[RFC][PATCH v4 26/32] objtool: mcount: Pre-allocate new ELF sections

[Matt Helsley]

Rather than allocating the ELF sections after collecting the
mcount locations and building the relocation entries, create
the empty sections beforehand. This has the benefit of
removing the memcpy() and just using the resulting libgelf
buffers directly.

Signed-off-by: Matt Helsley <mhelsley@xxxxxxxxxx>
---
 tools/objtool/recordmcount.h | 122 +++++++++++++++--------------------
 1 file changed, 52 insertions(+), 70 deletions(-)

diff --git a/tools/objtool/recordmcount.h b/tools/objtool/recordmcount.h
index 853c4006a34f..de31ff003032 100644
--- a/tools/objtool/recordmcount.h
+++ b/tools/objtool/recordmcount.h
@@ -17,7 +17,6 @@
  * This conversion to macros was done by:
  * Copyright 2010 Steven Rostedt <srostedt@xxxxxxxxxx>, Red Hat Inc.
  */
-#undef append_func
 #undef sift_rel_mcount
 #undef do_func
 #undef Elf_Shdr
@@ -31,7 +30,6 @@
 #undef _size
 
 #ifdef RECORD_MCOUNT_64
-# define append_func		append64
 # define sift_rel_mcount	sift64_rel_mcount
 # define do_func		do64
 # define Elf_Rel		Elf64_Rel
@@ -43,7 +41,6 @@
 # define _w			w8
 # define _size			8
 #else
-# define append_func		append32
 # define sift_rel_mcount	sift32_rel_mcount
 # define do_func		do32
 # define Elf_Rel		Elf32_Rel
@@ -62,60 +59,6 @@ static void fn_ELF_R_INFO(Elf_Rel *const rp, unsigned sym, unsigned type)
 }
 static void (*Elf_r_info)(Elf_Rel *const rp, unsigned sym, unsigned type) = fn_ELF_R_INFO;
 
-/* Append the new  __mcount_loc and its relocations. */
-static int append_func(uint_t const *const mloc0,
-			uint_t const *const mlocp,
-			Elf_Rel const *const mrel0,
-			Elf_Rel const *const mrelp,
-			unsigned int const loc_size,
-			unsigned int const rel_entsize,
-			unsigned int const symsec_sh_link)
-{
-	/* Begin constructing output file */
-	struct section *sec;
-	char const *mc_name = (sizeof(Elf_Rela) == rel_entsize)
-		? ".rela__mcount_loc"
-		:  ".rel__mcount_loc";
-	unsigned const old_shnum = lf->ehdr.e_shnum;
-
-	/* add section: __mcount_loc */
-	sec = elf_create_section(lf, mc_name + (sizeof(Elf_Rela) == rel_entsize) + strlen(".rel"), loc_size, mlocp - mloc0);
-	if (!sec)
-		return -1;
-
-	// created sec->sh.sh_size = (void *)mlocp - (void *)mloc0;
-	sec->sh.sh_link = 0;/* TODO objtool uses this? */
-	sec->sh.sh_info = 0;/* TODO objtool uses this? */
-	sec->sh.sh_addralign = loc_size;
-	// created sec->sh.sh_entsize = _size;
-
-	// assert sec->data->d_size == (void *)mlocp - (void *)mloc0
-	memcpy(sec->data->d_buf, mloc0, sec->data->d_size);
-	/* HACK link in Pre-assembled buffer ?
-	sec->data->d_buf = mloc0;
-	sec->data->d_size = sec->sh.sh_size;*/
-
-	/* add section .rel[a]__mcount_loc */
-	sec = elf_create_section(lf, mc_name, rel_entsize, mrelp - mrel0);
-	if (!sec)
-		return -1;
-	sec->sh.sh_type = (sizeof(Elf_Rela) == rel_entsize)
-				? SHT_RELA
-				: SHT_REL;
-	sec->sh.sh_flags = 0;
-	sec->sh.sh_link = find_section_by_name(lf, ".symtab")->idx;
-	sec->sh.sh_info = old_shnum;
-	sec->sh.sh_addralign = loc_size;
-
-	// assert sec->data->d_size == (void *)mrelp - (void *)mrel0
-	memcpy(sec->data->d_buf, mrel0, sec->data->d_size);
-	/* HACK link in Pre-assembled buffer ?
-	sec->data->d_buf = mrel0;
-	sec->data->d_size = sec->sh.sh_size;*/
-
-	return elf_write(lf);
-}
-
 /*
  * Look at the relocations in order to find the calls to mcount.
  * Accumulate the section offsets that are found, and their relocation info,
@@ -172,11 +115,13 @@ static int do_func(unsigned const reltype)
 	uint_t *      mlocp;
 
 	unsigned int rel_entsize = 0;
-	unsigned symsec_sh_link = 0;
 
