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]>

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/efi_loader/efi_runtime.c b/lib/efi_loader/efi_runtime.c
index 35eb6a77766..73d4097464c 100644
--- a/lib/efi_loader/efi_runtime.c
+++ b/lib/efi_loader/efi_runtime.c
@@ -210,6 +210,30 @@ void __efi_runtime efi_memcpy_runtime(void *dest, const void *src, size_t n)
 }
 
 /**
+ * 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
  *
  * @table:	EFI table