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, 165 insertions, 0 deletions

diff --git a/arch/alpha/lib/memchr.S b/arch/alpha/lib/memchr.S
new file mode 100644
index 000000000..45366e32f
--- /dev/null
+++ b/arch/alpha/lib/memchr.S
@@ -0,0 +1,165 @@
+/* Copyright (C) 1996 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by David Mosberger (davidm@cs.arizona.edu).
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Library General Public License as
+   published by the Free Software Foundation; either version 2 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If not,
+   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+   Boston, MA 02111-1307, USA.  */
+
+/* Finds characters in a memory area.  Optimized for the Alpha:
+
+      - memory accessed as aligned quadwords only
+      - uses cmpbge to compare 8 bytes in parallel
+      - does binary search to find 0 byte in last
+        quadword (HAKMEM needed 12 instructions to
+        do this instead of the 9 instructions that
+        binary search needs).
+
+For correctness consider that:
+
+      - only minimum number of quadwords may be accessed
+      - the third argument is an unsigned long
+*/
+#include <linux/export.h>
+        .set noreorder
+        .set noat
+
+	.globl memchr
+	.ent memchr
+memchr:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	# Hack -- if someone passes in (size_t)-1, hoping to just
+	# search til the end of the address space, we will overflow
+	# below when we find the address of the last byte.  Given
+	# that we will never have a 56-bit address space, cropping
+	# the length is the easiest way to avoid trouble.
+	zap	$18, 0x80, $5	#-e0	:
+
+	beq	$18, $not_found	# .. e1 :
+        ldq_u   $1, 0($16)	# e1	: load first quadword
+	insbl	$17, 1, $2	# .. e0 : $2 = 000000000000ch00
+	and	$17, 0xff, $17	#-e0    : $17 = 00000000000000ch
+	cmpult	$18, 9, $4	# .. e1 :
+	or	$2, $17, $17	# e0    : $17 = 000000000000chch
+        lda     $3, -1($31)	# .. e1 :
+	sll	$17, 16, $2	#-e0    : $2 = 00000000chch0000
+	addq	$16, $5, $5	# .. e1 :
+	or	$2, $17, $17	# e1    : $17 = 00000000chchchch
+	unop			#	:
+	sll	$17, 32, $2	#-e0    : $2 = chchchch00000000
+	or	$2, $17, $17	# e1	: $17 = chchchchchchchch
+	extql	$1, $16, $7	# e0    : 
+	beq	$4, $first_quad	# .. e1 :
+
+	ldq_u	$6, -1($5)	#-e1	: eight or less bytes to search
+	extqh	$6, $16, $6	# .. e0 :
+	mov	$16, $0		# e0	:
+	or	$7, $6, $1	# .. e1 : $1 = quadword starting at $16
+
+	# Deal with the case where at most 8 bytes remain to be searched
+	# in $1.  E.g.:
+	#	$18 = 6
+	#	$1 = ????c6c5c4c3c2c1
+$last_quad:
+	negq	$18, $6		#-e0	:
+        xor	$17, $1, $1	# .. e1 :
+	srl	$3, $6, $6	# e0    : $6 = mask of $18 bits set
+        cmpbge  $31, $1, $2	# .. e1 :
+	and	$2, $6, $2	#-e0	:
+        beq     $2, $not_found	# .. e1 :
+
+$found_it:
+	# Now, determine which byte matched:
+        negq    $2, $3		# e0	:
+        and     $2, $3, $2	# e1	:
+
+        and     $2, 0x0f, $1	#-e0	:
+        addq    $0, 4, $3	# .. e1 :
+        cmoveq  $1, $3, $0	# e0	:
+
+        addq    $0, 2, $3	# .. e1 :
+        and     $2, 0x33, $1	#-e0	:
+        cmoveq  $1, $3, $0	# .. e1 :
+
+        and     $2, 0x55, $1	# e0	:
+        addq    $0, 1, $3	# .. e1 :
+        cmoveq  $1, $3, $0	#-e0	:
+
+$done:	ret			# .. e1 :
+
+	# Deal with the case where $18 > 8 bytes remain to be
+	# searched.  $16 may not be aligned.
+	.align 4
+$first_quad:
+	andnot	$16, 0x7, $0	#-e1	:
+        insqh   $3, $16, $2	# .. e0	: $2 = 0000ffffffffffff ($16<0:2> ff)
+        xor	$1, $17, $1	# e0	:
+	or	$1, $2, $1	# e1	: $1 = ====ffffffffffff
+        cmpbge  $31, $1, $2	#-e0	:
+        bne     $2, $found_it	# .. e1 :
+
+	# At least one byte left to process.
+
+	ldq	$1, 8($0)	# e0	:
+	subq	$5, 1, $18	# .. e1 :
+	addq	$0, 8, $0	#-e0	:
+
+	# Make $18 point to last quad to be accessed (the
+	# last quad may or may not be partial).
+
+	andnot	$18, 0x7, $18	# .. e1 :
+	cmpult	$0, $18, $2	# e0	:
+	beq	$2, $final	# .. e1 :
+
+	# At least two quads remain to be accessed.
+
+	subq	$18, $0, $4	#-e0	: $4 <- nr quads to be processed
+	and	$4, 8, $4	# e1	: odd number of quads?
+	bne	$4, $odd_quad_count # e1 :
+
+	# At least three quads remain to be accessed
+
+	mov	$1, $4		# e0	: move prefetched value to correct reg
+
+	.align	4
+$unrolled_loop:
+	ldq	$1, 8($0)	#-e0	: prefetch $1
+	xor	$17, $4, $2	# .. e1 :
+	cmpbge	$31, $2, $2	# e0	:
+	bne	$2, $found_it	# .. e1 :
+
+	addq	$0, 8, $0	#-e0	:
+$odd_quad_count:
+	xor	$17, $1, $2	# .. e1 :
+	ldq	$4, 8($0)	# e0	: prefetch $4
+	cmpbge	$31, $2, $2	# .. e1 :
+	addq	$0, 8, $6	#-e0	:
+	bne	$2, $found_it	# .. e1	:
+
+	cmpult	$6, $18, $6	# e0	:
+	addq	$0, 8, $0	# .. e1 :
+	bne	$6, $unrolled_loop #-e1 :
+
+	mov	$4, $1		# e0	: move prefetched value into $1
+$final:	subq	$5, $0, $18	# .. e1	: $18 <- number of bytes left to do
+	bne	$18, $last_quad	# e1	:
+
+$not_found:
+	mov	$31, $0		#-e0	:
+	ret			# .. e1 :
+
+        .end memchr
+	EXPORT_SYMBOL(memchr)