efivarfs: fix statfs() on efivarfs

Some firmware (notably U-Boot) provides GetVariable() and
GetNextVariableName() but not QueryVariableInfo().

With commit d86ff333 ("efivarfs: expose used and total size") the
statfs syscall was broken for such firmware.

If QueryVariableInfo() does not exist or returns EFI_UNSUPPORTED, just
report the file system size as 0 as statfs_simple() previously did.

Fixes: d86ff333 ("efivarfs: expose used and total size")
Link: https://lore.kernel.org/all/20230910045445.41632-1-heinrich.schuchardt@canonical.com/Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
[ardb: log warning on QueryVariableInfo() failure]
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

fs/efivarfs/super.c

@@ -32,10 +32,16 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	u64 storage_space, remaining_space, max_variable_size;
 	efi_status_t status;
 
-	status = efivar_query_variable_info(attr, &storage_space, &remaining_space,
-					    &max_variable_size);
-	if (status != EFI_SUCCESS)
-		return efi_status_to_err(status);
+	/* Some UEFI firmware does not implement QueryVariableInfo() */
+	storage_space = remaining_space = 0;
+	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
+		status = efivar_query_variable_info(attr, &storage_space,
+						    &remaining_space,
+						    &max_variable_size);
+		if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
+			pr_warn_ratelimited("query_variable_info() failed: 0x%lx\n",
+					    status);
+	}
 
 	/*
 	 * This is not a normal filesystem, so no point in pretending it has a block