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 <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

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);

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

lib/efi_loader/efi_runtime.c

@@ -209,6 +209,30 @@ void __efi_runtime efi_memcpy_runtime(void *dest, const void *src, size_t n)
 		*d++ = *s++;
 }
 
+/**
+ * efi_memcmp_runtime() - compare memory areas
+ *
+ * At runtime memcmp() is not available.
+ *
+ * @s1:		first memory area
+ * @s2:		second memory area
+ * @n:		number of bytes to compare
+ * Return:	0 if equal, negative if s1 < s2, positive if s1 > s2
+ */
+int __efi_runtime efi_memcmp_runtime(const void *s1, const void *s2, size_t n)
+{
+	const u8 *pos1 = s1;
+	const u8 *pos2 = s2;
+
+	for (; n; --n) {
+		if (*pos1 != *pos2)
+			return *pos1 - *pos2;
+		++pos1;
+		++pos2;
+	}
+	return 0;
+}
+
 /**
  * efi_update_table_header_crc32() - Update crc32 in table header
  *

lib/efi_loader/efi_var_common.c

@@ -526,7 +526,7 @@ efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe)
 			continue;
 		ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
 				      var->length, data, 0, NULL,
-				      var->time);
+				      var->time, NULL);
 		if (ret != EFI_SUCCESS)
 			log_err("Failed to set EFI variable %ls\n", var->name);
 	}

lib/efi_loader/efi_var_mem.c

@@ -159,12 +159,39 @@ efi_status_t __efi_runtime efi_var_mem_ins(
 				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)
 {
 	u16 *data;
 	struct efi_var_entry *var;
 	u32 var_name_len;
 
+	if (changep)
+		*changep = true;
+
+	/*
+	 * If this is not an append (size2 == 0), check whether the variable
+	 * already exists with identical attributes and data. When nothing
+	 * changed we can skip the write and avoid superfluous erases.
+	 */
+	if (!size2 && changep) {
+		struct efi_var_entry *old;
+
+		old = efi_var_mem_find(vendor, variable_name, NULL);
+		if (old && old->attr == attributes &&
+		    old->length == size1 && old->time == time) {
+			u16 *old_data;
+
+			for (old_data = old->name; *old_data; ++old_data)
+				;
+			++old_data;
+
+			if (!efi_memcmp_runtime(old_data, data1, size1)) {
+				*changep = false;
+				return EFI_SUCCESS;
+			}
+		}
+	}
+
 	var = (struct efi_var_entry *)
 	      ((uintptr_t)efi_var_buf + efi_var_buf->length);
 	var_name_len = u16_strlen(variable_name) + 1;

lib/efi_loader/efi_variable.c

@@ -277,6 +277,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
 	struct efi_var_entry *var;
 	efi_uintn_t ret;
 	bool append, delete;
+	bool changed = false;
 	u64 time = 0;
 	enum efi_auth_var_type var_type;
 
@@ -366,6 +367,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
 	if (delete) {
 		/* EFI_NOT_FOUND has been handled before */
 		attributes = var->attr;
+		changed = true;
 		ret = EFI_SUCCESS;
 	} else if (append && var) {
 		/*
@@ -380,15 +382,19 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
 		ret = efi_var_mem_ins(variable_name, vendor,
 				      attributes & ~EFI_VARIABLE_APPEND_WRITE,
 				      var->length, old_data, data_size, data,
-				      time);
+				      time, &changed);
 	} else {
 		ret = efi_var_mem_ins(variable_name, vendor, attributes,
-				      data_size, data, 0, NULL, time);
+				      data_size, data, 0, NULL, time,
+				      &changed);
 	}
 
 	if (ret != EFI_SUCCESS)
 		return ret;
 
+	if (!changed)
+		return EFI_SUCCESS;
+
 	efi_var_mem_del(var);
 
 	if (var_type == EFI_AUTH_VAR_PK)
@@ -396,10 +402,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
 	else
 		ret = EFI_SUCCESS;
 
-	/*
-	 * Write non-volatile EFI variables
-	 * TODO: check if a value change has occured to avoid superfluous writes
-	 */
+	/* Write non-volatile EFI variables to storage */
 	if (attributes & EFI_VARIABLE_NON_VOLATILE) {
 		if (IS_ENABLED(CONFIG_EFI_VARIABLE_NO_STORE))
 			return EFI_SUCCESS;
@@ -498,6 +501,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
 	struct efi_var_entry *var;
 	efi_uintn_t ret;
 	bool append, delete;
+	bool changed = false;
 	u64 time = 0;
 
 	if (!IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE))
@@ -549,6 +553,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
 	if (delete) {
 		/* EFI_NOT_FOUND has been handled before */
 		attributes = var->attr;
+		changed = true;
 		ret = EFI_SUCCESS;
 	} else if (append && var) {
 		u16 *old_data = (void *)((uintptr_t)var->name +
@@ -556,15 +561,19 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
 
 		ret = efi_var_mem_ins(variable_name, vendor, attributes,
 				      var->length, old_data, data_size, data,
-				      time);
+				      time, &changed);
 	} else {
 		ret = efi_var_mem_ins(variable_name, vendor, attributes,
-				      data_size, data, 0, NULL, time);
+				      data_size, data, 0, NULL, time,
+				      &changed);
 	}
 
 	if (ret != EFI_SUCCESS)
 		return ret;
-	/* We are always inserting new variables, get rid of the old copy */
+
+	if (!changed)
+		return EFI_SUCCESS;
+
 	efi_var_mem_del(var);
 
 	return EFI_SUCCESS;