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, 354 insertions, 0 deletions

diff --git a/drivers/mtd/devices/slram.c b/drivers/mtd/devices/slram.c
new file mode 100644
index 000000000..aa38ecdff
--- /dev/null
+++ b/drivers/mtd/devices/slram.c
@@ -0,0 +1,354 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*======================================================================
+
+  This driver provides a method to access memory not used by the kernel
+  itself (i.e. if the kernel commandline mem=xxx is used). To actually
+  use slram at least mtdblock or mtdchar is required (for block or
+  character device access).
+
+  Usage:
+
+  if compiled as loadable module:
+    modprobe slram map=<name>,<start>,<end/offset>
+  if statically linked into the kernel use the following kernel cmd.line
+    slram=<name>,<start>,<end/offset>
+
+  <name>: name of the device that will be listed in /proc/mtd
+  <start>: start of the memory region, decimal or hex (0xabcdef)
+  <end/offset>: end of the memory region. It's possible to use +0x1234
+                to specify the offset instead of the absolute address
+
+  NOTE:
+  With slram it's only possible to map a contiguous memory region. Therefore
+  if there's a device mapped somewhere in the region specified slram will
+  fail to load (see kernel log if modprobe fails).
+
+  -
+
+  Jochen Schaeuble <psionic@psionic.de>
+
+======================================================================*/
+
+
+#include <linux/module.h>
+#include <linux/uaccess.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/ptrace.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/timer.h>
+#include <linux/major.h>
+#include <linux/fs.h>
+#include <linux/ioctl.h>
+#include <linux/init.h>
+#include <linux/io.h>
+
+#include <linux/mtd/mtd.h>
+
+#define SLRAM_MAX_DEVICES_PARAMS 6		/* 3 parameters / device */
+#define SLRAM_BLK_SZ 0x4000
+
+#define T(fmt, args...) printk(KERN_DEBUG fmt, ## args)
+#define E(fmt, args...) printk(KERN_NOTICE fmt, ## args)
+
+typedef struct slram_priv {
+	u_char *start;
+	u_char *end;
+} slram_priv_t;
+
+typedef struct slram_mtd_list {
+	struct mtd_info *mtdinfo;
+	struct slram_mtd_list *next;
+} slram_mtd_list_t;
+
+#ifdef MODULE
+static char *map[SLRAM_MAX_DEVICES_PARAMS];
+
+module_param_array(map, charp, NULL, 0);
+MODULE_PARM_DESC(map, "List of memory regions to map. \"map=<name>, <start>, <length / end>\"");
+#else
+static char *map;
+#endif
+
+static slram_mtd_list_t *slram_mtdlist = NULL;
+
+static int slram_erase(struct mtd_info *, struct erase_info *);
+static int slram_point(struct mtd_info *, loff_t, size_t, size_t *, void **,
+		resource_size_t *);
+static int slram_unpoint(struct mtd_info *, loff_t, size_t);
+static int slram_read(struct mtd_info *, loff_t, size_t, size_t *, u_char *);
+static int slram_write(struct mtd_info *, loff_t, size_t, size_t *, const u_char *);
+
+static int slram_erase(struct mtd_info *mtd, struct erase_info *instr)
+{
+	slram_priv_t *priv = mtd->priv;
+
+	memset(priv->start + instr->addr, 0xff, instr->len);
