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, 99 insertions, 0 deletions

diff --git a/fs/fuse/req.c b/fs/fuse/req.c
new file mode 100644
index 000000000..a01ee743d
--- /dev/null
+++ b/fs/fuse/req.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include "dev.h"
+#include "fuse_i.h"
+
+static int fuse_fill_creds(struct fuse_mount *fm, struct fuse_args *args, struct mnt_idmap *idmap)
+{
+	struct fuse_conn *fc = fm->fc;
+	bool no_idmap = !fm->sb || (fm->sb->s_iflags & SB_I_NOIDMAP);
+	kuid_t fsuid = mapped_fsuid(idmap, fc->user_ns);
+	kgid_t fsgid = mapped_fsgid(idmap, fc->user_ns);
+
+	args->pid = pid_nr_ns(task_pid(current), fc->pid_ns);
+
+	if (args->force) {
+		if (args->nocreds)
+			return 0;
+
+		if (no_idmap) {
+			args->uid = from_kuid_munged(fc->user_ns, current_fsuid());
+			args->gid = from_kgid_munged(fc->user_ns, current_fsgid());
+		} else {
+			args->uid = FUSE_INVALID_UIDGID;
+			args->gid = FUSE_INVALID_UIDGID;
+		}
+		return 0;
+	}
+
+	WARN_ON(args->nocreds);
+	/*
+	 * Keep the old behavior when idmappings support was not
+	 * declared by a FUSE server.
+	 *
+	 * For those FUSE servers who support idmapped mounts, we send UID/GID
+	 * only along with "inode creation" fuse requests, otherwise idmap ==
+	 * &invalid_mnt_idmap and req->in.h.{u,g}id will be equal to
+	 * FUSE_INVALID_UIDGID.
+	 */
+	if (no_idmap) {
+		fsuid = current_fsuid();
+		fsgid = current_fsgid();
+	}
+	args->uid = from_kuid(fc->user_ns, fsuid);
+	args->gid = from_kgid(fc->user_ns, fsgid);
+
+	if (no_idmap && unlikely(args->uid == ((uid_t)-1) || args->gid == ((gid_t)-1)))
+		return -EOVERFLOW;
+
+	return 0;
+}
+
+static int fuse_req_prep(struct fuse_mount *fm, struct fuse_args *args, struct mnt_idmap *idmap)
+{
+	if (!args->force && fm->fc->conn_error)
+		return -ECONNREFUSED;
+
+	return fuse_fill_creds(fm, args, idmap);
+}
+
+ssize_t __fuse_simple_request(struct mnt_idmap *idmap, struct fuse_mount *fm,
+			      struct fuse_args *args)
+{
+	struct fuse_conn *fc = fm->fc;
+	int err = fuse_req_prep(fm, args, idmap);
+
+	if (err)
+		return err;
+
+	return fuse_chan_send(fc->chan, args);
+}
+
+int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args, gfp_t gfp_flags)
+{
+	struct fuse_conn *fc = fm->fc;
+	int err;
+
+	WARN_ON(args->force && !args->nocreds);
+
+	err = fuse_req_prep(fm, args, &invalid_mnt_idmap);
+	if (err)
+		return err;
+
+	return fuse_chan_send_bg(fc->chan, args, gfp_flags);
+}
+EXPORT_SYMBOL_GPL(fuse_simple_background);
+
+int fuse_simple_notify_reply(struct fuse_mount *fm, struct fuse_args *args, u64 unique)
+{
+	struct fuse_conn *fc = fm->fc;
+	int err;
+
+	WARN_ON(args->force && !args->nocreds);
+
+	err = fuse_req_prep(fm, args, &invalid_mnt_idmap);
+	if (err)
+		return err;
+
+	return fuse_chan_send_notify_reply(fc->chan, args, unique);
+}