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, 191 insertions, 0 deletions

diff --git a/include/net/checksum.h b/include/net/checksum.h
new file mode 100644
index 000000000..3cbab35de
--- /dev/null
+++ b/include/net/checksum.h
@@ -0,0 +1,191 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * INET		An implementation of the TCP/IP protocol suite for the LINUX
+ *		operating system.  INET is implemented using the  BSD Socket
+ *		interface as the means of communication with the user level.
+ *
+ *		Checksumming functions for IP, TCP, UDP and so on
+ *
+ * Authors:	Jorge Cwik, <jorge@laser.satlink.net>
+ *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
+ *		Borrows very liberally from tcp.c and ip.c, see those
+ *		files for more names.
+ */
+
+#ifndef _CHECKSUM_H
+#define _CHECKSUM_H
+
+#include <linux/errno.h>
+#include <asm/types.h>
+#include <asm/byteorder.h>
+#include <asm/checksum.h>
+#if !defined(_HAVE_ARCH_COPY_AND_CSUM_FROM_USER) || !defined(HAVE_CSUM_COPY_USER)
+#include <linux/uaccess.h>
+#endif
+
+#ifndef _HAVE_ARCH_COPY_AND_CSUM_FROM_USER
+static __always_inline
+__wsum csum_and_copy_from_user (const void __user *src, void *dst,
+				      int len)
+{
+	if (copy_from_user(dst, src, len))
+		return 0;
+	return csum_partial(dst, len, ~0U);
+}
+#endif
+
+#ifndef HAVE_CSUM_COPY_USER
+static __always_inline __wsum csum_and_copy_to_user
+(const void *src, void __user *dst, int len)
+{
+	__wsum sum = csum_partial(src, len, ~0U);
+
+	if (copy_to_user(dst, src, len) == 0)
+		return sum;
+	return 0;
+}
+#endif
+
+#ifndef _HAVE_ARCH_CSUM_AND_COPY
+static __always_inline __wsum
+csum_partial_copy_nocheck(const void *src, void *dst, int len)
+{
+	memcpy(dst, src, len);
+	return csum_partial(dst, len, 0);
+}
+#endif
+
+#ifndef HAVE_ARCH_CSUM_ADD
+static __always_inline __wsum csum_add(__wsum csum, __wsum addend)
+{
+	u32 res = (__force u32)csum;
+	res += (__force u32)addend;
+	return (__force __wsum)(res + (res < (__force u32)addend));
+}
+#endif
+
+static __always_inline __wsum csum_sub(__wsum csum, __wsum addend)
+{
+	return csum_add(csum, ~addend);
+}
+
+static __always_inline __sum16 csum16_add(__sum16 csum, __be16 addend)
+{
+	u16 res = (__force u16)csum;
+
+	res += (__force u16)addend;
+	return (__force __sum16)(res + (res < (__force u16)addend));
+}
+
+static __always_inline __sum16 csum16_sub(__sum16 csum, __be16 addend)
+{
+	return csum16_add(csum, ~addend);
+}
+
+#ifndef HAVE_ARCH_CSUM_SHIFT
+static __always_inline __wsum csum_shift(__wsum sum, int offset)
+{
+	/* rotate sum to align it with a 16b boundary */
+	if (offset & 1)
+		return (__force __wsum)ror32((__force u32)sum, 8);
+	return sum;
+}
+#endif
+
+static __always_inline __wsum
+csum_block_add(__wsum csum, __wsum csum2, int offset)
+{
+	return csum_add(csum, csum_shift(csum2, offset));
+}
+
+static __always_inline __wsum
+csum_block_sub(__wsum csum, __wsum csum2, int offset)
+{
+	return csum_block_add(csum, ~csum2, offset);
+}
+
+static __always_inline __wsum csum_unfold(__sum16 n)
+{
+	return (__force __wsum)n;
+}
+
+#define CSUM_MANGLED_0 ((__force __sum16)0xffff)
+
+static __always_inline void csum_replace_by_diff(__sum16 *sum, __wsum diff)
+{
+	*sum = csum_fold(csum_add(diff, ~csum_unfold(*sum)));
+}
+
+static __always_inline void csum_replace4(__sum16 *sum, __be32 from, __be32 to)
+{
+	__wsum tmp = csum_sub(~csum_unfold(*sum), (__force __wsum)from);
+
+	*sum = csum_fold(csum_add(tmp, (__force __wsum)to));
+}
+
+/* Implements RFC 1624 (Incremental Internet Checksum)
+ * 3. Discussion states :
+ *     HC' = ~(~HC + ~m + m')
+ *  m : old value of a 16bit field
+ *  m' : new value of a 16bit field
+ */
+static __always_inline void csum_replace2(__sum16 *sum, __be16 old, __be16 new)
+{
+	*sum = ~csum16_add(csum16_sub(~(*sum), old), new);
+}
+
+static inline void csum_replace(__wsum *csum, __wsum old, __wsum new)
+{
+	*csum = csum_add(csum_sub(*csum, old), new);
+}
+
+static inline unsigned short csum_from32to16(unsigned int sum)
+{
+	sum += (sum >> 16) | (sum << 16);
+	return (unsigned short)(sum >> 16);
+}
+
+struct sk_buff;
+void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
+			      __be32 from, __be32 to, bool pseudohdr);
+void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
+			       const __be32 *from, const __be32 *to,
+			       bool pseudohdr);
+void inet_proto_csum_replace_by_diff(__sum16 *sum, struct sk_buff *skb,
+				     __wsum diff, bool pseudohdr, bool ipv6);
+
+static __always_inline
+void inet_proto_csum_replace2(__sum16 *sum, struct sk_buff *skb,
+			      __be16 from, __be16 to, bool pseudohdr)
+{
+	inet_proto_csum_replace4(sum, skb, (__force __be32)from,
+				 (__force __be32)to, pseudohdr);
+}
+
+static __always_inline __wsum remcsum_adjust(void *ptr, __wsum csum,
+					     int start, int offset)
+{
+	__sum16 *psum = (__sum16 *)(ptr + offset);
+	__wsum delta;
+
+	/* Subtract out checksum up to start */
+	csum = csum_sub(csum, csum_partial(ptr, start, 0));
+
+	/* Set derived checksum in packet */
+	delta = csum_sub((__force __wsum)csum_fold(csum),
+			 (__force __wsum)*psum);
+	*psum = csum_fold(csum);
+
+	return delta;
+}
+
+static __always_inline void remcsum_unadjust(__sum16 *psum, __wsum delta)
+{
+	*psum = csum_fold(csum_sub(delta, (__force __wsum)*psum));
+}
+
+static __always_inline __wsum wsum_negate(__wsum val)
+{
+	return (__force __wsum)-((__force u32)val);
+}
+#endif