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, 278 insertions, 0 deletions

diff --git a/net/6lowpan/debugfs.c b/net/6lowpan/debugfs.c
new file mode 100644
index 000000000..600b9563b
--- /dev/null
+++ b/net/6lowpan/debugfs.c
@@ -0,0 +1,278 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *
+ * Authors:
+ * (C) 2015 Pengutronix, Alexander Aring <aar@pengutronix.de>
+ * Copyright (c)  2015 Nordic Semiconductor. All Rights Reserved.
+ */
+
+#include <net/6lowpan.h>
+
+#include "6lowpan_i.h"
+
+#define LOWPAN_DEBUGFS_CTX_PFX_NUM_ARGS	8
+
+static struct dentry *lowpan_debugfs;
+
+static int lowpan_ctx_flag_active_set(void *data, u64 val)
+{
+	struct lowpan_iphc_ctx *ctx = data;
+
+	if (val != 0 && val != 1)
+		return -EINVAL;
+
+	if (val)
+		set_bit(LOWPAN_IPHC_CTX_FLAG_ACTIVE, &ctx->flags);
+	else
+		clear_bit(LOWPAN_IPHC_CTX_FLAG_ACTIVE, &ctx->flags);
+
+	return 0;
+}
+
+static int lowpan_ctx_flag_active_get(void *data, u64 *val)
+{
+	*val = lowpan_iphc_ctx_is_active(data);
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(lowpan_ctx_flag_active_fops,
+			 lowpan_ctx_flag_active_get,
+			 lowpan_ctx_flag_active_set, "%llu\n");
+
+static int lowpan_ctx_flag_c_set(void *data, u64 val)
+{
+	struct lowpan_iphc_ctx *ctx = data;
+
+	if (val != 0 && val != 1)
+		return -EINVAL;
+
+	if (val)
+		set_bit(LOWPAN_IPHC_CTX_FLAG_COMPRESSION, &ctx->flags);
+	else
+		clear_bit(LOWPAN_IPHC_CTX_FLAG_COMPRESSION, &ctx->flags);
+
+	return 0;
+}
+
+static int lowpan_ctx_flag_c_get(void *data, u64 *val)
+{
+	*val = lowpan_iphc_ctx_is_compression(data);
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(lowpan_ctx_flag_c_fops, lowpan_ctx_flag_c_get,
+			 lowpan_ctx_flag_c_set, "%llu\n");
+
+static int lowpan_ctx_plen_set(void *data, u64 val)
+{
+	struct lowpan_iphc_ctx *ctx = data;
+	struct lowpan_iphc_ctx_table *t =
+		container_of(ctx, struct lowpan_iphc_ctx_table, table[ctx->id]);
+
+	if (val > 128)
+		return -EINVAL;
+
+	spin_lock_bh(&t->lock);
+	ctx->plen = val;
+	spin_unlock_bh(&t->lock);
+
+	return 0;
+}
+
+static int lowpan_ctx_plen_get(void *data, u64 *val)
+{
+	struct lowpan_iphc_ctx *ctx = data;
+	struct lowpan_iphc_ctx_table *t =
+		container_of(ctx, struct lowpan_iphc_ctx_table, table[ctx->id]);
+
+	spin_lock_bh(&t->lock);
+	*val = ctx->plen;
+	spin_unlock_bh(&t->lock);
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(lowpan_ctx_plen_fops, lowpan_ctx_plen_get,
+			 lowpan_ctx_plen_set, "%llu\n");
+
+static int lowpan_ctx_pfx_show(struct seq_file *file, void *offset)
+{
+	struct lowpan_iphc_ctx *ctx = file->private;
+	struct lowpan_iphc_ctx_table *t =
+		container_of(ctx, struct lowpan_iphc_ctx_table, table[ctx->id]);
+
+	spin_lock_bh(&t->lock);
+	seq_printf(file, "%04x:%04x:%04x:%04x:%04x:%04x:%04x:%04x\n",
+		   be16_to_cpu(ctx->pfx.s6_addr16[0]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[1]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[2]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[3]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[4]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[5]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[6]),
+		   be16_to_cpu(ctx->pfx.s6_addr16[7]));
+	spin_unlock_bh(&t->lock);
+
+	return 0;
+}
+
+static int lowpan_ctx_pfx_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, lowpan_ctx_pfx_show, inode->i_private);
+}
+
+static ssize_t lowpan_ctx_pfx_write(struct file *fp,
+				    const char __user *user_buf, size_t count,
+				    loff_t *ppos)
+{
+	char buf[128] = {};
+	struct seq_file *file = fp->private_data;
+	struct lowpan_iphc_ctx *ctx = file->private;
+	struct lowpan_iphc_ctx_table *t =
+		container_of(ctx, struct lowpan_iphc_ctx_table, table[ctx->id]);
+	int status = count, n, i;
