xdp-loader: Make feature flags printing future-proof #528
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tohojo
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Oct 9, 2025
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Thank you for the patch! It makes sense to stop using the mask, but see below...
In netdev.h, NETDEV_XDP_ACT_MASK is declared as private, so it should only be used inside the kernel, and according to Jakub Kicinski then "user space should not care about it"[1]. linux$ git grep -B1 NETDEV_XDP_ACT_MASK include/ include/uapi/linux/netdev.h- /* private: */ include/uapi/linux/netdev.h: NETDEV_XDP_ACT_MASK = 127, In this specific case, then this code is buggy. If a new feature flag is added to the uapi header, and hence included in NETDEV_XDP_ACT_MASK, and xdp-tools is not updated, or an older version is re-build in a distro, with newer linux headers, then NETDEV_XDP_ACT_MASK can contain a new feature flag, and if the new flag is observed, then it will neither be checked nor trigger the unknown feature printing. This patch moves the flags into a constant array, and while looping over them in iface_print_xdp_features(), it builds a local mask consisting only of the tested flags. The local mask is used to replace NETDEV_XDP_ACT_MASK. [1] https://lore.kernel.org/netdev/[email protected]/ Signed-off-by: Asbjørn Sloth Tønnesen <[email protected]>
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In netdev.h, NETDEV_XDP_ACT_MASK is declared as private, so it should only be used inside the kernel, and according to Jakub Kicinski then "user space should not care about it"[1].
In this specific case, then this code is buggy. If a new feature flag is added to the uapi header, and hence included in NETDEV_XDP_ACT_MASK, and xdp-tools is not updated, or an older version is re-build in a distro, with newer linux headers, then NETDEV_XDP_ACT_MASK can contain a new feature flag, and if the new flag is observed, then it will neither be checked nor trigger the unknown feature printing.
This patch replaces the use of NETDEV_XDP_ACT_MASK, with a local mask consisting only of the tested flags.
[1] https://lore.kernel.org/netdev/[email protected]/