Re: [PATCH] usb: f_fs: Drop check on Reserved1 field on OS_DESC_EXT_COMPAT

[John Keeping]

On Mon, 13 Nov 2017 12:57:21 +0200, Felipe Balbi wrote:
> Good point. Then how about we just force the value to 1 in f_fs.c and
> remove the check?

That seems reasonable.  Something like this?

-- >8 --
Subject: [PATCH] usb: f_fs: Force Reserved1=1 in OS_DESC_EXT_COMPAT

The specification says that the Reserved1 field in OS_DESC_EXT_COMPAT
must have the value "1", but when this feature was first implemented we
rejected any non-zero values.

This was adjusted to accept all non-zero values (while now rejecting
zero) in commit 53642399aa71 ("usb: gadget: f_fs: Fix wrong check on
reserved1 of OS_DESC_EXT_COMPAT"), but that breaks any userspace
programs that worked previously by returning EINVAL when Reserved1 == 0
which was previously the only value that succeeded!

If we just set the field to "1" ourselves, both old and new userspace
programs continue to work correctly and, as a bonus, old programs are
now compliant with the specification without having to fix anything
themselves.

Fixes: 53642399aa71 ("usb: gadget: f_fs: Fix wrong check on reserved1 of OS_DESC_EXT_COMPAT")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
---
 drivers/usb/gadget/function/f_fs.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 652397eda6d6..520a96e7ef9a 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -2282,9 +2282,18 @@ static int __ffs_data_do_os_desc(enum ffs_os_desc_type type,
 		int i;
 
 		if (len < sizeof(*d) ||
-		    d->bFirstInterfaceNumber >= ffs->interfaces_count ||
-		    !d->Reserved1)
+		    d->bFirstInterfaceNumber >= ffs->interfaces_count)
 			return -EINVAL;
+		if (d->Reserved1 != 1) {
+			/*
+			 * According to the spec, Reserved1 must be set to 1
+			 * but older kernels incorrectly rejected non-zero
+			 * values.  We fix it here to avoid returning EINVAL
+			 * in response to values we used to accept.
+			 */
+			pr_debug("usb_ext_compat_desc::Reserved1 forced to 1\n");
+			d->Reserved1 = 1;
+		}
 		for (i = 0; i < ARRAY_SIZE(d->Reserved2); ++i)
 			if (d->Reserved2[i])
 				return -EINVAL;
-- 
2.15.0