x86/mem: Move memmove to out of line assembler

When building ARCH=i386 with CONFIG_LTO_CLANG_FULL=y, it's possible
(depending on additional configs which I have not been able to isolate)
to observe a failure during register allocation:

error: inline assembly requires more registers than available

when memmove is inlined into tcp_v4_fill_cb() or tcp_v6_fill_cb().

memmove is quite large and probably shouldn't be inlined due to size
alone. A noinline function attribute would be the simplest fix, but
there's a few things that stand out with the current definition:

In addition to having complex constraints that can't always be resolved,
the clobber list seems to be missing %bx. By using numbered operands
rather than symbolic operands, the constraints are quite obnoxious to
refactor.

Having a large function be 99% inline asm is a code smell that this
function should simply be written in stand-alone out-of-line assembler.

Moving this to out of line assembler guarantees that the
compiler cannot inline calls to memmove.

This has been done previously for 64b:
commit 9599ec0471de ("x86-64, mem: Convert memmove() to assembly file
and fix return value bug")

That gives the opportunity for other cleanups like fixing the
inconsistent use of tabs vs spaces and instruction suffixes, and the
label 3 appearing twice. Symbolic operands, local labels, and
additional comments would provide this code with a fresh coat of paint.

Finally, add a test that tickles the `rep movsl` implementation to test
it for correctness, since it has implicit operands.

Suggested-by: Ingo Molnar <[e-mail]>
Suggested-by: David Laight <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Kees Cook <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>
Link: https://lore.kernel.org/all/20221018172155.287409-1-ndesaulniers%40google.com

arch/x86/lib/Makefile

···
         lib-y += checksum_32.o
         lib-y += strstr_32.o
         lib-y += string_32.o
+        lib-y += memmove_32.o
 ifneq ($(CONFIG_X86_CMPXCHG64),y)
         lib-y += cmpxchg8b_emu.o atomic64_386_32.o
 endif