+	unsigned int addr[8];
+
+	if (copy_from_user(&buf, user_buf, min_t(size_t, sizeof(buf) - 1,
+						 count))) {
+		status = -EFAULT;
+		goto out;
+	}
+
+	n = sscanf(buf, "%04x:%04x:%04x:%04x:%04x:%04x:%04x:%04x",
+		   &addr[0], &addr[1], &addr[2], &addr[3], &addr[4],
+		   &addr[5], &addr[6], &addr[7]);
+	if (n != LOWPAN_DEBUGFS_CTX_PFX_NUM_ARGS) {
+		status = -EINVAL;
+		goto out;
+	}
+
+	spin_lock_bh(&t->lock);
+	for (i = 0; i < 8; i++)
+		ctx->pfx.s6_addr16[i] = cpu_to_be16(addr[i] & 0xffff);
+	spin_unlock_bh(&t->lock);
+
+out:
+	return status;
+}
+
+static const struct file_operations lowpan_ctx_pfx_fops = {
+	.open		= lowpan_ctx_pfx_open,
+	.read		= seq_read,
+	.write		= lowpan_ctx_pfx_write,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static void lowpan_dev_debugfs_ctx_init(struct net_device *dev,
+					struct dentry *ctx, u8 id)
+{
+	struct lowpan_dev *ldev = lowpan_dev(dev);
+	struct dentry *root;
+	char buf[32];
+
+	if (WARN_ON_ONCE(id >= LOWPAN_IPHC_CTX_TABLE_SIZE))
+		return;
+
+	sprintf(buf, "%d", id);
+
+	root = debugfs_create_dir(buf, ctx);
+
+	debugfs_create_file("active", 0644, root, &ldev->ctx.table[id],
+			    &lowpan_ctx_flag_active_fops);
+
+	debugfs_create_file("compression", 0644, root, &ldev->ctx.table[id],
+			    &lowpan_ctx_flag_c_fops);
+
+	debugfs_create_file("prefix", 0644, root, &ldev->ctx.table[id],
+			    &lowpan_ctx_pfx_fops);
+
+	debugfs_create_file("prefix_len", 0644, root, &ldev->ctx.table[id],
+			    &lowpan_ctx_plen_fops);
+}
+
+static int lowpan_context_show(struct seq_file *file, void *offset)
+{
+	struct lowpan_iphc_ctx_table *t = file->private;
+	int i;
+
+	seq_printf(file, "%3s|%-43s|%c\n", "cid", "prefix", 'C');
+	seq_puts(file, "-------------------------------------------------\n");
+
+	spin_lock_bh(&t->lock);
+	for (i = 0; i < LOWPAN_IPHC_CTX_TABLE_SIZE; i++) {
+		if (!lowpan_iphc_ctx_is_active(&t->table[i]))
+			continue;
+
+		seq_printf(file, "%3d|%39pI6c/%-3d|%d\n", t->table[i].id,
+			   &t->table[i].pfx, t->table[i].plen,
+			   lowpan_iphc_ctx_is_compression(&t->table[i]));
+	}
+	spin_unlock_bh(&t->lock);
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(lowpan_context);
+
+static int lowpan_short_addr_get(void *data, u64 *val)
+{
+	struct wpan_dev *wdev = data;
+
+	rtnl_lock();
+	*val = le16_to_cpu(wdev->short_addr);
+	rtnl_unlock();
+
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(lowpan_short_addr_fops, lowpan_short_addr_get, NULL,
+			 "0x%04llx\n");
+
+static void lowpan_dev_debugfs_802154_init(const struct net_device *dev,
+					  struct lowpan_dev *ldev)
+{
+	struct dentry *root;
+
+	if (!lowpan_is_ll(dev, LOWPAN_LLTYPE_IEEE802154))
+		return;
+
+	root = debugfs_create_dir("ieee802154", ldev->iface_debugfs);
+
+	debugfs_create_file("short_addr", 0444, root,
+			    lowpan_802154_dev(dev)->wdev->ieee802154_ptr,
+			    &lowpan_short_addr_fops);
+}
+
+void lowpan_dev_debugfs_init(struct net_device *dev)
+{
+	struct lowpan_dev *ldev = lowpan_dev(dev);
+	struct dentry *contexts;
+	int i;
+
+	/* creating the root */
+	ldev->iface_debugfs = debugfs_create_dir(dev->name, lowpan_debugfs);
+
+	contexts = debugfs_create_dir("contexts", ldev->iface_debugfs);
+
+	debugfs_create_file("show", 0644, contexts, &lowpan_dev(dev)->ctx,
+			    &lowpan_context_fops);
+
+	for (i = 0; i < LOWPAN_IPHC_CTX_TABLE_SIZE; i++)
+		lowpan_dev_debugfs_ctx_init(dev, contexts, i);
+
+	lowpan_dev_debugfs_802154_init(dev, ldev);
+}
+
+void lowpan_dev_debugfs_exit(struct net_device *dev)
+{
+	debugfs_remove_recursive(lowpan_dev(dev)->iface_debugfs);
+}
+
+void __init lowpan_debugfs_init(void)
+{
+	lowpan_debugfs = debugfs_create_dir("6lowpan", NULL);
+}
+
+void lowpan_debugfs_exit(void)
+{
+	debugfs_remove_recursive(lowpan_debugfs);
+}