s390/alternatives: remove padding generation code

clang fails to handle ".if" statements in inline assembly which are heavily
used in the alternatives code.

To work around this remove this code, and enforce that users of
alternatives must specify original and alternative instruction sequences
which have identical sizes. Add a compile time check with two ".org"
statements similar to arm64.

In result not only clang can handle this, but also quite a lot of code can
be removed.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1356
Link: https://lore.kernel.org/r/20220511120532.2228616-3-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

1 files changed, 2 insertions, 59 deletions

diff --git a/arch/s390/kernel/alternative.c b/arch/s390/kernel/alternative.c
index cce0ddee2d02..e7bca29f9c34 100644
--- a/arch/s390/kernel/alternative.c
+++ b/arch/s390/kernel/alternative.c
@@ -7,8 +7,6 @@
 #include <asm/facility.h>
 #include <asm/nospec-branch.h>
 
-#define MAX_PATCH_LEN (255 - 1)
-
 static int __initdata_or_module alt_instr_disabled;
 
 static int __init disable_alternative_instructions(char *str)
@@ -19,85 +17,30 @@ static int __init disable_alternative_instructions(char *str)
 
 early_param("noaltinstr", disable_alternative_instructions);
 
-struct brcl_insn {
-	u16 opc;
-	s32 disp;
-} __packed;
-
-static u16 __initdata_or_module nop16 = 0x0700;
-static u32 __initdata_or_module nop32 = 0x47000000;
-static struct brcl_insn __initdata_or_module nop48 = {
-	0xc004, 0
-};
-
-static const void *nops[] __initdata_or_module = {
-	&nop16,
-	&nop32,
-	&nop48
-};
-
-static void __init_or_module add_jump_padding(void *insns, unsigned int len)
-{
-	struct brcl_insn brcl = {
-		0xc0f4,
-		len / 2
-	};
-
-	memcpy(insns, &brcl, sizeof(brcl));
-	insns += sizeof(brcl);
-	len -= sizeof(brcl);
-
-	while (len > 0) {
-		memcpy(insns, &nop16, 2);
-		insns += 2;
-		len -= 2;
-	}
-}
-
-static void __init_or_module add_padding(void *insns, unsigned int len)
-{
-	if (len > 6)
-		add_jump_padding(insns, len);
-	else if (len >= 2)
-		memcpy(insns, nops[len / 2 - 1], len);
-}
-
 static void __init_or_module __apply_alternatives(struct alt_instr *start,
 						  struct alt_instr *end)
 {
 	struct alt_instr *a;
 	u8 *instr, *replacement;
-	u8 insnbuf[MAX_PATCH_LEN];
 
 	/*
 	 * The scan order should be from start to end. A later scanned
 	 * alternative code can overwrite previously scanned alternative code.
 	 */
 	for (a = start; a < end; a++) {
-		int insnbuf_sz = 0;
-
 		instr = (u8 *)&a->instr_offset + a->instr_offset;
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
 
 		if (!__test_facility(a->facility, alt_stfle_fac_list))
 			continue;
 
-		if (unlikely(a->instrlen % 2 || a->replacementlen % 2)) {
+		if (unlikely(a->instrlen % 2)) {
 			WARN_ONCE(1, "cpu alternatives instructions length is "
 				     "odd, skipping patching\n");
 			continue;
 		}
 
-		memcpy(insnbuf, replacement, a->replacementlen);
-		insnbuf_sz = a->replacementlen;
-
-		if (a->instrlen > a->replacementlen) {
-			add_padding(insnbuf + a->replacementlen,
-				    a->instrlen - a->replacementlen);
-			insnbuf_sz += a->instrlen - a->replacementlen;
-		}
-
-		s390_kernel_write(instr, insnbuf, insnbuf_sz);
+		s390_kernel_write(instr, replacement, a->instrlen);
 	}
 }