arch/x86/lib/memcpy_32.c

···
 	return __memset(s, c, count);
 }
 EXPORT_SYMBOL(memset);
-
-__visible void *memmove(void *dest, const void *src, size_t n)
-{
-	int d0,d1,d2,d3,d4,d5;
-	char *ret = dest;
-
-	__asm__ __volatile__(
-		/* Handle more 16 bytes in loop */
-		"cmp $0x10, %0\n\t"
-		"jb	1f\n\t"
-
-		/* Decide forward/backward copy mode */
-		"cmp %2, %1\n\t"
-		"jb	2f\n\t"
-
-		/*
-		 * movs instruction have many startup latency
-		 * so we handle small size by general register.
-		 */
-		"cmp  $680, %0\n\t"
-		"jb 3f\n\t"
-		/*
-		 * movs instruction is only good for aligned case.
-		 */
-		"mov %1, %3\n\t"
-		"xor %2, %3\n\t"
-		"and $0xff, %3\n\t"
-		"jz 4f\n\t"
-		"3:\n\t"
-		"sub $0x10, %0\n\t"
-
-		/*
-		 * We gobble 16 bytes forward in each loop.
-		 */
-		"3:\n\t"
-		"sub $0x10, %0\n\t"
-		"mov 0*4(%1), %3\n\t"
-		"mov 1*4(%1), %4\n\t"
-		"mov  %3, 0*4(%2)\n\t"
-		"mov  %4, 1*4(%2)\n\t"
-		"mov 2*4(%1), %3\n\t"
-		"mov 3*4(%1), %4\n\t"
-		"mov  %3, 2*4(%2)\n\t"
-		"mov  %4, 3*4(%2)\n\t"
-		"lea  0x10(%1), %1\n\t"
-		"lea  0x10(%2), %2\n\t"
-		"jae 3b\n\t"
-		"add $0x10, %0\n\t"
-		"jmp 1f\n\t"
-
-		/*
-		 * Handle data forward by movs.
-		 */
-		".p2align 4\n\t"
-		"4:\n\t"
-		"mov -4(%1, %0), %3\n\t"
-		"lea -4(%2, %0), %4\n\t"
-		"shr $2, %0\n\t"
-		"rep movsl\n\t"
-		"mov %3, (%4)\n\t"
-		"jmp 11f\n\t"
-		/*
-		 * Handle data backward by movs.
-		 */
-		".p2align 4\n\t"
-		"6:\n\t"
-		"mov (%1), %3\n\t"
-		"mov %2, %4\n\t"
-		"lea -4(%1, %0), %1\n\t"
-		"lea -4(%2, %0), %2\n\t"
-		"shr $2, %0\n\t"
-		"std\n\t"
-		"rep movsl\n\t"
-		"mov %3,(%4)\n\t"
-		"cld\n\t"
-		"jmp 11f\n\t"
-
-		/*
-		 * Start to prepare for backward copy.
-		 */
-		".p2align 4\n\t"
-		"2:\n\t"
-		"cmp  $680, %0\n\t"
-		"jb 5f\n\t"
-		"mov %1, %3\n\t"
-		"xor %2, %3\n\t"
-		"and $0xff, %3\n\t"
-		"jz 6b\n\t"
-
-		/*
-		 * Calculate copy position to tail.
-		 */
-		"5:\n\t"
-		"add %0, %1\n\t"
-		"add %0, %2\n\t"
-		"sub $0x10, %0\n\t"
-
-		/*
-		 * We gobble 16 bytes backward in each loop.
-		 */
-		"7:\n\t"
-		"sub $0x10, %0\n\t"
-
-		"mov -1*4(%1), %3\n\t"
-		"mov -2*4(%1), %4\n\t"
-		"mov  %3, -1*4(%2)\n\t"
-		"mov  %4, -2*4(%2)\n\t"
-		"mov -3*4(%1), %3\n\t"
-		"mov -4*4(%1), %4\n\t"
-		"mov  %3, -3*4(%2)\n\t"
-		"mov  %4, -4*4(%2)\n\t"
-		"lea  -0x10(%1), %1\n\t"
-		"lea  -0x10(%2), %2\n\t"
-		"jae 7b\n\t"
-		/*
-		 * Calculate copy position to head.
-		 */
-		"add $0x10, %0\n\t"
-		"sub %0, %1\n\t"
-		"sub %0, %2\n\t"
-
-		/*
-		 * Move data from 8 bytes to 15 bytes.
-		 */
-		".p2align 4\n\t"
-		"1:\n\t"
-		"cmp $8, %0\n\t"
-		"jb 8f\n\t"
-		"mov 0*4(%1), %3\n\t"
-		"mov 1*4(%1), %4\n\t"
-		"mov -2*4(%1, %0), %5\n\t"
-		"mov -1*4(%1, %0), %1\n\t"
-
-		"mov  %3, 0*4(%2)\n\t"
-		"mov  %4, 1*4(%2)\n\t"
-		"mov  %5, -2*4(%2, %0)\n\t"
-		"mov  %1, -1*4(%2, %0)\n\t"
-		"jmp 11f\n\t"
-
-		/*
-		 * Move data from 4 bytes to 7 bytes.
-		 */
-		".p2align 4\n\t"
-		"8:\n\t"
-		"cmp $4, %0\n\t"
-		"jb 9f\n\t"
-		"mov 0*4(%1), %3\n\t"
-		"mov -1*4(%1, %0), %4\n\t"
-		"mov  %3, 0*4(%2)\n\t"
-		"mov  %4, -1*4(%2, %0)\n\t"
-		"jmp 11f\n\t"
-
-		/*
-		 * Move data from 2 bytes to 3 bytes.
-		 */
-		".p2align 4\n\t"
-		"9:\n\t"
-		"cmp $2, %0\n\t"
-		"jb 10f\n\t"
-		"movw 0*2(%1), %%dx\n\t"
-		"movw -1*2(%1, %0), %%bx\n\t"
-		"movw %%dx, 0*2(%2)\n\t"
-		"movw %%bx, -1*2(%2, %0)\n\t"
-		"jmp 11f\n\t"
-
-		/*
-		 * Move data for 1 byte.
-		 */
-		".p2align 4\n\t"
-		"10:\n\t"
-		"cmp $1, %0\n\t"
-		"jb 11f\n\t"
-		"movb (%1), %%cl\n\t"
-		"movb %%cl, (%2)\n\t"
-		".p2align 4\n\t"
-		"11:"
-		: "=&c" (d0), "=&S" (d1), "=&D" (d2),
-		  "=r" (d3),"=r" (d4), "=r"(d5)
-		:"0" (n),
-		 "1" (src),
-		 "2" (dest)
-		:"memory");
-
-	return ret;
-
-}
-EXPORT_SYMBOL(memmove);

