Re: [PATCH 0/5] x86/kallsyms: Fix the call-thunk kallsyms fail

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 28, 2022 at 09:40:22PM +0200, Peter Zijlstra wrote:
> Hi all,
> 
> As reported here:
> 
>   https://lkml.kernel.org/r/202210241614.2ae4c1f5-yujie.liu@intel.com
> 
> The kallsyms change for call-thunks doesn't really work out as expected. These
> patches revert that change and propose an alternative.

Robot found it needed the below; have folded back and pushed out again.

---
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index cf743520a786..55066c493570 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3479,7 +3479,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
 			/* Ignore KCFI type preambles, which always fall through */
-			if (!strncmp(func->name, "__cfi_", 6))
+			if (!strncmp(func->name, "__cfi_", 6) ||
+			    !strncmp(func->name, "__pfx_", 6))
 				return 0;
 
 			WARN("%s() falls through to next function %s()",
@@ -4042,6 +4043,7 @@ static int add_prefix_symbol(struct objtool_file *file, struct symbol *func,
 
 	for (;;) {
 		struct instruction *prev = list_prev_entry(insn, list);
+		u64 offset;
 
 		if (&prev->list == &file->insn_list)
 			break;
@@ -4049,11 +4051,13 @@ static int add_prefix_symbol(struct objtool_file *file, struct symbol *func,
 		if (prev->type != INSN_NOP)
 			break;
 
-		insn = prev;
-		if (func->offset - prev->offset == opts.prefix) {
-			elf_create_prefix_symbol(file->elf, func, -opts.prefix);
+		offset = func->offset - prev->offset;
+		if (offset >= opts.prefix) {
+			if (offset == opts.prefix)
+				elf_create_prefix_symbol(file->elf, func, opts.prefix);
 			break;
 		}
+		insn = prev;
 	}
 
 	return 0;
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 9fbdad7a565d..3d636d12d679 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -822,7 +822,7 @@ elf_create_section_symbol(struct elf *elf, struct section *sec)
 static int elf_add_string(struct elf *elf, struct section *strtab, char *str);
 
 struct symbol *
-elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long addend)
+elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size)
 {
 	struct symbol *sym = calloc(1, sizeof(*sym));
 	size_t namelen = strlen(orig->name) + sizeof("__pfx_");
@@ -840,7 +840,8 @@ elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long addend)
 
 	sym->sym.st_name = elf_add_string(elf, NULL, name);
 	sym->sym.st_info = orig->sym.st_info;
-	sym->sym.st_value = orig->sym.st_value + addend;
+	sym->sym.st_value = orig->sym.st_value - size;
+	sym->sym.st_size = size;
 
 	sym = __elf_create_symbol(elf, sym);
 	if (sym)
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 9e3bd4717a11..b6974e3173aa 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -146,7 +146,7 @@ static inline bool has_multiple_files(struct elf *elf)
 struct elf *elf_open_read(const char *name, int flags);
 struct section *elf_create_section(struct elf *elf, const char *name, unsigned int sh_flags, size_t entsize, int nr);
 
-struct symbol *elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long addend);
+struct symbol *elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, long size);
 
 int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
 		  unsigned int type, struct symbol *sym, s64 addend);