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, 243 insertions, 0 deletions

diff --git a/net/sunrpc/auth_unix.c b/net/sunrpc/auth_unix.c
new file mode 100644
index 000000000..6c742a340
--- /dev/null
+++ b/net/sunrpc/auth_unix.c
@@ -0,0 +1,243 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/net/sunrpc/auth_unix.c
+ *
+ * UNIX-style authentication; no AUTH_SHORT support
+ *
+ * Copyright (C) 1996, Olaf Kirch <okir@monad.swb.de>
+ */
+
+#include <linux/slab.h>
+#include <linux/types.h>
+#include <linux/sched.h>
+#include <linux/module.h>
+#include <linux/mempool.h>
+#include <linux/sunrpc/clnt.h>
+#include <linux/sunrpc/auth.h>
+#include <linux/user_namespace.h>
+
+
+#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
+# define RPCDBG_FACILITY	RPCDBG_AUTH
+#endif
+
+static struct rpc_auth		unix_auth;
+static const struct rpc_credops	unix_credops;
+static mempool_t		*unix_pool;
+
+static struct rpc_auth *
+unx_create(const struct rpc_auth_create_args *args, struct rpc_clnt *clnt)
+{
+	refcount_inc(&unix_auth.au_count);
+	return &unix_auth;
+}
+
+static void
+unx_destroy(struct rpc_auth *auth)
+{
+}
+
+/*
+ * Lookup AUTH_UNIX creds for current process
+ */
+static struct rpc_cred *unx_lookup_cred(struct rpc_auth *auth,
+					struct auth_cred *acred, int flags)
+{
+	struct rpc_cred *ret;
+
+	ret = kmalloc_obj(*ret, rpc_task_gfp_mask());
+	if (!ret) {
+		if (!(flags & RPCAUTH_LOOKUP_ASYNC))
+			return ERR_PTR(-ENOMEM);
+		ret = mempool_alloc(unix_pool, GFP_NOWAIT);
+		if (!ret)
+			return ERR_PTR(-ENOMEM);
+	}
+	rpcauth_init_cred(ret, acred, auth, &unix_credops);
+	ret->cr_flags = 1UL << RPCAUTH_CRED_UPTODATE;
+	return ret;
+}
+
+static void
+unx_free_cred_callback(struct rcu_head *head)
+{
+	struct rpc_cred *rpc_cred = container_of(head, struct rpc_cred, cr_rcu);
+
+	put_cred(rpc_cred->cr_cred);
+	mempool_free(rpc_cred, unix_pool);
+}
+
+static void
+unx_destroy_cred(struct rpc_cred *cred)
+{
+	call_rcu(&cred->cr_rcu, unx_free_cred_callback);
+}
+
+/*
+ * Match credentials against current the auth_cred.
+ */
+static int
+unx_match(struct auth_cred *acred, struct rpc_cred *cred, int flags)
+{
+	unsigned int groups = 0;
+	unsigned int i;
+
+	if (cred->cr_cred == acred->cred)
+		return 1;
+
+	if (!uid_eq(cred->cr_cred->fsuid, acred->cred->fsuid) || !gid_eq(cred->cr_cred->fsgid, acred->cred->fsgid))
+		return 0;
+
+	if (acred->cred->group_info != NULL)
+		groups = acred->cred->group_info->ngroups;
+	if (groups > UNX_NGROUPS)
+		groups = UNX_NGROUPS;
+	if (cred->cr_cred->group_info == NULL)
+		return groups == 0;
+	if (groups != cred->cr_cred->group_info->ngroups)
+		return 0;
+
+	for (i = 0; i < groups ; i++)
+		if (!gid_eq(cred->cr_cred->group_info->gid[i], acred->cred->group_info->gid[i]))
+			return 0;
+	return 1;
+}
+
+/*
+ * Marshal credentials.
+ * Maybe we should keep a cached credential for performance reasons.
+ */
+static int
+unx_marshal(struct rpc_task *task, struct xdr_stream *xdr)
+{
+	struct rpc_clnt	*clnt = task->tk_client;
+	struct rpc_cred	*cred = task->tk_rqstp->rq_cred;
+	__be32		*p, *cred_len, *gidarr_len;
+	int		i;
+	struct group_info *gi = cred->cr_cred->group_info;
+	struct user_namespace *userns = clnt->cl_cred ?
