netdev - Re: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit

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Message-ID: <dfc94b21-0baa-4b1f-9261-725d8d5c66f0@intel.com>
Date: Mon, 27 Oct 2025 15:59:51 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Nathan Chancellor <nathan@...nel.org>
CC: Kees Cook <kees@...nel.org>, "David S. Miller" <davem@...emloft.net>,
	"Eric Dumazet" <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Nick Desaulniers
	<nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, "Justin
 Stitt" <justinstitt@...gle.com>, Sami Tolvanen <samitolvanen@...gle.com>,
	Russell King <linux@...linux.org.uk>, Tony Nguyen
	<anthony.l.nguyen@...el.com>, Michal Kubiak <michal.kubiak@...el.com>,
	<linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>,
	<linux-arm-kernel@...ts.infradead.org>, <netdev@...r.kernel.org>,
	<intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for
 libeth_xdp_tx_xmit_bulk()

From: Nathan Chancellor <nathan@...nel.org>
Date: Sat, 25 Oct 2025 21:53:20 +0100

> 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));
>         |         ^

I've been wondering why I keep getting these reports for some time
already as my CI bot's never been able to trigger this. So seems like
it's CFI...

>   ...
> 
> 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.
> 
> 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.

Hmmm,

For this patch:

Acked-by: Alexander Lobakin <aleksander.lobakin@...el.com>

However,

The XSk metadata infra in the kernel relies on that when we call
xsk_tx_metadata_request(), we pass a static const struct with our
callbacks and then the compiler makes all these calls direct.
This is not limited to libeth (although I realize that it triggered
this build failure due to the way how I pass these callbacks), every
driver which implements XSk Tx metadata and calls
xsk_tx_metadata_request() relies on that these calls will be direct,
otherwise there'll be such performance penalty that is unacceptable
for XSk speeds.
Maybe xsk_tx_metadata_request() should be __nocfi as well? Or all
the callers of it?

> 
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2124
> Fixes: 9705d6552f58 ("idpf: implement Rx path for AF_XDP")
> Signed-off-by: Nathan Chancellor <nathan@...nel.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),

Thanks,
Olek