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x86/alternatives: Optimize optimize_nops()

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Currently, optimize_nops() scans to see if the alternative starts with
NOPs. However, the emit pattern is:

  141:	\oldinstr
  142:	.skip (len-(142b-141b)), 0x90

That is, when 'oldinstr' is short, the tail is padded with NOPs. This case
never gets optimized.

Rewrite optimize_nops() to replace any trailing string of NOPs inside
the alternative to larger NOPs. Also run it irrespective of patching,
replacing NOPs in both the original and replaced code.

A direct consequence is that 'padlen' becomes superfluous, so remove it.

 [ bp:
   - Adjust commit message
   - remove a stale comment about needing to pad
   - add a comment in optimize_nops()
   - exit early if the NOP verif. loop catches a mismatch - function
     should not not add NOPs in that case
   - fix the "optimized NOPs" offsets output ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210326151259.442992235@infradead.org
This commit is contained in:
Peter Zijlstra 2021-03-26 16:12:01 +01:00 committed by Ingo Molnar
parent b1f480bc06
commit 23c1ad538f
3 changed files with 37 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
arch/x86/include/asm/alternative.h
View File

@ -65,7 +65,6 @@ struct alt_instr {
u16 cpuid; /* cpuid bit set for replacement */
u8 instrlen; /* length of original instruction */
u8 replacementlen; /* length of new instruction */
u8 padlen; /* length of build-time padding */
} __packed;
/*
@ -104,7 +103,6 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define alt_end_marker "663"
#define alt_slen "662b-661b"
#define alt_pad_len alt_end_marker"b-662b"
#define alt_total_slen alt_end_marker"b-661b"
#define alt_rlen(num) e_replacement(num)"f-"b_replacement(num)"f"
@ -151,8 +149,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
" .long " b_replacement(num)"f - .\n" /* new instruction */ \
" .word " __stringify(feature) "\n" /* feature bit */ \
" .byte " alt_total_slen "\n" /* source len */ \
" .byte " alt_rlen(num) "\n" /* replacement len */ \
" .byte " alt_pad_len "\n" /* pad len */
" .byte " alt_rlen(num) "\n" /* replacement len */
#define ALTINSTR_REPLACEMENT(newinstr, num) /* replacement */ \
"# ALT: replacement " #num "\n" \
@ -224,9 +221,6 @@ static inline int alternatives_text_reserved(void *start, void *end)
* Peculiarities:
* No memory clobber here.
* Argument numbers start with 1.
* Best is to use constraints that are fixed size (like (%1) ... "r")
* If you use variable sized constraints like "m" or "g" in the
* replacement make sure to pad to the worst case length.
* Leaving an unused argument 0 to keep API compatibility.
*/
#define alternative_input(oldinstr, newinstr, feature, input...) \
@ -315,13 +309,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
* enough information for the alternatives patching code to patch an
* instruction. See apply_alternatives().
*/
.macro altinstruction_entry orig alt feature orig_len alt_len pad_len
.macro altinstruction_entry orig alt feature orig_len alt_len
.long \orig - .
.long \alt - .
.word \feature
.byte \orig_len
.byte \alt_len
.byte \pad_len
.endm
/*
@ -338,7 +331,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
142:
.pushsection .altinstructions,"a"
altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f,142b-141b
altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f
.popsection
.pushsection .altinstr_replacement,"ax"
@ -375,8 +368,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
142:
.pushsection .altinstructions,"a"
altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f,142b-141b
altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f,142b-141b
altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f
altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f
.popsection
.pushsection .altinstr_replacement,"ax"

49
arch/x86/kernel/alternative.c
View File

@ -189,19 +189,35 @@ done:
static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
{
unsigned long flags;
int i;
struct insn insn;
int nop, i = 0;
for (i = 0; i < a->padlen; i++) {
if (instr[i] != 0x90)
/*
* Jump over the non-NOP insns, the remaining bytes must be single-byte
* NOPs, optimize them.
*/
for (;;) {
if (insn_decode_kernel(&insn, &instr[i]))
return;
if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
break;
if ((i += insn.length) >= a->instrlen)
return;
}
for (nop = i; i < a->instrlen; i++) {
if (WARN_ONCE(instr[i] != 0x90, "Not a NOP at 0x%px\n", &instr[i]))
return;
}
local_irq_save(flags);
add_nops(instr + (a->instrlen - a->padlen), a->padlen);
add_nops(instr + nop, i - nop);
local_irq_restore(flags);
DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
instr, a->instrlen - a->padlen, a->padlen);
instr, nop, a->instrlen);
}
/*
@ -247,19 +263,15 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
* - feature not present but ALTINSTR_FLAG_INV is set to mean,
* patch if feature is *NOT* present.
*/
if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) {
if (a->padlen > 1)
optimize_nops(a, instr);
if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV))
goto next;
continue;
}
DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d)",
(a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "",
feature >> 5,
feature & 0x1f,
instr, instr, a->instrlen,
replacement, a->replacementlen, a->padlen);
replacement, a->replacementlen);
DUMP_BYTES(instr, a->instrlen, "%px: old_insn: ", instr);
DUMP_BYTES(replacement, a->replacementlen, "%px: rpl_insn: ", replacement);
@ -283,14 +295,15 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
if (a->replacementlen && is_jmp(replacement[0]))
recompute_jump(a, instr, replacement, insn_buff);
if (a->instrlen > a->replacementlen) {
add_nops(insn_buff + a->replacementlen,
a->instrlen - a->replacementlen);
insn_buff_sz += a->instrlen - a->replacementlen;
}
for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
insn_buff[insn_buff_sz] = 0x90;
DUMP_BYTES(insn_buff, insn_buff_sz, "%px: final_insn: ", instr);
text_poke_early(instr, insn_buff, insn_buff_sz);
next:
optimize_nops(a, instr);
}
}

2
tools/objtool/arch/x86/include/arch/special.h
View File

@ -10,7 +10,7 @@
#define JUMP_ORIG_OFFSET 0
#define JUMP_NEW_OFFSET 4
#define ALT_ENTRY_SIZE 13
#define ALT_ENTRY_SIZE 12
#define ALT_ORIG_OFFSET 0
#define ALT_NEW_OFFSET 4
#define ALT_FEATURE_OFFSET 8