[PATCH v1 02/14] x86/alternatives: Add nested alternatives macros

From: Borislav Petkov
Date: Fri Jun 07 2024 - 07:17:52 EST

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From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Instead of making increasingly complicated ALTERNATIVE_n()
implementations, use a nested alternative expression.

The only difference between:

ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2)

and

ALTERNATIVE(ALTERNATIVE(oldinst, newinst1, flag1),
newinst2, flag2)

is that the outer alternative can add additional padding when the inner
alternative is the shorter one, which then results in
alt_instr::instrlen being inconsistent.

However, this is easily remedied since the alt_instr entries will be
consecutive and it is trivial to compute the max(alt_instr::instrlen) at
runtime while patching.

Specifically, after this the ALTERNATIVE_2 macro, after CPP expansion
(and manual layout), looks like this:

.macro ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2
740:
740: \oldinstr ;
741: .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;
742: .pushsection .altinstructions,"a" ;
altinstr_entry 740b,743f,\ft_flags1,742b-740b,744f-743f ;
.popsection ;
.pushsection .altinstr_replacement,"ax" ;
743: \newinstr1 ;
744: .popsection ; ;
741: .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;
742: .pushsection .altinstructions,"a" ;
altinstr_entry 740b,743f,\ft_flags2,742b-740b,744f-743f ;
.popsection ;
.pushsection .altinstr_replacement,"ax" ;
743: \newinstr2 ;
744: .popsection ;
.endm

The only label that is ambiguous is 740, however they all reference the
same spot, so that doesn't matter.

NOTE: obviously only @oldinstr may be an alternative; making @newinstr
an alternative would mean patching .altinstr_replacement which very
likely isn't what is intended, also the labels will be confused in that
case.

[ bp: Debug an issue where it would match the wrong two insns and
and consider them nested due to the same signed offsets in the
.alternative section and use instr_va() to compare the full virtual
addresses instead.

- Use new labels to denote that the new, nested
alternatives are being used when staring at preprocessed output.

- Use the %c constraint everywhere instead of %P and document the
difference for future reference. ]

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Co-developed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20230628104952.GA2439977@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
arch/x86/include/asm/alternative.h | 120 ++++++++++++++++++++++++++++-
arch/x86/kernel/alternative.c | 20 ++++-
tools/objtool/arch/x86/special.c | 23 ++++++
tools/objtool/special.c | 16 ++--
4 files changed, 167 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 6db78909180a..7d9ebc516bde 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -161,8 +161,13 @@ static inline int alternatives_text_reserved(void *start, void *end)

#define alt_end_marker "663"
#define alt_slen "662b-661b"
+#define n_alt_slen "772b-771b"
+
#define alt_total_slen alt_end_marker"b-661b"
+#define n_alt_total_slen "773b-771b"
+
#define alt_rlen(num) e_replacement(num)"f-"b_replacement(num)"f"
+#define n_alt_rlen "775f-774f"

#define OLDINSTR(oldinstr, num) \
"# ALT: oldnstr\n" \
@@ -172,6 +177,14 @@ static inline int alternatives_text_reserved(void *start, void *end)
"((" alt_rlen(num) ")-(" alt_slen ")),0x90\n" \
alt_end_marker ":\n"

+#define N_OLDINSTR(oldinstr) \
+ "# N_ALT: oldinstr\n" \
+ "771:\n\t" oldinstr "\n772:\n" \
+ "# N_ALT: padding\n" \
+ ".skip -(((" n_alt_rlen ")-(" n_alt_slen ")) > 0) * " \
+ "((" n_alt_rlen ")-(" n_alt_slen ")),0x90\n" \
+ "773:\n"
+
/*
* gas compatible max based on the idea from:
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
@@ -209,10 +222,25 @@ static inline int alternatives_text_reserved(void *start, void *end)
" .byte " alt_total_slen "\n" /* source len */ \
" .byte " alt_rlen(num) "\n" /* replacement len */

+#define N_ALTINSTR_ENTRY(ft_flags) \
+ ".pushsection .altinstructions,\"a\"\n" \
+ " .long 771b - .\n" /* label */ \
+ " .long 774f - .\n" /* new instruction */ \
+ " .4byte " __stringify(ft_flags) "\n" /* feature + flags */ \
+ " .byte " n_alt_total_slen "\n" /* source len */ \
+ " .byte " n_alt_rlen "\n" /* replacement len */ \
+ ".popsection\n"
+
#define ALTINSTR_REPLACEMENT(newinstr, num) /* replacement */ \
"# ALT: replacement " #num "\n" \
b_replacement(num)":\n\t" newinstr "\n" e_replacement(num) ":\n"

+#define N_ALTINSTR_REPLACEMENT(newinstr) /* replacement */ \
+ ".pushsection .altinstr_replacement, \"ax\"\n" \
+ "# N_ALT: replacement\n" \
+ "774:\n\t" newinstr "\n775:\n" \
+ ".popsection\n"
+
/* alternative assembly primitive: */
#define ALTERNATIVE(oldinstr, newinstr, ft_flags) \
OLDINSTR(oldinstr, 1) \
@@ -223,6 +251,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_REPLACEMENT(newinstr, 1) \
".popsection\n"