-	struct section *sec;
+	struct section *sec, *mlocs, *mrels;
+	unsigned int const old_shnum = lf->ehdr.e_shnum;
 
-	int result = 0;
+	int result = -1;
+	char const *mc_name;
+	bool is_rela;
 
 	if (find_section_by_name(lf, "__mcount_loc") != NULL)
 		return 0;
@@ -184,6 +129,7 @@ static int do_func(unsigned const reltype)
 	totrelsz = tot_relsize(&rel_entsize);
 	if (totrelsz == 0)
 		return 0;
+
 	mrel0 = malloc(totrelsz);
 	mrelp = mrel0;
 	if (!mrel0)
@@ -197,6 +143,32 @@ static int do_func(unsigned const reltype)
 		return -1;
 	}
 
+	is_rela = (sizeof(Elf_Rela) == rel_entsize);
+	mc_name = is_rela
+			? ".rela__mcount_loc"
+			:  ".rel__mcount_loc";
+
+	/* add section: __mcount_loc */
+	mlocs = elf_create_section(lf, mc_name + (is_rela ? 1 : 0) + strlen(".rel"), _size, 0);
+	if (!mlocs)
+		goto out;
+
+	mlocs->sh.sh_link = 0;
+	mlocs->sh.sh_info = 0;
+	mlocs->sh.sh_addralign = _size;
+
+	/* add section .rel[a]__mcount_loc */
+	mrels = elf_create_section(lf, mc_name, rel_entsize, 0);
+	if (!mrels)
+		goto out;
+	mrels->sh.sh_type = is_rela
+				? SHT_RELA
+				: SHT_REL;
+	mrels->sh.sh_flags = 0;
+	mrels->sh.sh_link = find_section_by_name(lf, ".symtab")->idx;
+	mrels->sh.sh_info = old_shnum;
+	mrels->sh.sh_addralign = _size;
+
 	list_for_each_entry(sec, &lf->sections, list) {
 		char const *txtname;
 
@@ -205,10 +177,8 @@ static int do_func(unsigned const reltype)
 			unsigned long recval = 0;
 			unsigned int recsym;
 
-			symsec_sh_link = sec->sh.sh_link;
-			result = find_section_sym_index(sec->sh.sh_info,
-						txtname, &recval, &recsym);
-			if (result)
+			if (find_section_sym_index(sec->sh.sh_info,
+						txtname, &recval, &recsym))
 				goto out;
 
 			mlocp = sift_rel_mcount(mlocp,
@@ -219,15 +189,27 @@ static int do_func(unsigned const reltype)
 			 * This section is ignored by ftrace, but still
 			 * has mcount calls. Convert them to nops now.
 			 */
-			if (nop_mcount(sec, txtname) < 0) {
-				result = -1;
+			if (nop_mcount(sec, txtname) < 0)
 				goto out;
-			}
 		}
 	}
-	if (!result && mloc0 != mlocp)
-		result = append_func(mloc0, mlocp, mrel0, mrelp,
-				     _size, rel_entsize, symsec_sh_link);
+
+	if (mloc0 != mlocp) {
+		/* Update the section sizes */
+		mlocs->sh.sh_size = (void *)mlocp - (void *)mloc0;
+		mlocs->len = mlocs->sh.sh_size;
+		mlocs->data->d_size = mlocs->len;
+		mlocs->data->d_buf = mloc0;
+
+		mrels->sh.sh_size = (void *)mrelp - (void *)mrel0;
+		mrels->len = mrels->sh.sh_size;
+		mrels->data->d_size = mrels->len;
+		mrels->data->d_buf = mrel0;
+
+		/* overwrite the ELF file */
+		result = elf_write(lf);
+	} else
+		result = 0;
 out:
 	free(mrel0);
 	free(mloc0);
-- 
2.20.1