Re: [RFC 28/31] x86/alternative: Create symbols for special section entries

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Sep 03, 2024 at 10:29:09AM +0200, Peter Zijlstra wrote:
> On Mon, Sep 02, 2024 at 09:00:11PM -0700, Josh Poimboeuf wrote:
> > Create a symbol for each special section entry.  This helps objtool
> > extract needed entries.
> 
> A little more explanation would be nice,..

Indeed!

From: Josh Poimboeuf <jpoimboe@kernel.org>
Subject: [PATCH] x86/alternative: Create symbols for special section entries

The kernel has a myriad of special sections including __bug_table,
.altinstructions, etc.  Each has its own distinct format, though each is
more or less an array of structs or pointers.

When creating a livepatch module, objtool extracts a subset of functions
out of the original object file and into a new one.  For that to work
properly, it also needs to extract a subset of each special section's
entries.  Specifically, it should only extract those entries which
reference the extracted functions.

One way to achieve that would be to hardcode intimate knowledge about
each special section and its entry sizes.  That's less than ideal,
especially for cases like .altinstr_replacement which has variable-sized
"structs" which are described by another section.

Take a more generic approach: for the "array of structs" style sections,
annotate each struct entry with a symbol containing the entry.  This
makes it easy for tooling to parse the data and avoids the fragility of
hardcoding section details.

(For the "array of pointers" style sections, no symbol is needed, as the
format is already self-evident.)

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/include/asm/alternative.h | 50 ++++++++++++++++++++----------
 arch/x86/include/asm/asm.h         | 24 +++++++++-----
 arch/x86/include/asm/bug.h         |  2 ++
 arch/x86/include/asm/cpufeature.h  |  2 ++
 arch/x86/include/asm/jump_label.h  |  2 ++
 include/linux/objtool.h            | 31 +++++++++++++++++-
 tools/objtool/check.c              | 22 +++++++++++--
 7 files changed, 104 insertions(+), 29 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index ba99ef75f56c..2617253bcb00 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -157,7 +157,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
 #define ALT_CALL_INSTR		"call BUG_func"
 
 #define b_replacement(num)	"664"#num
-#define e_replacement(num)	"665"#num
+
+#define __e_replacement(num)	__PASTE(665, num)
+#define e_replacement(num)	__stringify(__e_replacement(num))
 
 #define alt_end_marker		"663"
 #define alt_slen		"662b-661b"
@@ -203,15 +205,21 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	alt_end_marker ":\n"
 
 #define ALTINSTR_ENTRY(ft_flags, num)					      \
+	FAKE_SYMBOL(__alt_, 681f)					      \
 	" .long 661b - .\n"				/* label           */ \
 	" .long " b_replacement(num)"f - .\n"		/* new instruction */ \
 	" .4byte " __stringify(ft_flags) "\n"		/* feature + flags */ \
 	" .byte " alt_total_slen "\n"			/* source len      */ \
-	" .byte " alt_rlen(num) "\n"			/* replacement len */
+	" .byte " alt_rlen(num) "\n"			/* replacement len */ \
+	"681:\n"
 
-#define ALTINSTR_REPLACEMENT(newinstr, num)		/* replacement */	\
+#define ALTINSTR_REPLACEMENT(newinstr, num)					\
 	"# ALT: replacement " #num "\n"						\
-	b_replacement(num)":\n\t" newinstr "\n" e_replacement(num) ":\n"
+	FAKE_SYMBOL(__alt_instr_, __PASTE(__e_replacement(num), f))		\
+	b_replacement(num) ":\n"						\
+	"\t" newinstr "\n"							\
+	e_replacement(num) ":\n"
+
 
 /* alternative assembly primitive: */
 #define ALTERNATIVE(oldinstr, newinstr, ft_flags)			\
@@ -370,12 +378,20 @@ void nop_func(void);
  * enough information for the alternatives patching code to patch an
  * instruction. See apply_alternatives().
  */
-.macro altinstr_entry orig alt ft_flags orig_len alt_len
+.macro ALTINSTR_ENTRY orig alt ft_flags orig_len alt_len
+	FAKE_SYMBOL(__alt_, 681f)
 	.long \orig - .
 	.long \alt - .
 	.4byte \ft_flags
 	.byte \orig_len
 	.byte \alt_len
+	681:
+.endm
+
+.macro ALTINSTR_REPLACEMENT newinstr
+	FAKE_SYMBOL(__alt_instr_, 681f)
+	\newinstr
+	681:
 .endm
 