+/* Nested alternatives macro variant */
+#define N_ALTERNATIVE(oldinstr, newinstr, ft_flags) \
+ N_OLDINSTR(oldinstr) \
+ N_ALTINSTR_ENTRY(ft_flags) \
+ N_ALTINSTR_REPLACEMENT(newinstr)
+
#define ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) \
OLDINSTR_2(oldinstr, 1, 2) \
".pushsection .altinstructions,\"a\"\n" \
@@ -234,11 +268,21 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_REPLACEMENT(newinstr2, 2) \
".popsection\n"

+#define N_ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2) \
+ N_ALTERNATIVE(N_ALTERNATIVE(oldinst, newinst1, flag1), \
+ newinst2, flag2)
+
/* If @feature is set, patch in @newinstr_yes, otherwise @newinstr_no. */
#define ALTERNATIVE_TERNARY(oldinstr, ft_flags, newinstr_yes, newinstr_no) \
ALTERNATIVE_2(oldinstr, newinstr_no, X86_FEATURE_ALWAYS, \
newinstr_yes, ft_flags)

+/* If @feature is set, patch in @newinstr_yes, otherwise @newinstr_no. */
+#define N_ALTERNATIVE_TERNARY(oldinstr, ft_flags, newinstr_yes, newinstr_no) \
+ N_ALTERNATIVE_2(oldinstr, newinstr_no, X86_FEATURE_ALWAYS, \
+ newinstr_yes, ft_flags)
+
+
#define ALTERNATIVE_3(oldinsn, newinsn1, ft_flags1, newinsn2, ft_flags2, \
newinsn3, ft_flags3) \
OLDINSTR_3(oldinsn, 1, 2, 3) \
@@ -253,6 +297,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_REPLACEMENT(newinsn3, 3) \
".popsection\n"

+
+#define N_ALTERNATIVE_3(oldinst, newinst1, flag1, newinst2, flag2, \
+ newinst3, flag3) \
+ N_ALTERNATIVE(N_ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2), \
+ newinst3, flag3)
+
/*
* Alternative instructions for different CPU types or capabilities.
*
@@ -271,6 +321,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define alternative_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) \
asm_inline volatile(ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) ::: "memory")

+#define n_alternative(oldinstr, newinstr, ft_flags) \
+ asm_inline volatile (N_ALTERNATIVE(oldinstr, newinstr, ft_flags) : : : "memory")
+
+#define n_alternative_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) \
+ asm_inline volatile(N_ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) ::: "memory")
+
/*
* Alternative inline assembly with input.
*
@@ -288,11 +344,32 @@ static inline int alternatives_text_reserved(void *start, void *end)
asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, ft_flags) \
: output : "i" (0), ## input)

-/* Like alternative_io, but for replacing a direct call with another one. */
+#define n_alternative_io(oldinstr, newinstr, ft_flags, output, input...) \
+ asm_inline volatile (N_ALTERNATIVE(oldinstr, newinstr, ft_flags) \
+ : output : "i" (0), ## input)
+
+/*
+ * Like alternative_io, but for replacing a direct call with another one.
+ *
+ * Use the %c operand modifier which is the generic way to print a bare
+ * constant expression with all syntax-specific punctuation omitted. %P
+ * is the x86-specific variant which can handle constants too, for
+ * historical reasons, but it should be used primarily for PIC
+ * references: i.e., if used for a function, it would add the PLT
+ * suffix.
+ */
#define alternative_call(oldfunc, newfunc, ft_flags, output, input...) \
asm_inline volatile (ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
: output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)

+#define n_alternative_input(oldinstr, newinstr, ft_flags, input...) \
+ asm_inline volatile (N_ALTERNATIVE(oldinstr, newinstr, ft_flags) \
+ : : "i" (0), ## input)
+
+#define n_alternative_call(oldfunc, newfunc, ft_flags, output, input...) \
+ asm_inline volatile (N_ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
+ : output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)
+
/*
* Like alternative_call, but there are two features and respective functions.
* If CPU has feature2, function2 is used.
@@ -307,6 +384,15 @@ static inline int alternatives_text_reserved(void *start, void *end)
: [old] "i" (oldfunc), [new1] "i" (newfunc1), \
[new2] "i" (newfunc2), ## input)

+#define n_alternative_call_2(oldfunc, newfunc1, ft_flags1, newfunc2, ft_flags2, \
+ output, input...) \
+ asm_inline volatile (N_ALTERNATIVE_2("call %c[old]", "call %c[new1]", ft_flags1,\
+ "call %c[new2]", ft_flags2) \
+ : output, ASM_CALL_CONSTRAINT \
+ : [old] "i" (oldfunc), [new1] "i" (newfunc1), \
+ [new2] "i" (newfunc2), ## input)
+
+
/*
* use this macro(s) if you need more than one output parameter
* in alternative_io
@@ -403,6 +489,27 @@ void nop_func(void);
.popsection
.endm