arch/x86/lib/memmove_32.S

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/linkage.h>
+#include <asm/export.h>
+
+SYM_FUNC_START(memmove)
+/*
+ * void *memmove(void *dest_in, const void *src_in, size_t n)
+ * -mregparm=3 passes these in registers:
+ * dest_in: %eax
+ * src_in: %edx
+ * n: %ecx
+ * See also: arch/x86/entry/calling.h for description of the calling convention.
+ *
+ * n can remain in %ecx, but for `rep movsl`, we'll need dest in %edi and src
+ * in %esi.
+ */
+.set dest_in, %eax
+.set dest, %edi
+.set src_in, %edx
+.set src, %esi
+.set n, %ecx
+.set tmp0, %edx
+.set tmp0w, %dx
+.set tmp1, %ebx
+.set tmp1w, %bx
+.set tmp2, %eax
+.set tmp3b, %cl
+
+/*
+ * Save all callee-saved registers, because this function is going to clobber
+ * all of them:
+ */
+	pushl	%ebp
+	movl	%esp, %ebp	// set standard frame pointer
+
+	pushl	%ebx
+	pushl	%edi
+	pushl	%esi
+	pushl	%eax		// save 'dest_in' parameter [eax] as the return value
+
+	movl src_in, src
+	movl dest_in, dest
+
+	/* Handle more 16 bytes in loop */
+	cmpl	$0x10, n
+	jb	.Lmove_16B
+
+	/* Decide forward/backward copy mode */
+	cmpl	dest, src
+	jb	.Lbackwards_header
+
+	/*
+	 * movs instruction have many startup latency
+	 * so we handle small size by general register.
+	 */
+	cmpl	$680, n
+	jb	.Ltoo_small_forwards
+	/* movs instruction is only good for aligned case. */
+	movl	src, tmp0
+	xorl	dest, tmp0
+	andl	$0xff, tmp0
+	jz	.Lforward_movs
+.Ltoo_small_forwards:
+	subl	$0x10, n
+
+	/* We gobble 16 bytes forward in each loop. */
+.Lmove_16B_forwards_loop:
+	subl	$0x10, n
+	movl	0*4(src), tmp0
+	movl	1*4(src), tmp1
+	movl	tmp0, 0*4(dest)
+	movl	tmp1, 1*4(dest)
+	movl	2*4(src), tmp0
+	movl	3*4(src), tmp1
+	movl	tmp0, 2*4(dest)
+	movl	tmp1, 3*4(dest)
+	leal	0x10(src), src
+	leal	0x10(dest), dest
+	jae	.Lmove_16B_forwards_loop
+	addl	$0x10, n
+	jmp	.Lmove_16B
+
+	/* Handle data forward by movs. */
+.p2align 4
+.Lforward_movs:
+	movl	-4(src, n), tmp0
+	leal	-4(dest, n), tmp1
+	shrl	$2, n
+	rep	movsl
+	movl	tmp0, (tmp1)
+	jmp	.Ldone
+
+	/* Handle data backward by movs. */
+.p2align 4
+.Lbackwards_movs:
+	movl	(src), tmp0
+	movl	dest, tmp1
+	leal	-4(src, n), src
+	leal	-4(dest, n), dest
+	shrl	$2, n
+	std
+	rep	movsl
+	movl	tmp0,(tmp1)
+	cld
+	jmp	.Ldone
+
+	/* Start to prepare for backward copy. */
+.p2align 4
+.Lbackwards_header:
+	cmpl	$680, n
+	jb	.Ltoo_small_backwards
+	movl	src, tmp0
+	xorl	dest, tmp0
+	andl	$0xff, tmp0
+	jz	.Lbackwards_movs
+
+	/* Calculate copy position to tail. */
+.Ltoo_small_backwards:
+	addl	n, src
+	addl	n, dest
+	subl	$0x10, n
+
+	/* We gobble 16 bytes backward in each loop. */
+.Lmove_16B_backwards_loop:
+	subl	$0x10, n
+
+	movl	-1*4(src), tmp0
+	movl	-2*4(src), tmp1
+	movl	tmp0, -1*4(dest)
+	movl	tmp1, -2*4(dest)
+	movl	-3*4(src), tmp0
+	movl	-4*4(src), tmp1
+	movl	tmp0, -3*4(dest)
+	movl	tmp1, -4*4(dest)
+	leal	-0x10(src), src
+	leal	-0x10(dest), dest
+	jae	.Lmove_16B_backwards_loop
+	/* Calculate copy position to head. */
+	addl	$0x10, n
+	subl	n, src
+	subl	n, dest
+
+	/* Move data from 8 bytes to 15 bytes. */
+.p2align 4
+.Lmove_16B:
+	cmpl	$8, n
+	jb	.Lmove_8B
+	movl	0*4(src), tmp0
+	movl	1*4(src), tmp1
+	movl	-2*4(src, n), tmp2
+	movl	-1*4(src, n), src
+
+	movl	tmp0, 0*4(dest)
+	movl	tmp1, 1*4(dest)
+	movl	tmp2, -2*4(dest, n)
+	movl	src, -1*4(dest, n)
+	jmp	.Ldone
+
+	/* Move data from 4 bytes to 7 bytes. */
+.p2align 4
+.Lmove_8B:
+	cmpl	$4, n
+	jb	.Lmove_4B
+	movl	0*4(src), tmp0
+	movl	-1*4(src, n), tmp1
+	movl	tmp0, 0*4(dest)
+	movl	tmp1, -1*4(dest, n)
+	jmp	.Ldone
+
+	/* Move data from 2 bytes to 3 bytes. */
+.p2align 4
+.Lmove_4B:
+	cmpl	$2, n
+	jb	.Lmove_1B
+	movw	0*2(src), tmp0w
+	movw	-1*2(src, n), tmp1w
+	movw	tmp0w, 0*2(dest)
+	movw	tmp1w, -1*2(dest, n)
+	jmp	.Ldone
+
+	/* Move data for 1 byte. */
+.p2align 4
+.Lmove_1B:
+	cmpl	$1, n
+	jb	.Ldone
+	movb	(src), tmp3b
+	movb	tmp3b, (dest)
+.p2align 4
+.Ldone:
+	popl	dest_in	// restore 'dest_in' [eax] as the return value
+	/* Restore all callee-saved registers: */
+	popl	%esi
+	popl	%edi
+	popl	%ebx
+	popl	%ebp
+
+	RET
+SYM_FUNC_END(memmove)
+EXPORT_SYMBOL(memmove)