 .macro ALT_CALL_INSTR
@@ -396,12 +412,12 @@ void nop_func(void);
 142:
 
 	.pushsection .altinstructions,"a"
-	altinstr_entry 140b,143f,\ft_flags,142b-140b,144f-143f
+	ALTINSTR_ENTRY 140b,143f,\ft_flags,142b-140b,144f-143f
 	.popsection
 
 	.pushsection .altinstr_replacement,"ax"
 143:
-	\newinstr
+	ALTINSTR_REPLACEMENT "\newinstr"
 144:
 	.popsection
 .endm
@@ -435,15 +451,15 @@ void nop_func(void);
 142:
 
 	.pushsection .altinstructions,"a"
-	altinstr_entry 140b,143f,\ft_flags1,142b-140b,144f-143f
-	altinstr_entry 140b,144f,\ft_flags2,142b-140b,145f-144f
+	ALTINSTR_ENTRY 140b,143f,\ft_flags1,142b-140b,144f-143f
+	ALTINSTR_ENTRY 140b,144f,\ft_flags2,142b-140b,145f-144f
 	.popsection
 
 	.pushsection .altinstr_replacement,"ax"
 143:
-	\newinstr1
+	ALTINSTR_REPLACEMENT "\newinstr1"
 144:
-	\newinstr2
+	ALTINSTR_REPLACEMENT "\newinstr2"
 145:
 	.popsection
 .endm
@@ -457,18 +473,18 @@ void nop_func(void);
 142:
 
 	.pushsection .altinstructions,"a"
-	altinstr_entry 140b,143f,\ft_flags1,142b-140b,144f-143f
-	altinstr_entry 140b,144f,\ft_flags2,142b-140b,145f-144f
-	altinstr_entry 140b,145f,\ft_flags3,142b-140b,146f-145f
+	ALTINSTR_ENTRY 140b,143f,\ft_flags1,142b-140b,144f-143f
+	ALTINSTR_ENTRY 140b,144f,\ft_flags2,142b-140b,145f-144f
+	ALTINSTR_ENTRY 140b,145f,\ft_flags3,142b-140b,146f-145f
 	.popsection
 
 	.pushsection .altinstr_replacement,"ax"
 143:
-	\newinstr1
+	ALTINSTR_REPLACEMENT "\newinstr1"
 144:
-	\newinstr2
+	ALTINSTR_REPLACEMENT "\newinstr2"
 145:
-	\newinstr3
+	ALTINSTR_REPLACEMENT "\newinstr3"
 146:
 	.popsection
 .endm
diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 2bec0c89a95c..c337240d342c 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_ASM_H
 #define _ASM_X86_ASM_H
 
+#include <linux/objtool.h>
+
 #ifdef __ASSEMBLY__
 # define __ASM_FORM(x, ...)		x,## __VA_ARGS__
 # define __ASM_FORM_RAW(x, ...)		x,## __VA_ARGS__
@@ -149,9 +151,11 @@ static __always_inline __pure void *rip_rel_ptr(void *p)
 # define _ASM_EXTABLE_TYPE(from, to, type)			\
 	.pushsection "__ex_table","a" ;				\
 	.balign 4 ;						\
+	FAKE_SYMBOL(__ex_table_, 555f) ;			\
 	.long (from) - . ;					\
 	.long (to) - . ;					\
 	.long type ;						\
+	555: ;							\
 	.popsection
 
