[PATCH net] tools: ynl: c: correct reverse decode of empty attrs

[PATCH net] tools: ynl: c: correct reverse decode of empty attrs

[Jakub Kicinski]

netlink reports which attribute was incorrect by sending back
an attribute offset. Offset points to the address of struct nlattr,
but to interpret the type we also need the nesting path.
Attribute IDs have different meaning in different nests
of the same message.

Correct the condition for "is the offset within current attribute".
ynl_attr_data_len() does not include the attribute header,
so the end offset was off by 4 bytes.

This means that we'd always skip over flags and empty nests.

The devmem tests, for example, issues an invalid request with
empty queue nests, resulting in the following error:

  YNL failed: Kernel error: missing attribute: .queues.ifindex

The message is incorrect, "queues" nest does not have an "ifindex"
attribute defined. With this fix we decend correctly into the nest:

  YNL failed: Kernel error: missing attribute: .queues.id

Fixes: 86878f14d71a ("tools: ynl: user space helpers")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: donald.hunter@gmail.com
CC: nicolas.dichtel@6wind.com
CC: willemb@google.com
---
 tools/net/ynl/lib/ynl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/net/ynl/lib/ynl.c b/tools/net/ynl/lib/ynl.c
index e16cef160bc2..ce32cb35007d 100644
--- a/tools/net/ynl/lib/ynl.c
+++ b/tools/net/ynl/lib/ynl.c
@@ -95,7 +95,7 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
 
 	ynl_attr_for_each_payload(start, data_len, attr) {
 		astart_off = (char *)attr - (char *)start;
-		aend_off = astart_off + ynl_attr_data_len(attr);
+		aend_off = (char *)ynl_attr_data_end(attr) - (char *)start;
 		if (aend_off <= off)
 			continue;
 
-- 
2.48.1

Re: [PATCH net] tools: ynl: c: correct reverse decode of empty attrs

[Donald Hunter]

Jakub Kicinski <kuba@kernel.org> writes:

> netlink reports which attribute was incorrect by sending back
> an attribute offset. Offset points to the address of struct nlattr,
> but to interpret the type we also need the nesting path.
> Attribute IDs have different meaning in different nests
> of the same message.
>
> Correct the condition for "is the offset within current attribute".
> ynl_attr_data_len() does not include the attribute header,
> so the end offset was off by 4 bytes.
>
> This means that we'd always skip over flags and empty nests.
>
> The devmem tests, for example, issues an invalid request with
> empty queue nests, resulting in the following error:
>
>   YNL failed: Kernel error: missing attribute: .queues.ifindex
>
> The message is incorrect, "queues" nest does not have an "ifindex"
> attribute defined. With this fix we decend correctly into the nest:
>
>   YNL failed: Kernel error: missing attribute: .queues.id
>
> Fixes: 86878f14d71a ("tools: ynl: user space helpers")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> CC: donald.hunter@gmail.com
> CC: nicolas.dichtel@6wind.com
> CC: willemb@google.com
> ---
>  tools/net/ynl/lib/ynl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/net/ynl/lib/ynl.c b/tools/net/ynl/lib/ynl.c
> index e16cef160bc2..ce32cb35007d 100644
> --- a/tools/net/ynl/lib/ynl.c
> +++ b/tools/net/ynl/lib/ynl.c
> @@ -95,7 +95,7 @@ ynl_err_walk(struct ynl_sock *ys, void *start, void *end, unsigned int off,
>  
>  	ynl_attr_for_each_payload(start, data_len, attr) {
>  		astart_off = (char *)attr - (char *)start;
> -		aend_off = astart_off + ynl_attr_data_len(attr);
> +		aend_off = (char *)ynl_attr_data_end(attr) - (char *)start;
>  		if (aend_off <= off)
>  			continue;

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>

Re: [PATCH net] tools: ynl: c: correct reverse decode of empty attrs

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 23 Jan 2025 17:21:30 -0800 you wrote:
> netlink reports which attribute was incorrect by sending back
> an attribute offset. Offset points to the address of struct nlattr,
> but to interpret the type we also need the nesting path.
> Attribute IDs have different meaning in different nests
> of the same message.
> 
> Correct the condition for "is the offset within current attribute".
> ynl_attr_data_len() does not include the attribute header,
> so the end offset was off by 4 bytes.
> 
> [...]

Here is the summary with links:
  - [net] tools: ynl: c: correct reverse decode of empty attrs
    https://git.kernel.org/netdev/net/c/964417a5d4a0

You are awesome, thank you!
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