+#define __N_ALTERNATIVE(oldinst, newinst, flag) \
+740: \
+ oldinst ; \
+741: \
+ .skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90 ;\
+742: \
+ .pushsection .altinstructions,"a" ; \
+ altinstr_entry 740b,743f,flag,742b-740b,744f-743f ; \
+ .popsection ; \
+ .pushsection .altinstr_replacement,"ax" ; \
+743: \
+ newinst ; \
+744: \
+ .popsection ;
+
+
+.macro N_ALTERNATIVE oldinstr, newinstr, ft_flags
+ __N_ALTERNATIVE(\oldinstr, \newinstr, \ft_flags)
+.endm
+
+
#define old_len 141b-140b
#define new_len1 144f-143f
#define new_len2 145f-144f
@@ -417,7 +524,6 @@ void nop_func(void);
#define alt_max_2(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
#define alt_max_3(a, b, c) (alt_max_2(alt_max_2(a, b), c))

-
/*
* Same as ALTERNATIVE macro above but for two alternatives. If CPU
* has @feature1, it replaces @oldinstr with @newinstr1. If CPU has
@@ -445,6 +551,11 @@ void nop_func(void);
.popsection
.endm

+.macro N_ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2
+ __N_ALTERNATIVE(__N_ALTERNATIVE(\oldinstr, \newinstr1, \ft_flags1),
+ \newinstr2, \ft_flags2)
+.endm
+
.macro ALTERNATIVE_3 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2, newinstr3, ft_flags3
140:
\oldinstr
@@ -470,6 +581,11 @@ void nop_func(void);
.popsection
.endm

+.macro N_ALTERNATIVE_3 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2, newinstr3, ft_flags3
+ __N_ALTERNATIVE(N_ALTERNATIVE_2(\oldinstr, \newinstr1, \ft_flags1, \newinstr2, \ft_flags2),
+ \newinstr3, \ft_flags3)
+.endm
+
/* If @feature is set, patch in @newinstr_yes, otherwise @newinstr_no. */
#define ALTERNATIVE_TERNARY(oldinstr, ft_flags, newinstr_yes, newinstr_no) \
ALTERNATIVE_2 oldinstr, newinstr_no, X86_FEATURE_ALWAYS, \
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 89de61243272..37596a417094 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -432,6 +432,11 @@ static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a)
return 5;
}

+static inline u8 * instr_va(struct alt_instr *i)
+{
+ return (u8 *)&i->instr_offset + i->instr_offset;
+}
+
/*
* Replace instructions with better alternatives for this CPU type. This runs
* before SMP is initialized to avoid SMP problems with self modifying code.
@@ -447,7 +452,7 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
{
u8 insn_buff[MAX_PATCH_LEN];
u8 *instr, *replacement;
- struct alt_instr *a;
+ struct alt_instr *a, *b;

DPRINTK(ALT, "alt table %px, -> %px", start, end);

@@ -473,7 +478,18 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
for (a = start; a < end; a++) {
int insn_buff_sz = 0;

- instr = (u8 *)&a->instr_offset + a->instr_offset;
+ /*
+ * In case of nested ALTERNATIVE()s the outer alternative might
+ * add more padding. To ensure consistent patching find the max
+ * padding for all alt_instr entries for this site (nested
+ * alternatives result in consecutive entries).
+ */
+ for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) {
+ u8 len = max(a->instrlen, b->instrlen);
+ a->instrlen = b->instrlen = len;
+ }
+
+ instr = instr_va(a);
replacement = (u8 *)&a->repl_offset + a->repl_offset;
BUG_ON(a->instrlen > sizeof(insn_buff));
BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
index 4134d27c696b..4ea0f9815fda 100644
--- a/tools/objtool/arch/x86/special.c
+++ b/tools/objtool/arch/x86/special.c
@@ -9,6 +9,29 @@

void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
{
+ static struct special_alt *group, *prev;
+
+ /*
+ * Recompute orig_len for nested ALTERNATIVE()s.
+ */
+ if (group && group->orig_sec == alt->orig_sec &&
+ group->orig_off == alt->orig_off) {
+
+ struct special_alt *iter = group;
+ for (;;) {
+ unsigned int len = max(iter->orig_len, alt->orig_len);
+ iter->orig_len = alt->orig_len = len;
+
+ if (iter == prev)
+ break;
+
+ iter = list_next_entry(iter, list);
+ }
+
+ } else group = alt;
+
+ prev = alt;
+
switch (feature) {
case X86_FEATURE_SMAP:
/*
diff --git a/tools/objtool/special.c b/tools/objtool/special.c
index 91b1950f5bd8..097a69db82a0 100644
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -84,6 +84,14 @@ static int get_alt_entry(struct elf *elf, const struct special_entry *entry,
entry->new_len);
}

+ orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
+ if (!orig_reloc) {
+ WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
+ return -1;
+ }
+
+ reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
+
if (entry->feature) {
unsigned short feature;

@@ -94,14 +102,6 @@ static int get_alt_entry(struct elf *elf, const struct special_entry *entry,
arch_handle_alternative(feature, alt);
}

- orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
- if (!orig_reloc) {
- WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
- return -1;
- }
-
- reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
-
if (!entry->group || alt->new_len) {
new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
if (!new_reloc) {
--
2.43.0

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