 # ifdef CONFIG_KPROBES
@@ -196,19 +200,23 @@ static __always_inline __pure void *rip_rel_ptr(void *p)
 # define _ASM_EXTABLE_TYPE(from, to, type)			\
 	" .pushsection \"__ex_table\",\"a\"\n"			\
 	" .balign 4\n"						\
+	FAKE_SYMBOL(__ex_table_, 555f)			\
 	" .long (" #from ") - .\n"				\
 	" .long (" #to ") - .\n"				\
 	" .long " __stringify(type) " \n"			\
+	" 555:\n"						\
 	" .popsection\n"
 
-# define _ASM_EXTABLE_TYPE_REG(from, to, type, reg)				\
-	" .pushsection \"__ex_table\",\"a\"\n"					\
-	" .balign 4\n"								\
-	" .long (" #from ") - .\n"						\
-	" .long (" #to ") - .\n"						\
-	DEFINE_EXTABLE_TYPE_REG							\
-	"extable_type_reg reg=" __stringify(reg) ", type=" __stringify(type) " \n"\
-	UNDEFINE_EXTABLE_TYPE_REG						\
+# define _ASM_EXTABLE_TYPE_REG(from, to, type, reg)					\
+	" .pushsection \"__ex_table\",\"a\"\n"						\
+	" .balign 4\n"									\
+	FAKE_SYMBOL(__ex_table_, 555f)							\
+	" .long (" #from ") - .\n"							\
+	" .long (" #to ") - .\n"							\
+	DEFINE_EXTABLE_TYPE_REG								\
+	"extable_type_reg reg=" __stringify(reg) ", type=" __stringify(type) " \n"	\
+	UNDEFINE_EXTABLE_TYPE_REG							\
+	" 555:\n"									\
 	" .popsection\n"
 
 /* For C file, we already have NOKPROBE_SYMBOL macro */
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index a3ec87d198ac..86304d7d68f5 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -27,6 +27,7 @@
 do {									\
 	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
+		     FAKE_SYMBOL(__bug_table_, . + %c3)			\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
 		     "\t.word %c1"        "\t# bug_entry::line\n"	\
@@ -45,6 +46,7 @@ do {									\
 do {									\
 	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
+		     FAKE_SYMBOL(__bug_table_, . + %c1)			\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
 		     "\t.org 2b+%c1\n"					\
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 0b9611da6c53..9decf644d55a 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -178,9 +178,11 @@ static __always_inline bool _static_cpu_has(u16 bit)
 	asm goto(ALTERNATIVE_TERNARY("jmp 6f", %c[feature], "", "jmp %l[t_no]")
 		".pushsection .altinstr_aux,\"ax\"\n"
 		"6:\n"
+		FAKE_SYMBOL(__alt_aux_, 681f)
 		" testb %[bitnum], %a[cap_byte]\n"
 		" jnz %l[t_yes]\n"
 		" jmp %l[t_no]\n"
+		"681:\n"
 		".popsection\n"
 		 : : [feature]  "i" (bit),
 		     [bitnum]   "i" (1 << (bit & 7)),
diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index cbbef32517f0..731d7e69d244 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -15,9 +15,11 @@
 #define JUMP_TABLE_ENTRY				\
 	".pushsection __jump_table,  \"aw\" \n\t"	\
 	_ASM_ALIGN "\n\t"				\
+	FAKE_SYMBOL(__jump_table_, 2f)			\
 	".long 1b - . \n\t"				\
 	".long %l[l_yes] - . \n\t"			\
 	_ASM_PTR "%c0 + %c1 - .\n\t"			\
+	"2:\n\t"					\
 	".popsection \n\t"
 
 #ifdef CONFIG_HAVE_JUMP_LABEL_HACK
diff --git a/include/linux/objtool.h b/include/linux/objtool.h
index 5e66b6d26df5..ae5030cac10d 100644
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h
@@ -10,9 +10,27 @@
 
