Date: Fri, 31 May 2024 16:30:31 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra
 <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros


> From: Peter Zijlstra <peterz@...radead.org>
> 
> 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. ]
> 
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Co-developed-by: Borislav Petkov (AMD) <bp@...en8.de>
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> Link: https://lkml.kernel.org/r/20230628104952.GA2439977@hirez.programming.kicks-ass.net
> ---
>   arch/x86/include/asm/alternative.h | 110 ++++++++++++++++++++++++++++-
>   arch/x86/kernel/alternative.c      |  20 +++++-
>   tools/objtool/arch/x86/special.c   |  23 ++++++
>   tools/objtool/special.c            |  16 ++---
>   4 files changed, 158 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index 6db78909180a..8554c2c339ff 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 +344,24 @@ static inline int alternatives_text_reserved(void *start, void *end)
>   	asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, ft_flags)	\
>   		: output : "i" (0), ## input)
>   
> +#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. */
>   #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 %P[old]", "call %P[new]", ft_flags) \
> +		: output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)

Please don't resurrect the %P modifier, use %c instead.

> +
>   /*
>    * Like alternative_call, but there are two features and respective functions.
>    * If CPU has feature2, function2 is used.
> @@ -307,6 +376,15 @@ 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 %P[old]", "call %P[new1]", ft_flags1,\
> +		"call %P[new2]", ft_flags2)				      \
> +		: output, ASM_CALL_CONSTRAINT				      \
> +		: [old] "i" (oldfunc), [new1] "i" (newfunc1),		      \
> +		  [new2] "i" (newfunc2), ## input)

Here too, %P should be avoided in favor of %c.

Uros.

> +
>   /*
>    * use this macro(s) if you need more than one output parameter
>    * in alternative_io
> @@ -403,6 +481,27 @@ 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 +516,6 @@ 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 +543,11 @@ 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 +573,11 @@ 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) {

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