[PATCH] platform/surface: aggregator: Fix braces in if condition with unlikely() macro

[PATCH] platform/surface: aggregator: Fix braces in if condition with unlikely() macro

[Maximilian Luz]

The braces of the unlikely() macro inside the if condition only cover
the subtraction part, not the whole statement. This causes the result of
the subtraction to be converted to zero or one. While that still works
in this context, it causes static analysis tools to complain (and is
just plain wrong).

Fix the bracket placement and, while at it, simplify the if-condition.
Also add a comment to the if-condition explaining what we expect the
result to be and what happens on the failure path, as it seems to have
caused a bit of confusion.

This commit should not cause any difference in behavior or generated
code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: c167b9c7e3d6 ("platform/surface: Add Surface Aggregator subsystem")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
---
 .../surface/aggregator/ssh_packet_layer.c     | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/platform/surface/aggregator/ssh_packet_layer.c b/drivers/platform/surface/aggregator/ssh_packet_layer.c
index 74f0faaa2b27..583315db8b02 100644
--- a/drivers/platform/surface/aggregator/ssh_packet_layer.c
+++ b/drivers/platform/surface/aggregator/ssh_packet_layer.c
@@ -1694,7 +1694,24 @@ static size_t ssh_ptl_rx_eval(struct ssh_ptl *ptl, struct ssam_span *source)
 	/* Find SYN. */
 	syn_found = sshp_find_syn(source, &aligned);
 
-	if (unlikely(aligned.ptr - source->ptr) > 0) {
+	if (unlikely(aligned.ptr != source->ptr)) {
+		/*
+		 * We expect aligned.ptr == source->ptr. If this is not the
+		 * case, then aligned.ptr > source->ptr and we've encountered
+		 * some unexpected data where we'd expect the start of a new
+		 * message (i.e. the SYN sequence).
+		 *
+		 * This can happen when a CRC check for the previous message
+		 * failed and we start actively searching for the next one
+		 * (via the call to sshp_find_syn() above), or the first bytes
+		 * of a message got dropped or corrupted.
+		 *
+		 * In any case, we issue a warning, send a NAK to the EC to
+		 * request re-transmission of any data we haven't acknowledged
+		 * yet, and finally, skip everything up to the next SYN
+		 * sequence.
+		 */
+
 		ptl_warn(ptl, "rx: parser: invalid start of frame, skipping\n");
 
 		/*
-- 
2.30.0

Re: [PATCH] platform/surface: aggregator: Fix braces in if condition with unlikely() macro

[Hans de Goede]

Hi,

On 1/26/21 6:22 PM, Maximilian Luz wrote:
> The braces of the unlikely() macro inside the if condition only cover
> the subtraction part, not the whole statement. This causes the result of
> the subtraction to be converted to zero or one. While that still works
> in this context, it causes static analysis tools to complain (and is
> just plain wrong).
> 
> Fix the bracket placement and, while at it, simplify the if-condition.
> Also add a comment to the if-condition explaining what we expect the
> result to be and what happens on the failure path, as it seems to have
> caused a bit of confusion.
> 
> This commit should not cause any difference in behavior or generated
> code.
> 
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Fixes: c167b9c7e3d6 ("platform/surface: Add Surface Aggregator subsystem")
> Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>

Thank you for your patch, I've applied this patch to my review-hans 
branch:
https://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git/log/?h=review-hans

Note it will show up in my review-hans branch once I've pushed my
local branch there, which might take a while.

Once I've run some tests on this branch the patches there will be
added to the platform-drivers-x86/for-next branch and eventually
will be included in the pdx86 pull-request to Linus for the next
merge-window.

Regards,

Hans


> ---
>  .../surface/aggregator/ssh_packet_layer.c     | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/platform/surface/aggregator/ssh_packet_layer.c b/drivers/platform/surface/aggregator/ssh_packet_layer.c
> index 74f0faaa2b27..583315db8b02 100644
> --- a/drivers/platform/surface/aggregator/ssh_packet_layer.c
> +++ b/drivers/platform/surface/aggregator/ssh_packet_layer.c
> @@ -1694,7 +1694,24 @@ static size_t ssh_ptl_rx_eval(struct ssh_ptl *ptl, struct ssam_span *source)
>  	/* Find SYN. */
>  	syn_found = sshp_find_syn(source, &aligned);
>  
> -	if (unlikely(aligned.ptr - source->ptr) > 0) {
> +	if (unlikely(aligned.ptr != source->ptr)) {
> +		/*
> +		 * We expect aligned.ptr == source->ptr. If this is not the
> +		 * case, then aligned.ptr > source->ptr and we've encountered
> +		 * some unexpected data where we'd expect the start of a new
> +		 * message (i.e. the SYN sequence).
> +		 *
> +		 * This can happen when a CRC check for the previous message
> +		 * failed and we start actively searching for the next one
> +		 * (via the call to sshp_find_syn() above), or the first bytes
> +		 * of a message got dropped or corrupted.
> +		 *
> +		 * In any case, we issue a warning, send a NAK to the EC to
> +		 * request re-transmission of any data we haven't acknowledged
> +		 * yet, and finally, skip everything up to the next SYN
> +		 * sequence.
> +		 */
> +
>  		ptl_warn(ptl, "rx: parser: invalid start of frame, skipping\n");
>  
>  		/*
>