonathan Corbet --- Documentation/filesystems/efivarfs.rst | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/Documentation/filesystems/efivarfs.rst b/Documentation/filesystems/efivarfs.rst index 90ac65683e7e..0551985821b8 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