efi_loader: avoid superfluous variable store writes on unchanged data

Every SetVariable() call triggers efi_var_mem_ins() followed by
efi_var_to_storage(), even when the variable value is not actually
changing. This is unfriendly to flash-backed stores that suffer
wear from unnecessary erase/write cycles.

Add a change-detection path to efi_var_mem_ins(): when size2 == 0
(i.e. not an append) and the caller passes a non-NULL changep flag,
look up the existing variable and compare attributes, length, time
and data byte-by-byte. If everything matches, set *changep = false
and return EFI_SUCCESS without touching the variable buffer.

Both efi_set_variable_int() and efi_set_variable_runtime() now
check the flag and skip efi_var_mem_del() / efi_var_to_storage()
when nothing changed.

Introduce efi_memcmp_runtime() - a runtime-safe byte-by-byte memory
comparison helper, following the same pattern as the existing
efi_memcpy_runtime(). The standard memcmp() is not available after
ExitBootServices() and calling it from Linux will crash.

Tested-by: Heinrich Schuchardt <[email protected]>
Reviewed-by: Heinrich Schuchardt <[email protected]>
Signed-off-by: Michal Simek <[email protected]>
Reviewed-by: Ilias Apalodimas <[email protected]>

2 files changed, 10 insertions, 1 deletions

diff --git a/include/efi_loader.h b/include/efi_loader.h
index 0ebc80e0af0..3a4d502631c 100644
--- a/include/efi_loader.h
+++ b/include/efi_loader.h
@@ -1148,6 +1148,9 @@ struct pkcs7_message *efi_parse_pkcs7_header(const void *buf,
 /* runtime implementation of memcpy() */
 void efi_memcpy_runtime(void *dest, const void *src, size_t n);
 
+/* runtime implementation of memcmp() */
+int efi_memcmp_runtime(const void *s1, const void *s2, size_t n);
+
 /* commonly used helper functions */
 u16 *efi_create_indexed_name(u16 *buffer, size_t buffer_size, const char *name,
 			     unsigned int index);
diff --git a/include/efi_variable.h b/include/efi_variable.h
index fc1184e5ca1..c3229c717d8 100644
--- a/include/efi_variable.h
+++ b/include/efi_variable.h
@@ -216,6 +216,11 @@ void efi_var_mem_del(struct efi_var_entry *var);
  * The variable is appended without checking if a variable of the same name
  * already exists. The two data buffers are concatenated.
  *
+ * When @changep is non-NULL and @size2 is 0, the function compares the new
+ * value against an existing variable with the same name and vendor. If
+ * attributes and data are identical the insertion is skipped and *@changep
+ * is set to false, avoiding superfluous writes.
+ *
  * @variable_name:	variable name
  * @vendor:		GUID
  * @attributes:		variable attributes
@@ -224,13 +229,14 @@ void efi_var_mem_del(struct efi_var_entry *var);
  * @size2:		size of the second data field
  * @data2:		second data buffer
  * @time:		time of authentication (as seconds since start of epoch)
+ * @changep:		pointer to change flag (may be NULL)
  * Result:		status code
  */
 efi_status_t efi_var_mem_ins(const u16 *variable_name,
 			     const efi_guid_t *vendor, u32 attributes,
 			     const efi_uintn_t size1, const void *data1,
 			     const efi_uintn_t size2, const void *data2,
-			     const u64 time);
+			     const u64 time, bool *changep);
 
 /**
  * efi_var_mem_free() - determine free memory for variables