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, 62 insertions, 0 deletions

diff --git a/fs/hpfs/dentry.c b/fs/hpfs/dentry.c
new file mode 100644
index 000000000..89a36fdc6
--- /dev/null
+++ b/fs/hpfs/dentry.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/fs/hpfs/dentry.c
+ *
+ *  Mikulas Patocka (mikulas@artax.karlin.mff.cuni.cz), 1998-1999
+ *
+ *  dcache operations
+ */
+
+#include "hpfs_fn.h"
+
+/*
+ * Note: the dentry argument is the parent dentry.
+ */
+
+static int hpfs_hash_dentry(const struct dentry *dentry, struct qstr *qstr)
+{
+	unsigned long	 hash;
+	int		 i;
+	unsigned l = qstr->len;
+
+	if (l == 1) if (qstr->name[0]=='.') goto x;
+	if (l == 2) if (qstr->name[0]=='.' || qstr->name[1]=='.') goto x;
+	hpfs_adjust_length(qstr->name, &l);
+	/*if (hpfs_chk_name(qstr->name,&l))*/
+		/*return -ENAMETOOLONG;*/
+		/*return -ENOENT;*/
+	x:
+
+	hash = init_name_hash(dentry);
+	for (i = 0; i < l; i++)
+		hash = partial_name_hash(hpfs_upcase(hpfs_sb(dentry->d_sb)->sb_cp_table,qstr->name[i]), hash);
+	qstr->hash = end_name_hash(hash);
+
+	return 0;
+}
+
+static int hpfs_compare_dentry(const struct dentry *dentry,
+		unsigned int len, const char *str, const struct qstr *name)
+{
+	unsigned al = len;
+	unsigned bl = name->len;
+
+	hpfs_adjust_length(str, &al);
+	/*hpfs_adjust_length(b->name, &bl);*/
+
+	/*
+	 * 'str' is the nane of an already existing dentry, so the name
+	 * must be valid. 'name' must be validated first.
+	 */
+
+	if (hpfs_chk_name(name->name, &bl))
+		return 1;
+	if (hpfs_compare_names(dentry->d_sb, str, al, name->name, bl, 0))
+		return 1;
+	return 0;
+}
+
+const struct dentry_operations hpfs_dentry_operations = {
+	.d_hash		= hpfs_hash_dentry,
+	.d_compare	= hpfs_compare_dentry,
+};