kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 188 insertions, 0 deletions

diff --git a/usr/include/Makefile b/usr/include/Makefile
new file mode 100644
index 000000000..ee69dd9d9
--- /dev/null
+++ b/usr/include/Makefile
@@ -0,0 +1,188 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+# Unlike the kernel space, exported headers are written in standard C.
+#  - Forbid C++ style comments
+#  - Use '__inline__', '__asm__' instead of 'inline', 'asm'
+#
+# -std=c90 (equivalent to -ansi) catches the violation of those.
+# We cannot go as far as adding -Wpedantic since it emits too many warnings.
+UAPI_CFLAGS := -std=c90 -Werror=implicit-function-declaration
+UAPI_ARCH := $(or $(HEADER_ARCH),$(SRCARCH))
+
+override c_flags = $(KBUILD_USERCFLAGS) $(UAPI_CFLAGS) -Wp,-MMD,$(depfile)
+cxx_flags = $(filter-out -Wmissing-prototypes -Wstrict-prototypes -std=%, $(KBUILD_USERCFLAGS)) -std=c++98
+
+# The following are excluded for now because they fail to build.
+#
+# Do not add a new header to the blacklist without legitimate reason.
+# Please consider to fix the header first.
+#
+# Sorted alphabetically.
+no-header-test += asm/ucontext.h
+no-header-test += drm/vmwgfx_drm.h
+no-header-test += linux/am437x-vpfe.h
+no-header-test += linux/coda.h
+no-header-test += linux/cyclades.h
+no-header-test += linux/errqueue.h
+no-header-test += linux/hdlc/ioctl.h
+no-header-test += linux/ivtv.h
+no-header-test += linux/matroxfb.h
+no-header-test += linux/omap3isp.h
+no-header-test += linux/omapfb.h
+no-header-test += linux/patchkey.h
+no-header-test += linux/phonet.h
+no-header-test += linux/sctp.h
+no-header-test += linux/sysctl.h
+no-header-test += linux/usb/audio.h
+no-header-test += linux/v4l2-mediabus.h
+no-header-test += linux/v4l2-subdev.h
+no-header-test += linux/videodev2.h
+no-header-test += linux/vm_sockets.h
+no-header-test += sound/asequencer.h
+no-header-test += sound/asoc.h
+no-header-test += sound/asound.h
+no-header-test += sound/compress_offload.h
+no-header-test += sound/emu10k1.h
+no-header-test += sound/sfnt_info.h
+no-header-test += xen/evtchn.h
+no-header-test += xen/gntdev.h
+no-header-test += xen/privcmd.h
+
+# More headers are broken in some architectures
+
+ifneq ($(filter arc openrisc xtensa nios2, $(UAPI_ARCH)),)
+no-header-test += linux/bpf_perf_event.h
+endif
+
+ifeq ($(UAPI_ARCH),sparc)
+no-header-test += asm/uctx.h
+no-header-test += asm/fbio.h
+endif
+
+# asm-generic/*.h is used by asm/*.h, and should not be included directly
+no-header-test += asm-generic/%
+
+# The following are not compatible with C++.
+#
+# Do not add a new header to the list without legitimate reason.
+# Please consider to fix the header first.
+#
+# Sorted alphabetically.
+no-header-test-cxx += linux/auto_dev-ioctl.h
+no-header-test-cxx += linux/map_to_14segment.h
+no-header-test-cxx += linux/map_to_7segment.h
+no-header-test-cxx += linux/netfilter/xt_sctp.h
+no-header-test-cxx += linux/target_core_user.h
+no-header-test-cxx += linux/vhost.h
+no-header-test-cxx += linux/vhost_types.h
+no-header-test-cxx += linux/virtio_net.h
+no-header-test-cxx += linux/virtio_ring.h
+no-header-test-cxx += scsi/fc/fc_els.h
+
+ifeq ($(UAPI_ARCH),x86)
+no-header-test-cxx += asm/elf.h
+endif
+
+# The following are using libc header and types.
+#
+# Do not add a new header to the list without legitimate reason.
+# Please consider to fix the header first.
+#
+# Sorted alphabetically.
