[tip: x86/core] x86/alternatives: Optimize optimize_nops()

From: tip-bot2 for Peter Zijlstra
Date: Thu Apr 01 2021 - 14:01:11 EST

The following commit has been merged into the x86/core branch of tip:

Commit-ID: b4da5166b084f3fac01d68e0e67cbf3bf78a3e12
Gitweb: https://git.kernel.org/tip/b4da5166b084f3fac01d68e0e67cbf3bf78a3e12
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Fri, 26 Mar 2021 16:12:01 +01:00
Committer: Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 31 Mar 2021 20:30:04 +02:00

x86/alternatives: Optimize optimize_nops()

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@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20210326151259.442992235@xxxxxxxxxxxxx
---
arch/x86/include/asm/alternative.h | 17 +-----
arch/x86/kernel/alternative.c | 49 +++++++++++-------
tools/objtool/arch/x86/include/arch/special.h | 2 +-
3 files changed, 37 insertions(+), 31 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 17b3609..a3c2315 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -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"
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index f902f28..1298d58 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -345,19 +345,35 @@ 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;
+
+ /*
+ * 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 (i = 0; i < a->padlen; i++) {
- if (instr[i] != 0x90)
+ 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);
}

/*
@@ -403,19 +419,15 @@ 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);
-
- continue;
- }
+ if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV))
+ goto next;

- 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);
@@ -439,14 +451,15 @@ 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);
}
}

diff --git a/tools/objtool/arch/x86/include/arch/special.h b/tools/objtool/arch/x86/include/arch/special.h
index d818b2b..14271cc 100644
--- a/tools/objtool/arch/x86/include/arch/special.h
+++ b/tools/objtool/arch/x86/include/arch/special.h
@@ -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