[PATCH 1/2] net: usb: cdc_ncm: convert sysfs show() callbacks to sysfs_emit()

[PATCH 1/2] net: usb: cdc_ncm: convert sysfs show() callbacks to sysfs_emit()

[Omer Zalman]

sysfs_emit() was introduced in v5.10 as the preferred replacement for
sprintf() in sysfs show() handlers. It bounds output to PAGE_SIZE and
avoids potential buffer overruns.

Convert all show() callbacks and the NCM_PARM_ATTR macro from sprintf()
to sysfs_emit().

Signed-off-by: Omer Zalman <omerzalman42@gmail.com>
---
 drivers/net/usb/cdc_ncm.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 7057c6c0cfc6..6fcb4d711a64 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -204,7 +204,7 @@ static ssize_t min_tx_pkt_show(struct device *d,
 	struct usbnet *dev = netdev_priv(to_net_dev(d));
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 
-	return sprintf(buf, "%u\n", ctx->min_tx_pkt);
+	return sysfs_emit(buf, "%u\n", ctx->min_tx_pkt);
 }
 
 static ssize_t rx_max_show(struct device *d,
@@ -213,7 +213,7 @@ static ssize_t rx_max_show(struct device *d,
 	struct usbnet *dev = netdev_priv(to_net_dev(d));
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 
-	return sprintf(buf, "%u\n", ctx->rx_max);
+	return sysfs_emit(buf, "%u\n", ctx->rx_max);
 }
 
 static ssize_t tx_max_show(struct device *d,
@@ -222,7 +222,7 @@ static ssize_t tx_max_show(struct device *d,
 	struct usbnet *dev = netdev_priv(to_net_dev(d));
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 
-	return sprintf(buf, "%u\n", ctx->tx_max);
+	return sysfs_emit(buf, "%u\n", ctx->tx_max);
 }
 
 static ssize_t tx_timer_usecs_show(struct device *d,
@@ -231,7 +231,7 @@ static ssize_t tx_timer_usecs_show(struct device *d,
 	struct usbnet *dev = netdev_priv(to_net_dev(d));
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 
-	return sprintf(buf, "%u\n", ctx->timer_interval / (u32)NSEC_PER_USEC);
+	return sysfs_emit(buf, "%u\n", ctx->timer_interval / (u32)NSEC_PER_USEC);
 }
 
 static ssize_t min_tx_pkt_store(struct device *d,
@@ -313,7 +313,7 @@ static ssize_t ndp_to_end_show(struct device *d, struct device_attribute *attr,
 	struct usbnet *dev = netdev_priv(to_net_dev(d));
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 
-	return sprintf(buf, "%c\n", ctx->drvflags & CDC_NCM_FLAG_NDP_TO_END ? 'Y' : 'N');
+	return sysfs_emit(buf, "%c\n", ctx->drvflags & CDC_NCM_FLAG_NDP_TO_END ? 'Y' : 'N');
 }
 
 static ssize_t ndp_to_end_store(struct device *d,  struct device_attribute *attr, const char *buf, size_t len)
@@ -362,7 +362,7 @@ static ssize_t cdc_ncm_show_##name(struct device *d, struct device_attribute *at
 { \
 	struct usbnet *dev = netdev_priv(to_net_dev(d)); \
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0]; \
-	return sprintf(buf, format "\n", tocpu(ctx->ncm_parm.name));	\
+	return sysfs_emit(buf, format "\n", tocpu(ctx->ncm_parm.name));	\
 } \
 static DEVICE_ATTR(name, 0444, cdc_ncm_show_##name, NULL)
 
-- 
2.39.5 (Apple Git-154)

[PATCH 2/2] net: usb: cdc_ncm: prevent silent u16 truncation in min_tx_pkt_store()

[Omer Zalman]

min_tx_pkt is a u16 field but the sysfs store handler accepted any
unsigned long and assigned it directly. Values that do not fit in 16
bits silently truncate: writing 65537 wraps to 1, turning a
"never pad" intent into "always pad".

ctx->tx_max can itself reach 65537 because cdc_ncm_update_rxtx_max()
increments the negotiated NTB size by one pad byte when it is an exact
multiple of the USB maxpacket size.

Clamp the stored value to min(tx_max, U16_MAX). Any value above tx_max
is already semantically equivalent to tx_max since frame lengths never
exceed it; the U16_MAX cap ensures the result is always representable
in the u16 field without wrapping.

