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, 42 insertions, 0 deletions

diff --git a/include/uapi/asm-generic/poll.h b/include/uapi/asm-generic/poll.h
new file mode 100644
index 000000000..f9c520ce4
--- /dev/null
+++ b/include/uapi/asm-generic/poll.h
@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef __ASM_GENERIC_POLL_H
+#define __ASM_GENERIC_POLL_H
+
+/* These are specified by iBCS2 */
+#define POLLIN		0x0001
+#define POLLPRI		0x0002
+#define POLLOUT		0x0004
+#define POLLERR		0x0008
+#define POLLHUP		0x0010
+#define POLLNVAL	0x0020
+
+/* The rest seem to be more-or-less nonstandard. Check them! */
+#define POLLRDNORM	0x0040
+#define POLLRDBAND	0x0080
+#ifndef POLLWRNORM
+#define POLLWRNORM	0x0100
+#endif
+#ifndef POLLWRBAND
+#define POLLWRBAND	0x0200
+#endif
+#ifndef POLLMSG
+#define POLLMSG		0x0400
+#endif
+#ifndef POLLREMOVE
+#define POLLREMOVE	0x1000
+#endif
+#ifndef POLLRDHUP
+#define POLLRDHUP       0x2000
+#endif
+
+#define POLLFREE	(__force __poll_t)0x4000
+
+#define POLL_BUSY_LOOP	(__force __poll_t)0x8000
+
+struct pollfd {
+	int fd;
+	short events;
+	short revents;
+};
+
+#endif	/* __ASM_GENERIC_POLL_H */