+
+	return(0);
+}
+
+static int slram_point(struct mtd_info *mtd, loff_t from, size_t len,
+		size_t *retlen, void **virt, resource_size_t *phys)
+{
+	slram_priv_t *priv = mtd->priv;
+
+	*virt = priv->start + from;
+	*retlen = len;
+	return(0);
+}
+
+static int slram_unpoint(struct mtd_info *mtd, loff_t from, size_t len)
+{
+	return 0;
+}
+
+static int slram_read(struct mtd_info *mtd, loff_t from, size_t len,
+		size_t *retlen, u_char *buf)
+{
+	slram_priv_t *priv = mtd->priv;
+
+	memcpy(buf, priv->start + from, len);
+	*retlen = len;
+	return(0);
+}
+
+static int slram_write(struct mtd_info *mtd, loff_t to, size_t len,
+		size_t *retlen, const u_char *buf)
+{
+	slram_priv_t *priv = mtd->priv;
+
+	memcpy(priv->start + to, buf, len);
+	*retlen = len;
+	return(0);
+}
+
+/*====================================================================*/
+
+static int register_device(char *name, unsigned long start, unsigned long length)
+{
+	slram_mtd_list_t **curmtd;
+	slram_mtd_list_t *new_mtd;
+	struct mtd_info *mtdinfo;
+	slram_priv_t *priv;
+	int ret = -ENOMEM;
+
+	curmtd = &slram_mtdlist;
+	while (*curmtd) {
+		curmtd = &(*curmtd)->next;
+	}
+
+	new_mtd = kmalloc_obj(slram_mtd_list_t);
+	if (!new_mtd) {
+		E("slram: Cannot allocate new MTD device.\n");
+		return(-ENOMEM);
+	}
+	new_mtd->next = NULL;
+
+	mtdinfo = kzalloc_obj(struct mtd_info);
+	if (!mtdinfo) {
+		E("slram: Cannot allocate new MTD device.\n");
+		goto err_free_list;
+	}
+	new_mtd->mtdinfo = mtdinfo;
+
+	priv = kzalloc_obj(slram_priv_t);
+	if (!priv) {
+		E("slram: Cannot allocate new MTD device.\n");
+		goto err_free_mtdinfo;
+	}
+	mtdinfo->priv = priv;
+
+	priv->start = memremap(start, length,
+			       MEMREMAP_WB | MEMREMAP_WT | MEMREMAP_WC);
+	if (!priv->start) {
+		E("slram: memremap failed\n");
+		ret = -EIO;
+		goto err_free_priv;
+	}
+	priv->end = priv->start + length;
+
+	mtdinfo->name = name;
+	mtdinfo->size = length;
+	mtdinfo->flags = MTD_CAP_RAM;
+	mtdinfo->_erase = slram_erase;
+	mtdinfo->_point = slram_point;
+	mtdinfo->_unpoint = slram_unpoint;
+	mtdinfo->_read = slram_read;
+	mtdinfo->_write = slram_write;
+	mtdinfo->owner = THIS_MODULE;
+	mtdinfo->type = MTD_RAM;
+	mtdinfo->erasesize = SLRAM_BLK_SZ;
+	mtdinfo->writesize = 1;
+
+	if (mtd_device_register(mtdinfo, NULL, 0)) {
+		E("slram: Failed to register new device\n");
+		ret = -EAGAIN;
+		goto err_unmap;
+	}
+	*curmtd = new_mtd;
+	T("slram: Registered device %s from %luKiB to %luKiB\n", name,
+			(start / 1024), ((start + length) / 1024));
+	T("slram: Mapped from 0x%p to 0x%p\n", priv->start, priv->end);
+	return 0;
+
+err_unmap:
+	memunmap(priv->start);
+err_free_priv:
+	kfree(priv);
+err_free_mtdinfo:
+	kfree(mtdinfo);
+err_free_list:
+	kfree(new_mtd);
+	return ret;
+}
+
+static void unregister_devices(void)
+{
+	slram_mtd_list_t *nextitem;
+
+	while (slram_mtdlist) {
+		nextitem = slram_mtdlist->next;
+		mtd_device_unregister(slram_mtdlist->mtdinfo);
+		memunmap(((slram_priv_t *)slram_mtdlist->mtdinfo->priv)->start);
+		kfree(slram_mtdlist->mtdinfo->priv);