Fixes: 39eb7e0e8c88 ("net: cdc_ncm: allow tuning min_tx_pkt")
Signed-off-by: Omer Zalman <omerzalman42@gmail.com>
---
 drivers/net/usb/cdc_ncm.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 6fcb4d711a64..edd69f4e1596 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -242,11 +242,17 @@ static ssize_t min_tx_pkt_store(struct device *d,
 	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
 	unsigned long val;
 
-	/* no need to restrict values - anything from 0 to infinity is OK */
 	if (kstrtoul(buf, 0, &val))
 		return -EINVAL;
 
-	ctx->min_tx_pkt = val;
+	/* Clamp to tx_max: the frame length can never exceed tx_max,
+	 * so any threshold above it has the same effect (padding is
+	 * never applied).  Also cap at U16_MAX since min_tx_pkt is
+	 * a u16 - without this, values like 65537 silently wrap to 1
+	 * and invert the intended behavior.
+	 */
+	ctx->min_tx_pkt = min_t(unsigned long, val,
+				min_t(u32, ctx->tx_max, U16_MAX));
 	return len;
 }
 
-- 
2.39.5 (Apple Git-154)

Re: [PATCH 1/2] net: usb: cdc_ncm: convert sysfs show() callbacks to sysfs_emit()

[Greg KH]

On Tue, Mar 17, 2026 at 11:06:31PM -0500, Omer Zalman wrote:
> sysfs_emit() was introduced in v5.10 as the preferred replacement for
> sprintf() in sysfs show() handlers. It bounds output to PAGE_SIZE and
> avoids potential buffer overruns.
> 
> Convert all show() callbacks and the NCM_PARM_ATTR macro from sprintf()
> to sysfs_emit().

There is no need to change existing show functions to sysfs_emit(),
unless you are doing some other work in the same area.  Otherwise it
would be a huge churn all across the tree.

So no need for this patch, sorry.

thanks,

greg k-h

Re: [PATCH 2/2] net: usb: cdc_ncm: prevent silent u16 truncation in min_tx_pkt_store()

[Greg KH]

On Tue, Mar 17, 2026 at 11:06:32PM -0500, Omer Zalman wrote:
> min_tx_pkt is a u16 field but the sysfs store handler accepted any
> unsigned long and assigned it directly. Values that do not fit in 16
> bits silently truncate: writing 65537 wraps to 1, turning a
> "never pad" intent into "always pad".
> 
> ctx->tx_max can itself reach 65537 because cdc_ncm_update_rxtx_max()
> increments the negotiated NTB size by one pad byte when it is an exact
> multiple of the USB maxpacket size.
> 
> Clamp the stored value to min(tx_max, U16_MAX). Any value above tx_max
> is already semantically equivalent to tx_max since frame lengths never
> exceed it; the U16_MAX cap ensures the result is always representable
> in the u16 field without wrapping.
> 
> Fixes: 39eb7e0e8c88 ("net: cdc_ncm: allow tuning min_tx_pkt")
> Signed-off-by: Omer Zalman <omerzalman42@gmail.com>
> ---
>  drivers/net/usb/cdc_ncm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
> index 6fcb4d711a64..edd69f4e1596 100644
> --- a/drivers/net/usb/cdc_ncm.c
> +++ b/drivers/net/usb/cdc_ncm.c
> @@ -242,11 +242,17 @@ static ssize_t min_tx_pkt_store(struct device *d,
>  	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
>  	unsigned long val;
>  
> -	/* no need to restrict values - anything from 0 to infinity is OK */
>  	if (kstrtoul(buf, 0, &val))
>  		return -EINVAL;
>  
> -	ctx->min_tx_pkt = val;
> +	/* Clamp to tx_max: the frame length can never exceed tx_max,
> +	 * so any threshold above it has the same effect (padding is
> +	 * never applied).  Also cap at U16_MAX since min_tx_pkt is
> +	 * a u16 - without this, values like 65537 silently wrap to 1
> +	 * and invert the intended behavior.
> +	 */
> +	ctx->min_tx_pkt = min_t(unsigned long, val,
> +				min_t(u32, ctx->tx_max, U16_MAX));
>  	return len;
>  }
>  
> -- 
> 2.39.5 (Apple Git-154)
> 
> 

Hi,

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