lib/memcpy_kunit.c

···
 #undef TEST_OP
 }
 
+static unsigned char larger_array [2048];
+
 static void memmove_test(struct kunit *test)
 {
 #define TEST_OP "memmove"
···
 	ptr = &overlap.data[2];
 	memmove(ptr, overlap.data, 5);
 	compare("overlapping write", overlap, overlap_expected);
+
+	/* Verify larger overlapping moves. */
+	larger_array[256] = 0xAAu;
+	/*
+	 * Test a backwards overlapping memmove first. 256 and 1024 are
+	 * important for i386 to use rep movsl.
+	 */
+	memmove(larger_array, larger_array + 256, 1024);
+	KUNIT_ASSERT_EQ(test, larger_array[0], 0xAAu);
+	KUNIT_ASSERT_EQ(test, larger_array[256], 0x00);
+	KUNIT_ASSERT_NULL(test,
+		memchr(larger_array + 1, 0xaa, ARRAY_SIZE(larger_array) - 1));
+	/* Test a forwards overlapping memmove. */
+	larger_array[0] = 0xBBu;
+	memmove(larger_array + 256, larger_array, 1024);
+	KUNIT_ASSERT_EQ(test, larger_array[0], 0xBBu);
+	KUNIT_ASSERT_EQ(test, larger_array[256], 0xBBu);
+	KUNIT_ASSERT_NULL(test, memchr(larger_array + 1, 0xBBu, 256 - 1));
+	KUNIT_ASSERT_NULL(test,
+		memchr(larger_array + 257, 0xBBu, ARRAY_SIZE(larger_array) - 257));
 #undef TEST_OP
 }