[RFC][PATCH v4 09/32] objtool: mcount: Use symbol structs to find mcount relocations

From: Matt Helsley <mhelsley@vmware.com>
To: <linux-kernel@vger.kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Julien Thierry <jthierry@redhat.com>,
	Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
	Matt Helsley <mhelsley@vmware.com>
Subject: [RFC][PATCH v4 09/32] objtool: mcount: Use symbol structs to find mcount relocations
Date: Tue, 2 Jun 2020 12:50:02 -0700	[thread overview]
Message-ID: <d8771d3c340dae48b78b301b1c05c2aafc7f0897.1591125127.git.mhelsley@vmware.com> (raw)
In-Reply-To: <cover.1591125127.git.mhelsley@vmware.com>

Rather than open coding symbol name lookups in get_sym_and_*()
we rename the structure and use objtool's lookup function to
get the symbol name for a relocation. We also change the name
of the function to better reflect its purpose.

Signed-off-by: Matt Helsley <mhelsley@vmware.com>
---
 tools/objtool/elf.c          |  2 +-
 tools/objtool/elf.h          |  1 +
 tools/objtool/recordmcount.h | 54 ++++++++++--------------------------
 3 files changed, 17 insertions(+), 40 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 2e8f5f90e264..107f89660e45 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -149,7 +149,7 @@ struct section *find_section_by_index(const struct elf *elf, unsigned int idx)
 	return NULL;
 }
 
-static struct symbol *find_symbol_by_index(struct elf *elf, unsigned int idx)
+struct symbol *find_symbol_by_index(const struct elf *elf, unsigned int idx)
 {
 	struct symbol *sym;
 
diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
index ae82479a8259..235719730392 100644
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -129,6 +129,7 @@ struct section *find_section_by_index(const struct elf *elf, unsigned int idx);
 struct symbol *find_func_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_name(const struct elf *elf, const char *name);
+struct symbol *find_symbol_by_index(const struct elf *elf, unsigned int idx);
 struct symbol *find_symbol_containing(const struct section *sec, unsigned long offset);
 struct reloc *find_reloc_by_dest(const struct elf *elf, struct section *sec, unsigned long offset);
 struct reloc *find_reloc_by_dest_range(const struct elf *elf, struct section *sec,
diff --git a/tools/objtool/recordmcount.h b/tools/objtool/recordmcount.h
index dba1c385106f..d49da1e32315 100644
--- a/tools/objtool/recordmcount.h
+++ b/tools/objtool/recordmcount.h
@@ -28,7 +28,7 @@
 #undef has_rel_mcount
 #undef tot_relsize
 #undef get_mcountsym
-#undef get_sym_str_and_relp
+#undef get_relp
 #undef do_func
 #undef Elf_Addr
 #undef Elf_Ehdr
@@ -56,7 +56,7 @@
 # define find_section_sym_index	find64_section_sym_index
 # define has_rel_mcount		has64_rel_mcount
 # define tot_relsize		tot64_relsize
-# define get_sym_str_and_relp	get_sym_str_and_relp_64
+# define get_relp		get_relp_64
 # define do_func		do64
 # define get_mcountsym		get_mcountsym_64
 # define is_fake_mcount		is_fake_mcount64
@@ -88,7 +88,7 @@
 # define find_section_sym_index	find32_section_sym_index
 # define has_rel_mcount		has32_rel_mcount
 # define tot_relsize		tot32_relsize
-# define get_sym_str_and_relp	get_sym_str_and_relp_32
+# define get_relp		get_relp_32
 # define do_func		do32
 # define get_mcountsym		get_mcountsym_32
 # define is_fake_mcount		is_fake_mcount32
@@ -261,15 +261,10 @@ static int append_func(Elf_Ehdr *const ehdr,
 	return elf_write(lf);
 }
 
