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, 215 insertions, 0 deletions

diff --git a/drivers/tty/tty_baudrate.c b/drivers/tty/tty_baudrate.c
new file mode 100644
index 000000000..f9b49939c
--- /dev/null
+++ b/drivers/tty/tty_baudrate.c
@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
+ */
+
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/termios.h>
+#include <linux/tty.h>
+#include <linux/export.h>
+#include "tty.h"
+
+
+/*
+ * Routine which returns the baud rate of the tty
+ *
+ * Note that the baud_table needs to be kept in sync with the
+ * include/asm/termbits.h file.
+ */
+static const speed_t baud_table[] = {
+	0, 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800, 2400,
+	4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800,
+#ifdef __sparc__
+	76800, 153600, 307200, 614400, 921600, 500000, 576000,
+	1000000, 1152000, 1500000, 2000000
+#else
+	500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000,
+	2500000, 3000000, 3500000, 4000000
+#endif
+};
+
+static const tcflag_t baud_bits[] = {
+	B0, B50, B75, B110, B134, B150, B200, B300, B600, B1200, B1800, B2400,
+	B4800, B9600, B19200, B38400, B57600, B115200, B230400, B460800,
+#ifdef __sparc__
+	B76800, B153600, B307200, B614400, B921600, B500000, B576000,
+	B1000000, B1152000, B1500000, B2000000
+#else
+	B500000, B576000, B921600, B1000000, B1152000, B1500000, B2000000,
+	B2500000, B3000000, B3500000, B4000000
+#endif
+};
+
+static int n_baud_table = ARRAY_SIZE(baud_table);
+
+/**
+ *	tty_termios_baud_rate
+ *	@termios: termios structure
+ *
+ *	Convert termios baud rate data into a speed. This should be called
+ *	with the termios lock held if this termios is a terminal termios
+ *	structure. Device drivers can call this function but should use
+ *	->c_[io]speed directly as they are updated.
+ *
+ *	Locking: none
+ */
+
+speed_t tty_termios_baud_rate(const struct ktermios *termios)
+{
+	unsigned int cbaud;
+
+	cbaud = termios->c_cflag & CBAUD;
+
+	/* Magic token for arbitrary speed via c_ispeed/c_ospeed */
+	if (cbaud == BOTHER)
+		return termios->c_ospeed;
+
+	if (cbaud & CBAUDEX) {
+		cbaud &= ~CBAUDEX;
+		cbaud += 15;
+	}
+	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
+}
+EXPORT_SYMBOL(tty_termios_baud_rate);
+
+/**
+ *	tty_termios_input_baud_rate
+ *	@termios: termios structure
+ *
+ *	Convert termios baud rate data into a speed. This should be called
+ *	with the termios lock held if this termios is a terminal termios
+ *	structure. Device drivers can call this function but should use
+ *	->c_[io]speed directly as they are updated.
+ *
+ *	Locking: none
+ */
+
+speed_t tty_termios_input_baud_rate(const struct ktermios *termios)
+{
+	unsigned int cbaud = (termios->c_cflag >> IBSHIFT) & CBAUD;
+
+	if (cbaud == B0)
+		return tty_termios_baud_rate(termios);
+
+	/* Magic token for arbitrary speed via c_ispeed */
+	if (cbaud == BOTHER)
+		return termios->c_ispeed;
+
+	if (cbaud & CBAUDEX) {
+		cbaud &= ~CBAUDEX;
+		cbaud += 15;
+	}
+	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
+}
+EXPORT_SYMBOL(tty_termios_input_baud_rate);
+
+/**
+ *	tty_termios_encode_baud_rate
+ *	@termios: ktermios structure holding user requested state
+ *	@ibaud: input speed
+ *	@obaud: output speed
+ *
+ *	Encode the speeds set into the passed termios structure. This is
+ *	used as a library helper for drivers so that they can report back
+ *	the actual speed selected when it differs from the speed requested
+ *
+ *	For maximal back compatibility with legacy SYS5/POSIX *nix behaviour
+ *	we need to carefully set the bits when the user does not get the
+ *	desired speed. We allow small margins and preserve as much of possible
+ *	of the input intent to keep compatibility.
+ *
+ *	Locking: Caller should hold termios lock. This is already held
+ *	when calling this function from the driver termios handler.
+ *
+ *	The ifdefs deal with platforms whose owners have yet to update them
+ *	and will all go away once this is done.
+ */
+
+void tty_termios_encode_baud_rate(struct ktermios *termios,
+				  speed_t ibaud, speed_t obaud)
+{
+	int i = 0;
+	int ifound = -1, ofound = -1;
+	int iclose = ibaud/50, oclose = obaud/50;
+	int ibinput = 0;
+
+	if (obaud == 0)			/* CD dropped */
+		ibaud = 0;		/* Clear ibaud to be sure */
+
+	termios->c_ispeed = ibaud;
+	termios->c_ospeed = obaud;
+
+	if (((termios->c_cflag >> IBSHIFT) & CBAUD) != B0)
+		ibinput = 1;	/* An input speed was specified */
+
+	/* If the user asked for a precise weird speed give a precise weird
+	 * answer. If they asked for a Bfoo speed they may have problems
+	 * digesting non-exact replies so fuzz a bit.
+	 */
+
+	if ((termios->c_cflag & CBAUD) == BOTHER) {
+		oclose = 0;
+		if (!ibinput)
+			iclose = 0;
+	}
+	if (((termios->c_cflag >> IBSHIFT) & CBAUD) == BOTHER)
+		iclose = 0;
+
+	termios->c_cflag &= ~CBAUD;
+	termios->c_cflag &= ~(CBAUD << IBSHIFT);
+
+	/*
+	 *	Our goal is to find a close match to the standard baud rate
+	 *	returned. Walk the baud rate table and if we get a very close
+	 *	match then report back the speed as a POSIX Bxxxx value by
+	 *	preference
+	 */
+
+	do {
+		if (obaud - oclose <= baud_table[i] &&
+		    obaud + oclose >= baud_table[i]) {
+			termios->c_cflag |= baud_bits[i];
+			ofound = i;
+		}
+		if (ibaud - iclose <= baud_table[i] &&
+		    ibaud + iclose >= baud_table[i]) {
+			/* For the case input == output don't set IBAUD bits
+			 * if the user didn't do so.
+			 */
+			if (ofound == i && !ibinput) {
+				ifound  = i;
+			} else {
+				ifound = i;
+				termios->c_cflag |= (baud_bits[i] << IBSHIFT);
+			}
+		}
+	} while (++i < n_baud_table);
+
+	/* If we found no match then use BOTHER. */
+	if (ofound == -1)
+		termios->c_cflag |= BOTHER;
+	/* Set exact input bits only if the input and output differ or the
+	 * user already did.
+	 */
+	if (ifound == -1 && (ibaud != obaud || ibinput))
+		termios->c_cflag |= (BOTHER << IBSHIFT);
+}
+EXPORT_SYMBOL_GPL(tty_termios_encode_baud_rate);
+
+/**
+ *	tty_encode_baud_rate		-	set baud rate of the tty
+ *	@tty:   terminal device
+ *	@ibaud: input baud rate
+ *	@obaud: output baud rate
+ *
+ *	Update the current termios data for the tty with the new speed
+ *	settings. The caller must hold the termios_rwsem for the tty in
+ *	question.
+ */
+
+void tty_encode_baud_rate(struct tty_struct *tty, speed_t ibaud, speed_t obaud)
+{
+	tty_termios_encode_baud_rate(&tty->termios, ibaud, obaud);
+}
+EXPORT_SYMBOL_GPL(tty_encode_baud_rate);