[RFC PATCH] firmware: qemu_fw_cfg.c: default to listing user-facing items only

From: Gabriel Somlo
Date: Tue May 03 2016 - 14:34:24 EST

Per the QEMU fw_cfg documentation, items targeted at guest-side
userspace should have names beginning with the string "opt/".
This patch limits the default sysfs fw_cfg listing to items
named "opt/*".

The "opt/" prefix on a fw_cfg item name may be interpreted as
being analogous to bit 2 in ACPI's _STA object return value
(i.e., "show in UI").

A full listing of all fw_cfg items can still be made available
by enabling the 'list_all' module parameter, also introduced by
this patch.

Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Gabriel Somlo <somlo@xxxxxxx>

Sorry it took me this long to whip together the few lines of code
below -- end-of-semester craziness happened, and I'm just beginning
to recover... :)

I've (somewhat reluctantly) modified the fw-cfg sysfs driver to
only show blobs named "opt/*" by default, at Michael's request.

Even though my instinct tells me hardcoding "opt/" in the source
feels a bit like mixing in policy, the argument that "opt/" is
analogous to the ACPI "hide from u/i" bit does make a lot of sense.

Personally, I can live with non-"opt/" blobs being hidden by default,
particularly since I'm still allowing them to be listed via the
"list-all" module parameter.

I'm cc-ing everyone I can remember showing any interest in this
driver and/or offering help and feedback, for a chance to comment
(with apologies if I left out anyone).

Absent any further objections, I'm personally OK with defaulting
to "opt/"-only blobs being shown to userspace.

Thanks again for all the help and feedback,

drivers/firmware/qemu_fw_cfg.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index d999fe3..c580b73 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -37,6 +37,10 @@ MODULE_AUTHOR("Gabriel L. Somlo <somlo@xxxxxxx>");
MODULE_DESCRIPTION("QEMU fw_cfg sysfs support");

+static int modparam_list_all;
+module_param_named(list_all, modparam_list_all, int, 0444);
+MODULE_PARM_DESC(list_all, "List all fw-cfg items under by_name sysfs folder.");
/* selector key values for "well-known" fw_cfg entries */
#define FW_CFG_SIGNATURE 0x00
#define FW_CFG_ID 0x01
@@ -452,6 +456,10 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
int err;
struct fw_cfg_sysfs_entry *entry;

+ /* skip listing item if name does not begin with "opt/" */
+ if (!modparam_list_all && strncmp(f->name, "opt/", 4))
+ return 0;
/* allocate new entry */
entry = kzalloc(sizeof(*entry), GFP_KERNEL);
if (!entry)