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Subject[PATCH v2] 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@vger.kernel.org
Signed-off-by: John Keeping <john@metanate.com>
---
drivers/usb/gadget/function/f_fs.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

On Mon, 27 Nov 2017 13:34:20 +0200, Felipe Balbi wrote:

> exactly like that. Care to send as a formal patch so I can apply?

Here it is.

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 97ea059a7aa4..7537cc1a7d79 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
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