linux-kernel - [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit

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Message-Id: <20251025-idpf-fix-arm-kcfi-build-error-v1-3-ec57221153ae@kernel.org>
Date: Sat, 25 Oct 2025 21:53:20 +0100
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>, 
 Alexander Lobakin <aleksander.lobakin@...el.com>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Nathan Chancellor <nathan@...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: [PATCH 3/3] libeth: xdp: Disable generic kCFI pass for
 libeth_xdp_tx_xmit_bulk()

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.

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@...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),

-- 
2.51.1