 #ifndef __ASSEMBLY__
 
-#define UNWIND_HINT(type, sp_reg, sp_offset, signal)	\
+#define DEFINE_FAKE_SYMBOL						\
+	".macro fake_symbol name, end\n\t"				\
+	"#ifdef __x86_64__\n"						\
+	".type \\name\\@, @object\n"					\
+	".size \\name\\@, \\end - .\n"					\
+	"\\name\\@:\n"							\
+	"#endif\n"							\
+	".endm\n\t"
+
+#define UNDEFINE_FAKE_SYMBOL						\
+	".purgem fake_symbol\n\t"
+
+#define FAKE_SYMBOL(name, end)							\
+	DEFINE_FAKE_SYMBOL							\
+	"fake_symbol " __stringify(name) ", \"" __stringify(end) "\"\n\t"	\
+	UNDEFINE_FAKE_SYMBOL
+
+#define UNWIND_HINT(type, sp_reg, sp_offset, signal)		\
 	"987: \n\t"						\
 	".pushsection .discard.unwind_hints\n\t"		\
+	FAKE_SYMBOL(__unwind_hint_, 988f)			\
 	/* struct unwind_hint */				\
 	".long 987b - .\n\t"					\
 	".short " __stringify(sp_offset) "\n\t"			\
@@ -20,6 +38,7 @@
 	".byte " __stringify(type) "\n\t"			\
 	".byte " __stringify(signal) "\n\t"			\
 	".balign 4 \n\t"					\
+	"988:\n\t"						\
 	".popsection\n\t"
 
 /*
@@ -60,6 +79,14 @@
 
 #else /* __ASSEMBLY__ */
 
+.macro fake_symbol name, end
+	.type \name\@, @object
+	.size \name\@, \end - .
+	\name\@:
+.endm
+
+#define FAKE_SYMBOL(name, end) fake_symbol name, __stringify(end)
+
 /*
  * This macro indicates that the following intra-function call is valid.
  * Any non-annotated intra-function call will cause objtool to issue a warning.
@@ -94,6 +121,7 @@
 .macro UNWIND_HINT type:req sp_reg=0 sp_offset=0 signal=0
 .Lhere_\@:
 	.pushsection .discard.unwind_hints
+		FAKE_SYMBOL(__unwind_hint_, .Lend_\@)
 		/* struct unwind_hint */
 		.long .Lhere_\@ - .
 		.short \sp_offset
@@ -101,6 +129,7 @@
 		.byte \type
 		.byte \signal
 		.balign 4
+		.Lend_\@:
 	.popsection
 .endm
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 3c8d0903dfa7..5dd78a7f75c3 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -633,6 +633,18 @@ static void add_dead_ends(struct objtool_file *file)
 	}
 }
 
+static void create_fake_symbol(struct objtool_file *file, const char *name_pfx,
+			       struct section *sec, unsigned long offset,
+			       size_t size)
+{
+	char name[256];
+	static int ctr;
+
+	snprintf(name, 256, "%s_%d", name_pfx, ctr++);
+
+	elf_create_symbol(file->elf, name, sec, STB_LOCAL, STT_OBJECT, offset, size);
+}
+
 static void create_static_call_sections(struct objtool_file *file)
 {
 	struct static_call_site *site;
@@ -664,10 +676,11 @@ static void create_static_call_sections(struct objtool_file *file)
 
 	idx = 0;
 	list_for_each_entry(insn, &file->static_call_list, call_node) {
+		unsigned long offset = idx * sizeof(*site);
 
 		/* populate reloc for 'addr' */
-		elf_init_reloc_text_sym(file->elf, sec, idx * sizeof(*site),
-					idx * 2, insn->sec, insn->offset);
+		elf_init_reloc_text_sym(file->elf, sec, offset, idx * 2,
+					insn->sec, insn->offset);
 
 		/* find key symbol */
 		key_name = strdup(insn_call_dest(insn)->name);
@@ -698,10 +711,13 @@ static void create_static_call_sections(struct objtool_file *file)
 		free(key_name);
 
 		/* populate reloc for 'key' */
-		elf_init_reloc_data_sym(file->elf, sec, idx * sizeof(*site) + 4,
+		elf_init_reloc_data_sym(file->elf, sec, offset + 4,
 					(idx * 2) + 1, key_sym,
 					is_sibling_call(insn) * STATIC_CALL_SITE_TAIL);
 
+		create_fake_symbol(file, "__static_call_site_", sec,
+				   offset, sizeof(*site));
+
 		idx++;
 	}
 }
-- 
2.46.0