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 <[email protected]> --- 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),
