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, 366 insertions, 0 deletions

diff --git a/drivers/dibs/dibs_loopback.c b/drivers/dibs/dibs_loopback.c
new file mode 100644
index 000000000..649e4e375
--- /dev/null
+++ b/drivers/dibs/dibs_loopback.c
@@ -0,0 +1,366 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  Functions for dibs loopback/loopback-ism device.
+ *
+ *  Copyright (c) 2024, Alibaba Inc.
+ *
+ *  Author: Wen Gu <guwen@linux.alibaba.com>
+ *          Tony Lu <tonylu@linux.alibaba.com>
+ *
+ */
+
+#include <linux/bitops.h>
+#include <linux/device.h>
+#include <linux/dibs.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/types.h>
+
+#include "dibs_loopback.h"
+
+#define DIBS_LO_SUPPORT_NOCOPY	0x1
+#define DIBS_DMA_ADDR_INVALID	(~(dma_addr_t)0)
+
+static const char dibs_lo_dev_name[] = "lo";
+/* global loopback device */
+static struct dibs_lo_dev *lo_dev;
+
+static u16 dibs_lo_get_fabric_id(struct dibs_dev *dibs)
+{
+	return DIBS_LOOPBACK_FABRIC;
+}
+
+static int dibs_lo_query_rgid(struct dibs_dev *dibs, const uuid_t *rgid,
+			      u32 vid_valid, u32 vid)
+{
+	/* rgid should be the same as lgid */
+	if (!uuid_equal(rgid, &dibs->gid))
+		return -ENETUNREACH;
+	return 0;
+}
+
+static int dibs_lo_max_dmbs(void)
+{
+	return DIBS_LO_MAX_DMBS;
+}
+
+static int dibs_lo_register_dmb(struct dibs_dev *dibs, struct dibs_dmb *dmb,
+				struct dibs_client *client)
+{
+	struct dibs_lo_dmb_node *dmb_node, *tmp_node;
+	struct dibs_lo_dev *ldev;
+	struct folio *folio;
+	unsigned long flags;
+	int sba_idx, rc;
+
+	ldev = dibs->drv_priv;
+	sba_idx = dmb->idx;
+	/* check space for new dmb */
+	for_each_clear_bit(sba_idx, ldev->sba_idx_mask, DIBS_LO_MAX_DMBS) {
+		if (!test_and_set_bit(sba_idx, ldev->sba_idx_mask))
+			break;
+	}
+	if (sba_idx == DIBS_LO_MAX_DMBS)
+		return -ENOSPC;
+
+	dmb_node = kzalloc_obj(*dmb_node);
+	if (!dmb_node) {
+		rc = -ENOMEM;
+		goto err_bit;
+	}
+
+	dmb_node->sba_idx = sba_idx;
+	dmb_node->len = dmb->dmb_len;
+
+	/* not critical; fail under memory pressure and fallback to TCP */
+	folio = folio_alloc(GFP_KERNEL | __GFP_NOWARN | __GFP_NOMEMALLOC |
+			    __GFP_NORETRY | __GFP_ZERO,
+			    get_order(dmb_node->len));
+	if (!folio) {
+		rc = -ENOMEM;
+		goto err_node;
+	}
+	dmb_node->cpu_addr = folio_address(folio);
+	dmb_node->dma_addr = DIBS_DMA_ADDR_INVALID;
+	refcount_set(&dmb_node->refcnt, 1);
+
+again:
+	/* add new dmb into hash table */
+	get_random_bytes(&dmb_node->token, sizeof(dmb_node->token));
+	write_lock_bh(&ldev->dmb_ht_lock);
+	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, dmb_node->token) {
+		if (tmp_node->token == dmb_node->token) {
+			write_unlock_bh(&ldev->dmb_ht_lock);
+			goto again;
+		}
+	}
+	hash_add(ldev->dmb_ht, &dmb_node->list, dmb_node->token);
+	write_unlock_bh(&ldev->dmb_ht_lock);
+	atomic_inc(&ldev->dmb_cnt);
+
+	dmb->idx = dmb_node->sba_idx;
+	dmb->dmb_tok = dmb_node->token;
+	dmb->cpu_addr = dmb_node->cpu_addr;
+	dmb->dma_addr = dmb_node->dma_addr;
+	dmb->dmb_len = dmb_node->len;
