There are two different ways that LLVM can expand kCFI operand bundles in LLVM IR: generically in the middle end or using an architecture specific sequence when lowering LLVM IR to machine code in the backend. The generic pass allows any architecture to take advantage of kCFI but the expansion of these bundles in the middle end can mess with optimizations that may turn indirect calls into direct calls when the call target is known at compile time, such as after inlining. Add __nocfi_generic, dependent on an architecture selecting CONFIG_ARCH_USES_CFI_GENERIC_LLVM_PASS, to disable kCFI bundle generation in functions where only the generic kCFI pass may cause problems. Link: https://github.com/ClangBuiltLinux/linux/issues/2124 Signed-off-by: Nathan Chancellor --- arch/Kconfig | 7 +++++++ include/linux/compiler_types.h | 6 ++++++ 2 files changed, 13 insertions(+) diff --git a/arch/Kconfig b/arch/Kconfig index 74ff01133532..61130b88964b 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -917,6 +917,13 @@ config ARCH_USES_CFI_TRAPS An architecture should select this option if it requires the .kcfi_traps section for KCFI trap handling. +config ARCH_USES_CFI_GENERIC_LLVM_PASS + bool + help + An architecture should select this option if it uses the generic + KCFIPass in LLVM to expand kCFI bundles instead of architecture-specific + lowering. + config CFI bool "Use Kernel Control Flow Integrity (kCFI)" default CFI_CLANG diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 59288a2c1ad2..1414be493738 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -461,6 +461,12 @@ struct ftrace_likely_data { # define __nocfi #endif +#if defined(CONFIG_ARCH_USES_CFI_GENERIC_LLVM_PASS) +# define __nocfi_generic __nocfi +#else +# define __nocfi_generic +#endif + /* * Any place that could be marked with the "alloc_size" attribute is also * a place to be marked with the "malloc" attribute, except those that may -- 2.51.1 Prior to clang 22.0.0 [1], ARM did not have an architecture specific kCFI bundle lowering in the backend, which may cause issues. Select CONFIG_ARCH_USES_CFI_GENERIC_LLVM_PASS to enable use of __nocfi_generic. Link: https://github.com/llvm/llvm-project/commit/d130f402642fba3d065aacb506cb061c899558de [1] Link: https://github.com/ClangBuiltLinux/linux/issues/2124 Signed-off-by: Nathan Chancellor --- arch/arm/Kconfig | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 2e3f93b690f4..4fb985b76e97 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -44,6 +44,8 @@ config ARM select ARCH_USE_BUILTIN_BSWAP select ARCH_USE_CMPXCHG_LOCKREF select ARCH_USE_MEMTEST + # https://github.com/llvm/llvm-project/commit/d130f402642fba3d065aacb506cb061c899558de + select ARCH_USES_CFI_GENERIC_LLVM_PASS if CLANG_VERSION < 220000 select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU select ARCH_WANT_GENERAL_HUGETLB select ARCH_WANT_IPC_PARSE_VERSION -- 2.51.1 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 --- 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