-static unsigned get_mcountsym(Elf_Sym const *const sym0,
-			      Elf_Rel const *relp,
-			      char const *const str0)
+static unsigned get_mcountsym(Elf_Rel const *relp)
 {
-	unsigned mcountsym = 0;
-
-	Elf_Sym const *const symp =
-		&sym0[Elf_r_sym(relp)];
-	char const *symname = &str0[w(symp->st_name)];
+	struct symbol *sym = find_symbol_by_index(lf, Elf_r_sym(relp));
+	char const *symname = sym->name;
 	char const *mcount = gpfx == '_' ? "_mcount" : "mcount";
 	char const *fentry = "__fentry__";
 
@@ -278,31 +273,16 @@ static unsigned get_mcountsym(Elf_Sym const *const sym0,
 	if (strcmp(mcount, symname) == 0 ||
 	    (altmcount && strcmp(altmcount, symname) == 0) ||
 	    (strcmp(fentry, symname) == 0))
-		mcountsym = Elf_r_sym(relp);
-
-	return mcountsym;
+		return Elf_r_sym(relp);
+	return 0;
 }
 
-static void get_sym_str_and_relp(const struct section * const rels,
-				 Elf_Ehdr const *const ehdr,
-				 Elf_Sym const **sym0,
-				 char const **str0,
-				 Elf_Rel const **relp)
+static void get_relp(const struct section * const rels,
+			Elf_Ehdr const *const ehdr,
+			Elf_Rel const **relp)
 {
-	Elf_Shdr *const shdr0 = (Elf_Shdr *)(_w(ehdr->e_shoff)
-		+ (void *)ehdr);
-	unsigned const symsec_sh_link = rels->sh.sh_link;
-	Elf_Shdr const *const symsec = &shdr0[symsec_sh_link];
-	Elf_Shdr const *const strsec = &shdr0[w(symsec->sh_link)];
 	Elf_Rel const *const rel0 = (Elf_Rel const *)(rels->sh.sh_offset
 		+ (void *)ehdr);
-
-	*sym0 = (Elf_Sym const *)(_w(symsec->sh_offset)
-				  + (void *)ehdr);
-
-	*str0 = (char const *)(_w(strsec->sh_offset)
-			       + (void *)ehdr);
-
 	*relp = rel0;
 }
 
@@ -322,19 +302,17 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
 {
 	uint_t *const mloc0 = mlocp;
 	Elf_Rel *mrelp = *mrelpp;
-	Elf_Sym const *sym0;
-	char const *str0;
 	Elf_Rel const *relp;
 	unsigned int rel_entsize = rels->sh.sh_entsize;
 	unsigned const nrel = rels->sh.sh_size / rel_entsize;
 	unsigned mcountsym = 0;
 	unsigned t;
 
-	get_sym_str_and_relp(rels, ehdr, &sym0, &str0, &relp);
+	get_relp(rels, ehdr, &relp);
 
 	for (t = nrel; t; --t) {
 		if (!mcountsym)
-			mcountsym = get_mcountsym(sym0, relp, str0);
+			mcountsym = get_mcountsym(relp);
 
 		if (mcountsym && mcountsym == Elf_r_sym(relp) &&
 				!is_fake_mcount(relp)) {
@@ -368,8 +346,6 @@ static int nop_mcount(const struct section * const rels,
 {
 	Elf_Shdr *const shdr0 = (Elf_Shdr *)(_w(ehdr->e_shoff)
 		+ (void *)ehdr);
-	Elf_Sym const *sym0;
-	char const *str0;
 	Elf_Rel const *relp;
 	Elf_Shdr const *const shdr = &shdr0[rels->sh.sh_info];
 	unsigned rel_entsize = rels->sh.sh_entsize;
@@ -378,13 +354,13 @@ static int nop_mcount(const struct section * const rels,
 	unsigned t;
 	int once = 0;
 
-	get_sym_str_and_relp(rels, ehdr, &sym0, &str0, &relp);
+	get_relp(rels, ehdr, &relp);
 
 	for (t = nrel; t; --t) {
 		int ret = -1;
 
 		if (!mcountsym)
-			mcountsym = get_mcountsym(sym0, relp, str0);
+			mcountsym = get_mcountsym(relp);
 
 		if (mcountsym == Elf_r_sym(relp) && !is_fake_mcount(relp)) {
 			if (make_nop)
-- 
2.20.1

