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[net,v5,7/7] usbnet: ipheth: document scope of NCM implementation

Message ID 20250125235409.3106594-8-forst@pen.gy (mailing list archive)
State New
Delegated to: Netdev Maintainers
Headers show
Series usbnet: ipheth: prevent OoB reads of NDP16 | expand

Checks

Context Check Description
netdev/series_format success Posting correctly formatted
netdev/tree_selection success Clearly marked for net
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag present in non-next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 0 this patch: 0
netdev/build_tools success No tools touched, skip
netdev/cc_maintainers warning 1 maintainers not CCed: andrew+netdev@lunn.ch
netdev/build_clang success Errors and warnings before: 1 this patch: 1
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 0 this patch: 0
netdev/checkpatch warning WARNING: The commit message has 'stable@', perhaps it also needs a 'Fixes:' tag?
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
netdev/contest success net-next-2025-01-26--03-00 (tests: 885)

Commit Message

Foster Snowhill Jan. 25, 2025, 11:54 p.m. UTC 
Clarify that the "NCM" implementation in `ipheth` is very limited, as
iOS devices aren't compatible with the CDC NCM specification in regular
tethering mode.

For a standards-compliant implementation, one shall turn to
the `cdc_ncm` module.

Cc: stable@vger.kernel.org # 6.5.x
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
---
v5:
    No code changes. Added Cc to stable and Reviewed-by Jakub tags.
v4: https://lore.kernel.org/netdev/20250105010121.12546-8-forst@pen.gy/
    No changes.
v3: https://lore.kernel.org/netdev/20241123235432.821220-6-forst@pen.gy/
    This comment was part of the commit message for v2. With v3, given
    how the patches are split up, it makes more sense to add the comment
    directly in code.
v2: https://lore.kernel.org/netdev/20240912211817.1707844-1-forst@pen.gy/
    No code changes. Update commit message to further clarify that
    `ipheth` is not and does not aim to be a complete or spec-compliant
    CDC NCM implementation.
v1: n/a
---
 drivers/net/usb/ipheth.c | 8 ++++++++
 1 file changed, 8 insertions(+)
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Patch

diff --git a/drivers/net/usb/ipheth.c b/drivers/net/usb/ipheth.c
index 5347cd7e295b..a19789b57190 100644
--- a/drivers/net/usb/ipheth.c
+++ b/drivers/net/usb/ipheth.c
@@ -218,6 +218,14 @@  static int ipheth_rcvbulk_callback_legacy(struct urb *urb)
 	return ipheth_consume_skb(buf, len, dev);
 }
 
+/* In "NCM mode", the iOS device encapsulates RX (phone->computer) traffic
+ * in NCM Transfer Blocks (similarly to CDC NCM). However, unlike reverse
+ * tethering (handled by the `cdc_ncm` driver), regular tethering is not
+ * compliant with the CDC NCM spec, as the device is missing the necessary
+ * descriptors, and TX (computer->phone) traffic is not encapsulated
+ * at all. Thus `ipheth` implements a very limited subset of the spec with
+ * the sole purpose of parsing RX URBs.
+ */
 static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 {
 	struct usb_cdc_ncm_nth16 *ncmh;