+
+	spin_lock_irqsave(&dibs->lock, flags);
+	dibs->dmb_clientid_arr[sba_idx] = client->id;
+	spin_unlock_irqrestore(&dibs->lock, flags);
+
+	return 0;
+
+err_node:
+	kfree(dmb_node);
+err_bit:
+	clear_bit(sba_idx, ldev->sba_idx_mask);
+	return rc;
+}
+
+static void dibs_lo_free_dmb(struct dibs_lo_dev *ldev,
+			     struct dibs_lo_dmb_node *dmb_node)
+{
+	clear_bit(dmb_node->sba_idx, ldev->sba_idx_mask);
+	folio_put(virt_to_folio(dmb_node->cpu_addr));
+	kfree(dmb_node);
+
+	if (atomic_dec_and_test(&ldev->dmb_cnt))
+		wake_up(&ldev->ldev_release);
+}
+
+static int dibs_lo_unregister_dmb(struct dibs_dev *dibs, struct dibs_dmb *dmb)
+{
+	struct dibs_lo_dmb_node *dmb_node = NULL, *tmp_node;
+	struct dibs_lo_dev *ldev;
+	unsigned long flags;
+	bool last;
+
+	ldev = dibs->drv_priv;
+
+	/* find dmb from hash table */
+	write_lock_bh(&ldev->dmb_ht_lock);
+	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, dmb->dmb_tok) {
+		if (tmp_node->token == dmb->dmb_tok) {
+			dmb_node = tmp_node;
+			break;
+		}
+	}
+	if (!dmb_node) {
+		write_unlock_bh(&ldev->dmb_ht_lock);
+		return -EINVAL;
+	}
+	last = refcount_dec_and_test(&dmb_node->refcnt);
+	if (last)
+		hash_del(&dmb_node->list);
+	write_unlock_bh(&ldev->dmb_ht_lock);
+
+	if (last) {
+		spin_lock_irqsave(&dibs->lock, flags);
+		dibs->dmb_clientid_arr[dmb_node->sba_idx] = NO_DIBS_CLIENT;
+		spin_unlock_irqrestore(&dibs->lock, flags);
+
+		dibs_lo_free_dmb(ldev, dmb_node);
+	}
+	return 0;
+}
+
+static int dibs_lo_support_dmb_nocopy(struct dibs_dev *dibs)
+{
+	return DIBS_LO_SUPPORT_NOCOPY;
+}
+
+static int dibs_lo_attach_dmb(struct dibs_dev *dibs, struct dibs_dmb *dmb)
+{
+	struct dibs_lo_dmb_node *dmb_node = NULL, *tmp_node;
+	struct dibs_lo_dev *ldev;
+
+	ldev = dibs->drv_priv;
+
+	/* find dmb_node according to dmb->dmb_tok */
+	read_lock_bh(&ldev->dmb_ht_lock);
+	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, dmb->dmb_tok) {
+		if (tmp_node->token == dmb->dmb_tok) {
+			dmb_node = tmp_node;
+			break;
+		}
+	}
+	if (!dmb_node) {
+		read_unlock_bh(&ldev->dmb_ht_lock);
+		return -EINVAL;
+	}
+	refcount_inc(&dmb_node->refcnt);
+	read_unlock_bh(&ldev->dmb_ht_lock);
+
+	/* provide dmb information */
+	dmb->idx = dmb_node->sba_idx;
+	dmb->dmb_tok = dmb_node->token;
+	dmb->cpu_addr = dmb_node->cpu_addr;
+	dmb->dma_addr = dmb_node->dma_addr;
+	dmb->dmb_len = dmb_node->len;
+	return 0;
+}
+
+static int dibs_lo_detach_dmb(struct dibs_dev *dibs, u64 token)
+{
+	struct dibs_lo_dmb_node *dmb_node = NULL, *tmp_node;
+	struct dibs_lo_dev *ldev;
+	bool last;
+
+	ldev = dibs->drv_priv;
+
+	/* find dmb_node according to dmb->dmb_tok */
+	write_lock_bh(&ldev->dmb_ht_lock);
+	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, token) {
+		if (tmp_node->token == token) {
+			dmb_node = tmp_node;
+			break;
+		}
+	}
+	if (!dmb_node) {
+		write_unlock_bh(&ldev->dmb_ht_lock);
+		return -EINVAL;
+	}
+	last = refcount_dec_and_test(&dmb_node->refcnt);
+	if (last)
+		hash_del(&dmb_node->list);
+	write_unlock_bh(&ldev->dmb_ht_lock);