+uses-libc += linux/a.out.h
+uses-libc += linux/atmbr2684.h
+uses-libc += linux/auto_dev-ioctl.h
+uses-libc += linux/auto_fs.h
+uses-libc += linux/auto_fs4.h
+uses-libc += linux/btrfs_tree.h
+uses-libc += linux/cec-funcs.h
+uses-libc += linux/cec.h
+uses-libc += linux/dvb/dmx.h
+uses-libc += linux/dvb/video.h
+uses-libc += linux/ethtool.h
+uses-libc += linux/ethtool_netlink.h
+uses-libc += linux/fuse.h
+uses-libc += linux/gsmmux.h
+uses-libc += linux/icmp.h
+uses-libc += linux/idxd.h
+uses-libc += linux/if.h
+uses-libc += linux/if_arp.h
+uses-libc += linux/if_bonding.h
+uses-libc += linux/if_pppox.h
+uses-libc += linux/if_tunnel.h
+uses-libc += linux/input.h
+uses-libc += linux/ip6_tunnel.h
+uses-libc += linux/joystick.h
+uses-libc += linux/llc.h
+uses-libc += linux/mctp.h
+uses-libc += linux/mdio.h
+uses-libc += linux/mii.h
+uses-libc += linux/mptcp.h
+uses-libc += linux/netdevice.h
+uses-libc += linux/netfilter/xt_RATEEST.h
+uses-libc += linux/netfilter/xt_hashlimit.h
+uses-libc += linux/netfilter/xt_physdev.h
+uses-libc += linux/netfilter/xt_rateest.h
+uses-libc += linux/netfilter_arp/arp_tables.h
+uses-libc += linux/netfilter_arp/arpt_mangle.h
+uses-libc += linux/netfilter_bridge.h
+uses-libc += linux/netfilter_bridge/ebtables.h
+uses-libc += linux/netfilter_ipv4.h
+uses-libc += linux/netfilter_ipv4/ip_tables.h
+uses-libc += linux/netfilter_ipv6.h
+uses-libc += linux/netfilter_ipv6/ip6_tables.h
+uses-libc += linux/route.h
+uses-libc += linux/shm.h
+uses-libc += linux/soundcard.h
+uses-libc += linux/string.h
+uses-libc += linux/tipc_config.h
+uses-libc += linux/uhid.h
+uses-libc += linux/uinput.h
+uses-libc += linux/vhost.h
+uses-libc += linux/vhost_types.h
+uses-libc += linux/virtio_ring.h
+uses-libc += linux/wireless.h
+uses-libc += regulator/regulator.h
+uses-libc += scsi/fc/fc_els.h
+
+ifeq ($(UAPI_ARCH),hexagon)
+uses-libc += asm/sigcontext.h
+endif
+
+ifeq ($(UAPI_ARCH),nios2)
+uses-libc += asm/ptrace.h
+endif
+
+ifeq ($(UAPI_ARCH),s390)
+uses-libc += asm/chpid.h
+uses-libc += asm/chsc.h
+endif
+
+always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
+
+# $(cc-option) forces '-x c' which breaks '-x c++' detection.
+cc-can-compile-cxx := $(call try-run,$(CC) $(CLANG_FLAGS) -c -x c++ /dev/null -o "$$TMP", 1)
+
+target-libc = $(filter $(uses-libc), $*.h)
+target-can-compile = $(filter-out $(no-header-test), $*.h)
+target-can-compile-cxx = $(and $(cc-can-compile-cxx), $(target-can-compile), $(filter-out $(no-header-test-cxx), $*.h))
+
+hdrtest-flags = -fsyntax-only -Werror \
+		-nostdinc -I $(obj) $(if $(target-libc), -I $(srctree)/usr/dummy-include)
+
+# Include the header twice to detect missing include guard.
+quiet_cmd_hdrtest = HDRTEST $<
+      cmd_hdrtest = \
+		$(CC) $(c_flags) $(hdrtest-flags) -x c /dev/null \
+			$(if $(target-can-compile), -include $< -include $<); \
+		$(if $(target-can-compile-cxx), \
+			$(CC) $(cxx_flags) $(hdrtest-flags) -x c++ /dev/null -include $<;) \
+		$(PERL) $(src)/headers_check.pl $(obj) $<; \
+		touch $@
+
+$(obj)/%.hdrtest: $(obj)/%.h $(src)/headers_check.pl FORCE
+	$(call if_changed_dep,hdrtest)
+
+# Since GNU Make 4.3, $(patsubst $(obj)/%/,%,$(wildcard $(obj)/*/)) works.
+# To support older Make versions, use a somewhat tedious way.
+clean-files += $(filter-out Makefile headers_check.pl, $(notdir $(wildcard $(obj)/*)))