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/fs/netfs/read_single.c b/fs/netfs/read_single.c
new file mode 100644
index 000000000..8833550d2
--- /dev/null
+++ b/fs/netfs/read_single.c
@@ -0,0 +1,191 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Single, monolithic object support (e.g. AFS directory).
+ *
+ * Copyright (C) 2024 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/export.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/slab.h>
+#include <linux/uio.h>
+#include <linux/sched/mm.h>
+#include <linux/task_io_accounting_ops.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+/**
+ * netfs_single_mark_inode_dirty - Mark a single, monolithic object inode dirty
+ * @inode: The inode to mark
+ *
+ * Mark an inode that contains a single, monolithic object as dirty so that its
+ * writepages op will get called.  If set, the SINGLE_NO_UPLOAD flag indicates
+ * that the object will only be written to the cache and not uploaded (e.g. AFS
+ * directory contents).
+ */
+void netfs_single_mark_inode_dirty(struct inode *inode)
+{
+	struct netfs_inode *ictx = netfs_inode(inode);
+	bool cache_only = test_bit(NETFS_ICTX_SINGLE_NO_UPLOAD, &ictx->flags);
+	bool caching = fscache_cookie_enabled(netfs_i_cookie(netfs_inode(inode)));
+
+	if (cache_only && !caching)
+		return;
+
+	mark_inode_dirty(inode);
+
+	if (caching && !(inode_state_read_once(inode) & I_PINNING_NETFS_WB)) {
+		bool need_use = false;
+
+		spin_lock(&inode->i_lock);
+		if (!(inode_state_read(inode) & I_PINNING_NETFS_WB)) {
+			inode_state_set(inode, I_PINNING_NETFS_WB);
+			need_use = true;
+		}
+		spin_unlock(&inode->i_lock);
+
+		if (need_use)
+			fscache_use_cookie(netfs_i_cookie(ictx), true);
+	}
+
+}
+EXPORT_SYMBOL(netfs_single_mark_inode_dirty);
+
+static int netfs_single_begin_cache_read(struct netfs_io_request *rreq, struct netfs_inode *ctx)
+{
+	return fscache_begin_read_operation(&rreq->cache_resources, netfs_i_cookie(ctx));
+}
+
+static void netfs_single_cache_prepare_read(struct netfs_io_request *rreq,
+					    struct netfs_io_subrequest *subreq)
+{
+	struct netfs_cache_resources *cres = &rreq->cache_resources;
+
+	if (!cres->ops) {
+		subreq->source = NETFS_DOWNLOAD_FROM_SERVER;
+		return;
+	}
+	subreq->source = cres->ops->prepare_read(subreq, rreq->i_size);
+	trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
+
+}
+
+static void netfs_single_read_cache(struct netfs_io_request *rreq,
+				    struct netfs_io_subrequest *subreq)
+{
+	struct netfs_cache_resources *cres = &rreq->cache_resources;
+
+	_enter("R=%08x[%x]", rreq->debug_id, subreq->debug_index);
+	netfs_stat(&netfs_n_rh_read);
+	cres->ops->read(cres, subreq->start, &subreq->io_iter, NETFS_READ_HOLE_FAIL,
+			netfs_cache_read_terminated, subreq);
+}
+
+/*
+ * Perform a read to a buffer from the cache or the server.  Only a single
+ * subreq is permitted as the object must be fetched in a single transaction.
+ */
+static int netfs_single_dispatch_read(struct netfs_io_request *rreq)
+{
+	struct netfs_io_subrequest *subreq;
+	int ret = 0;
+
+	subreq = netfs_alloc_subrequest(rreq);
+	if (!subreq)
+		return -ENOMEM;
+
+	subreq->source	= NETFS_SOURCE_UNKNOWN;
+	subreq->start	= 0;
+	subreq->len	= rreq->len;
+	subreq->io_iter	= rreq->buffer.iter;
+
+	netfs_queue_read(rreq, subreq);
+
+	netfs_single_cache_prepare_read(rreq, subreq);
+	switch (subreq->source) {
+	case NETFS_DOWNLOAD_FROM_SERVER:
+		netfs_stat(&netfs_n_rh_download);
+		if (rreq->netfs_ops->prepare_read) {
+			ret = rreq->netfs_ops->prepare_read(subreq);
+			if (ret < 0)
+				goto cancel;
+		}
+
+		smp_wmb(); /* Write lists before ALL_QUEUED. */
+		set_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags);
+		rreq->netfs_ops->issue_read(subreq);
+		rreq->submitted += subreq->len;
+		break;
+	case NETFS_READ_FROM_CACHE:
+		smp_wmb(); /* Write lists before ALL_QUEUED. */
+		set_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags);
+		trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
+		netfs_single_read_cache(rreq, subreq);
+		rreq->submitted += subreq->len;
+		ret = 0;
+		break;
+	default:
+		pr_warn("Unexpected single-read source %u\n", subreq->source);
+		WARN_ON_ONCE(true);
+		ret = -EIO;
+		goto cancel;
+	}
+
+	return ret;
+cancel:
+	netfs_cancel_read(subreq, ret);
+	smp_wmb(); /* Write lists before ALL_QUEUED. */
+	set_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags);
+	netfs_wake_collector(rreq);
+	return ret;
+}
+
+/**
+ * netfs_read_single - Synchronously read a single blob of pages.
+ * @inode: The inode to read from.
+ * @file: The file we're using to read or NULL.
+ * @iter: The buffer we're reading into.
+ *
+ * Fulfil a read request for a single monolithic object by drawing data from
+ * the cache if possible, or the netfs if not.  The buffer may be larger than
+ * the file content; unused beyond the EOF will be zero-filled.  The content
+ * will be read with a single I/O request (though this may be retried).
+ *
+ * The calling netfs must initialise a netfs context contiguous to the vfs
+ * inode before calling this.
+ *
+ * This is usable whether or not caching is enabled.  If caching is enabled,
+ * the data will be stored as a single object into the cache.
+ */
+ssize_t netfs_read_single(struct inode *inode, struct file *file, struct iov_iter *iter)
+{
+	struct netfs_io_request *rreq;
+	struct netfs_inode *ictx = netfs_inode(inode);
+	ssize_t ret;
+
+	rreq = netfs_alloc_request(inode->i_mapping, file, 0, iov_iter_count(iter),
+				   NETFS_READ_SINGLE);
+	if (IS_ERR(rreq))
+		return PTR_ERR(rreq);
+
+	ret = netfs_single_begin_cache_read(rreq, ictx);
+	if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
+		goto cleanup_free;
+
+	netfs_stat(&netfs_n_rh_read_single);
+	trace_netfs_read(rreq, 0, rreq->len, netfs_read_trace_read_single);
+
+	rreq->buffer.iter = *iter;
+	netfs_single_dispatch_read(rreq);
+
+	ret = netfs_wait_for_read(rreq);
+	netfs_put_request(rreq, netfs_rreq_trace_put_return);
+	return ret;
+
+cleanup_free:
+	netfs_put_failed_request(rreq);
+	return ret;
+}
+EXPORT_SYMBOL(netfs_read_single);