+
+	if (last)
+		dibs_lo_free_dmb(ldev, dmb_node);
+
+	return 0;
+}
+
+static int dibs_lo_move_data(struct dibs_dev *dibs, u64 dmb_tok,
+			     unsigned int idx, bool sf, unsigned int offset,
+			     void *data, unsigned int size)
+{
+	struct dibs_lo_dmb_node *rmb_node = NULL, *tmp_node;
+	struct dibs_lo_dev *ldev;
+	u16 s_mask;
+	u8 client_id;
+	u32 sba_idx;
+
+	ldev = dibs->drv_priv;
+
+	read_lock_bh(&ldev->dmb_ht_lock);
+	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, dmb_tok) {
+		if (tmp_node->token == dmb_tok) {
+			rmb_node = tmp_node;
+			break;
+		}
+	}
+	if (!rmb_node) {
+		read_unlock_bh(&ldev->dmb_ht_lock);
+		return -EINVAL;
+	}
+	if ((u64)offset + size > rmb_node->len) {
+		read_unlock_bh(&ldev->dmb_ht_lock);
+		return -EINVAL;
+	}
+
+	memcpy((char *)rmb_node->cpu_addr + offset, data, size);
+	sba_idx = rmb_node->sba_idx;
+	read_unlock_bh(&ldev->dmb_ht_lock);
+
+	if (!sf)
+		return 0;
+
+	spin_lock(&dibs->lock);
+	client_id = dibs->dmb_clientid_arr[sba_idx];
+	s_mask = ror16(0x1000, idx);
+	if (likely(client_id != NO_DIBS_CLIENT && dibs->subs[client_id]))
+		dibs->subs[client_id]->ops->handle_irq(dibs, sba_idx, s_mask);
+	spin_unlock(&dibs->lock);
+
+	return 0;
+}
+
+static const struct dibs_dev_ops dibs_lo_ops = {
+	.get_fabric_id = dibs_lo_get_fabric_id,
+	.query_remote_gid = dibs_lo_query_rgid,
+	.max_dmbs = dibs_lo_max_dmbs,
+	.register_dmb = dibs_lo_register_dmb,
+	.unregister_dmb = dibs_lo_unregister_dmb,
+	.move_data = dibs_lo_move_data,
+	.support_mmapped_rdmb = dibs_lo_support_dmb_nocopy,
+	.attach_dmb = dibs_lo_attach_dmb,
+	.detach_dmb = dibs_lo_detach_dmb,
+};
+
+static void dibs_lo_dev_init(struct dibs_lo_dev *ldev)
+{
+	rwlock_init(&ldev->dmb_ht_lock);
+	hash_init(ldev->dmb_ht);
+	atomic_set(&ldev->dmb_cnt, 0);
+	init_waitqueue_head(&ldev->ldev_release);
+}
+
+static void dibs_lo_dev_exit(struct dibs_lo_dev *ldev)
+{
+	if (atomic_read(&ldev->dmb_cnt))
+		wait_event(ldev->ldev_release, !atomic_read(&ldev->dmb_cnt));
+}
+
+static int dibs_lo_dev_probe(void)
+{
+	struct dibs_lo_dev *ldev;
+	struct dibs_dev *dibs;
+	int ret;
+
+	ldev = kzalloc_obj(*ldev);
+	if (!ldev)
+		return -ENOMEM;
+
+	dibs = dibs_dev_alloc();
+	if (!dibs) {
+		kfree(ldev);
+		return -ENOMEM;
+	}
+
+	ldev->dibs = dibs;
+	dibs->drv_priv = ldev;
+	dibs_lo_dev_init(ldev);
+	uuid_gen(&dibs->gid);
+	dibs->ops = &dibs_lo_ops;
+
+	dibs->dev.parent = NULL;
+	dev_set_name(&dibs->dev, "%s", dibs_lo_dev_name);
+
+	ret = dibs_dev_add(dibs);
+	if (ret)
+		goto err_reg;
+	lo_dev = ldev;
+	return 0;
+
+err_reg:
+	/* pairs with dibs_dev_alloc() */
+	put_device(&dibs->dev);
+	kfree(ldev);
+
+	return ret;
+}
+
+static void dibs_lo_dev_remove(void)
+{
+	if (!lo_dev)
+		return;
+
+	dibs_dev_del(lo_dev->dibs);
+	dibs_lo_dev_exit(lo_dev);
+	/* pairs with dibs_dev_alloc() */
+	put_device(&lo_dev->dibs->dev);
+	kfree(lo_dev);
+	lo_dev = NULL;
+}
+
+int dibs_loopback_init(void)
+{
+	return dibs_lo_dev_probe();
+}
+
+void dibs_loopback_exit(void)
+{
+	dibs_lo_dev_remove();
+}