Re: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit_bulk()

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 10/25/25 22:53, Nathan Chancellor wrote:
> When building drivers/net/ethernet/intel/idpf/xsk.c for ARCH=arm with
> CONFIG_CFI=y using a version of LLVM prior to 22.0.0, there is a
> BUILD_BUG_ON failure:
> 
>    $ cat arch/arm/configs/repro.config
>    CONFIG_BPF_SYSCALL=y
>    CONFIG_CFI=y
>    CONFIG_IDPF=y
>    CONFIG_XDP_SOCKETS=y
> 
>    $ make -skj"$(nproc)" ARCH=arm LLVM=1 clean defconfig repro.config drivers/net/ethernet/intel/idpf/xsk.o
>    In file included from drivers/net/ethernet/intel/idpf/xsk.c:4:
>    include/net/libeth/xsk.h:205:2: error: call to '__compiletime_assert_728' declared with 'error' attribute: BUILD_BUG_ON failed: !__builtin_constant_p(tmo == libeth_xsktmo)
>      205 |         BUILD_BUG_ON(!__builtin_constant_p(tmo == libeth_xsktmo));
>          |         ^
>    ...
> 
> libeth_xdp_tx_xmit_bulk() indirectly calls libeth_xsk_xmit_fill_buf()
> but these functions are marked as __always_inline so that the compiler
> can turn these indirect calls into direct ones and see that the tmo
> parameter to __libeth_xsk_xmit_fill_buf_md() is ultimately libeth_xsktmo
> from idpf_xsk_xmit().
> 
> Unfortunately, the generic kCFI pass in LLVM expands the kCFI bundles
> from the indirect calls in libeth_xdp_tx_xmit_bulk() in such a way that
> later optimizations cannot turn these calls into direct ones, making the
> BUILD_BUG_ON fail because it cannot be proved at compile time that tmo
> is libeth_xsktmo.
> 

sorry, but from regular driver developer perspective, just after reading
your commit messages, I'm unable to tell what the fix is about, and from
that follows a bigger issue: how to write code in the future to avoid
such issues (it would be infeasible to always wait for a LLVM specialist
to come up with a fix ;))

was the tricky case to call __always_inline func from another that was
marked the same? Would it be also the case if one of the functions would
not be marked with __always_inline attribute, but still end up inlined?

what would be the cost of the alternative naive solution, to always add
__nocfi_generic to functions marked __always_inline?
(technically you would redefine __always_inline to have also
__nocfi_generic for the config combinations that require that)

sorry for my ignorance of not reading any of the attached URLs

> Disable the generic kCFI pass for libeth_xdp_tx_xmit_bulk() to ensure
> these indirect calls can always be turned into direct calls to avoid
> this error.
> 
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2124
> Fixes: 9705d6552f58 ("idpf: implement Rx path for AF_XDP")
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>   include/net/libeth/xdp.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/libeth/xdp.h b/include/net/libeth/xdp.h
> index bc3507edd589..898723ab62e8 100644
> --- a/include/net/libeth/xdp.h
> +++ b/include/net/libeth/xdp.h
> @@ -513,7 +513,7 @@ struct libeth_xdp_tx_desc {
>    * can't fail, but can send less frames if there's no enough free descriptors
>    * available. The actual free space is returned by @prep from the driver.
>    */
> -static __always_inline u32
> +static __always_inline __nocfi_generic u32
>   libeth_xdp_tx_xmit_bulk(const struct libeth_xdp_tx_frame *bulk, void *xdpsq,
>   			u32 n, bool unroll, u64 priv,
>   			u32 (*prep)(void *xdpsq, struct libeth_xdpsq *sq),
>