+		kfree(slram_mtdlist->mtdinfo);
+		kfree(slram_mtdlist);
+		slram_mtdlist = nextitem;
+	}
+}
+
+static unsigned long handle_unit(unsigned long value, char *unit)
+{
+	if ((*unit == 'M') || (*unit == 'm')) {
+		return(value * 1024 * 1024);
+	} else if ((*unit == 'K') || (*unit == 'k')) {
+		return(value * 1024);
+	}
+	return(value);
+}
+
+static int parse_cmdline(char *devname, char *szstart, char *szlength)
+{
+	char *buffer;
+	unsigned long devstart;
+	unsigned long devlength;
+
+	if ((!devname) || (!szstart) || (!szlength)) {
+		unregister_devices();
+		return(-EINVAL);
+	}
+
+	devstart = simple_strtoul(szstart, &buffer, 0);
+	devstart = handle_unit(devstart, buffer);
+
+	if (*(szlength) != '+') {
+		devlength = simple_strtoul(szlength, &buffer, 0);
+		devlength = handle_unit(devlength, buffer);
+		if (devlength < devstart)
+			goto err_out;
+
+		devlength -= devstart;
+	} else {
+		devlength = simple_strtoul(szlength + 1, &buffer, 0);
+		devlength = handle_unit(devlength, buffer);
+	}
+	T("slram: devname=%s, devstart=0x%lx, devlength=0x%lx\n",
+			devname, devstart, devlength);
+	if (devlength % SLRAM_BLK_SZ != 0)
+		goto err_out;
+
+	if ((devstart = register_device(devname, devstart, devlength))){
+		unregister_devices();
+		return((int)devstart);
+	}
+	return(0);
+
+err_out:
+	E("slram: Illegal length parameter.\n");
+	return(-EINVAL);
+}
+
+#ifndef MODULE
+
+static int __init mtd_slram_setup(char *str)
+{
+	map = str;
+	return(1);
+}
+
+__setup("slram=", mtd_slram_setup);
+
+#endif
+
+static int __init init_slram(void)
+{
+	char *devname;
+
+#ifndef MODULE
+	char *devstart;
+	char *devlength;
+
+	if (!map) {
+		E("slram: not enough parameters.\n");
+		return(-EINVAL);
+	}
+	while (map) {
+		devname = devstart = devlength = NULL;
+
+		if (!(devname = strsep(&map, ","))) {
+			E("slram: No devicename specified.\n");
+			break;
+		}
+		T("slram: devname = %s\n", devname);
+		if ((!map) || (!(devstart = strsep(&map, ",")))) {
+			E("slram: No devicestart specified.\n");
+			break;
+		}
+		T("slram: devstart = %s\n", devstart);
+		if ((!map) || (!(devlength = strsep(&map, ",")))) {
+			E("slram: No devicelength / -end specified.\n");
+			break;
+		}
+		T("slram: devlength = %s\n", devlength);
+		if (parse_cmdline(devname, devstart, devlength) != 0) {
+			return(-EINVAL);
+		}
+	}
+#else
+	int count;
+	int i;
+
+	for (count = 0; count < SLRAM_MAX_DEVICES_PARAMS && map[count];
+			count++) {
+	}
+
+	if ((count % 3 != 0) || (count == 0)) {
+		E("slram: not enough parameters.\n");
+		return(-EINVAL);
+	}
+	for (i = 0; i < (count / 3); i++) {
+		devname = map[i * 3];
+
+		if (parse_cmdline(devname, map[i * 3 + 1], map[i * 3 + 2])!=0) {
+			return(-EINVAL);
+		}
+
+	}
+#endif /* !MODULE */
+
+	return(0);
+}
+
+static void __exit cleanup_slram(void)
+{
+	unregister_devices();
+}
+
+module_init(init_slram);
+module_exit(cleanup_slram);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Jochen Schaeuble <psionic@psionic.de>");
+MODULE_DESCRIPTION("MTD driver for uncached system RAM");