+		clnt->cl_cred->user_ns : &init_user_ns;
+
+	/* Credential */
+
+	p = xdr_reserve_space(xdr, 3 * sizeof(*p));
+	if (!p)
+		goto marshal_failed;
+	*p++ = rpc_auth_unix;
+	cred_len = p++;
+	*p++ = xdr_zero;	/* stamp */
+	if (xdr_stream_encode_opaque(xdr, clnt->cl_nodename,
+				     clnt->cl_nodelen) < 0)
+		goto marshal_failed;
+	p = xdr_reserve_space(xdr, 3 * sizeof(*p));
+	if (!p)
+		goto marshal_failed;
+	*p++ = cpu_to_be32(from_kuid_munged(userns, cred->cr_cred->fsuid));
+	*p++ = cpu_to_be32(from_kgid_munged(userns, cred->cr_cred->fsgid));
+
+	gidarr_len = p++;
+	if (gi)
+		for (i = 0; i < UNX_NGROUPS && i < gi->ngroups; i++)
+			*p++ = cpu_to_be32(from_kgid_munged(userns, gi->gid[i]));
+	*gidarr_len = cpu_to_be32(p - gidarr_len - 1);
+	*cred_len = cpu_to_be32((p - cred_len - 1) << 2);
+	p = xdr_reserve_space(xdr, (p - gidarr_len - 1) << 2);
+	if (!p)
+		goto marshal_failed;
+
+	/* Verifier */
+
+	p = xdr_reserve_space(xdr, 2 * sizeof(*p));
+	if (!p)
+		goto marshal_failed;
+	*p++ = rpc_auth_null;
+	*p   = xdr_zero;
+
+	return 0;
+
+marshal_failed:
+	return -EMSGSIZE;
+}
+
+/*
+ * Refresh credentials. This is a no-op for AUTH_UNIX
+ */
+static int
+unx_refresh(struct rpc_task *task)
+{
+	set_bit(RPCAUTH_CRED_UPTODATE, &task->tk_rqstp->rq_cred->cr_flags);
+	return 0;
+}
+
+static int
+unx_validate(struct rpc_task *task, struct xdr_stream *xdr)
+{
+	struct rpc_auth *auth = task->tk_rqstp->rq_cred->cr_auth;
+	__be32 *p;
+	u32 size;
+
+	p = xdr_inline_decode(xdr, 2 * sizeof(*p));
+	if (!p)
+		return -EIO;
+	switch (*p++) {
+	case rpc_auth_null:
+	case rpc_auth_unix:
+	case rpc_auth_short:
+		break;
+	default:
+		return -EIO;
+	}
+	size = be32_to_cpup(p);
+	if (size > RPC_MAX_AUTH_SIZE)
+		return -EIO;
+	p = xdr_inline_decode(xdr, size);
+	if (!p)
+		return -EIO;
+
+	auth->au_verfsize = XDR_QUADLEN(size) + 2;
+	auth->au_rslack = XDR_QUADLEN(size) + 2;
+	auth->au_ralign = XDR_QUADLEN(size) + 2;
+	return 0;
+}
+
+int __init rpc_init_authunix(void)
+{
+	unix_pool = mempool_create_kmalloc_pool(16, sizeof(struct rpc_cred));
+	return unix_pool ? 0 : -ENOMEM;
+}
+
+void rpc_destroy_authunix(void)
+{
+	mempool_destroy(unix_pool);
+}
+
+const struct rpc_authops authunix_ops = {
+	.owner		= THIS_MODULE,
+	.au_flavor	= RPC_AUTH_UNIX,
+	.au_name	= "UNIX",
+	.create		= unx_create,
+	.destroy	= unx_destroy,
+	.lookup_cred	= unx_lookup_cred,
+};
+
+static
+struct rpc_auth		unix_auth = {
+	.au_cslack	= UNX_CALLSLACK,
+	.au_rslack	= NUL_REPLYSLACK,
+	.au_verfsize	= NUL_REPLYSLACK,
+	.au_ops		= &authunix_ops,
+	.au_flavor	= RPC_AUTH_UNIX,
+	.au_count	= REFCOUNT_INIT(1),
+};
+
+static
+const struct rpc_credops unix_credops = {
+	.cr_name	= "AUTH_UNIX",
+	.crdestroy	= unx_destroy_cred,
+	.crmatch	= unx_match,
+	.crmarshal	= unx_marshal,
+	.crwrap_req	= rpcauth_wrap_req_encode,
+	.crrefresh	= unx_refresh,
+	.crvalidate	= unx_validate,
+	.crunwrap_resp	= rpcauth_unwrap_resp_decode,
+};