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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>
44 lines
1.6 KiB
ReStructuredText
44 lines
1.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=======================================
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efivarfs - a (U)EFI variable filesystem
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=======================================
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The efivarfs filesystem was created to address the shortcomings of
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using entries in sysfs to maintain EFI variables. The old sysfs EFI
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variables code only supported variables of up to 1024 bytes. This
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limitation existed in version 0.99 of the EFI specification, but was
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removed before any full releases. Since variables can now be larger
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than a single page, sysfs isn't the best interface for this.
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Variables can be created, deleted and modified with the efivarfs
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filesystem.
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efivarfs is typically mounted like this::
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mount -t efivarfs none /sys/firmware/efi/efivars
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Due to the presence of numerous firmware bugs where removing non-standard
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UEFI variables causes the system firmware to fail to POST, efivarfs
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files that are not well-known standardized variables are created
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as immutable files. This doesn't prevent removal - "chattr -i" will work -
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but it does prevent this kind of failure from being accomplished
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accidentally.
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.. warning ::
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When a content of an UEFI variable in /sys/firmware/efi/efivars is
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displayed, for example using "hexdump", pay attention that the first
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4 bytes of the output represent the UEFI variable attributes,
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in little-endian format.
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Practically the output of each efivar is composed of:
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+-----------------------------------+
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|4_bytes_of_attributes + efivar_data|
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+-----------------------------------+
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*See also:*
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- Documentation/admin-guide/acpi/ssdt-overlays.rst
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- Documentation/ABI/stable/sysfs-firmware-efi-vars
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