[PATCH v2] objtool: Support multiple rodata sections.

[PATCH v2] objtool: Support multiple rodata sections.

[Allan Xavier]

This commit adds support for processing switch jump tables in objects
with multiple .rodata sections, such as those created when using
-ffunction-sections and -fdata-sections.  Currently, objtool always
looks in .rodata for jump table information which results in many
"sibling call from callable instruction with modified stack frame"
warnings with objects compiled using those flags.

The fix is comprised of three parts:

1. Flagging all .rodata sections when importing elf information for
easier checking later.

2. Keeping a reference to the section each relocation is from in order
to get the list_head for the other relocations in that section.

3. Finding jump tables by following relocations to .rodata sections,
rather than always referencing a single global .rodata section.

The patch has been tested without data sections enabled and no
differences in the resulting orc unwind information were seen.

Note that as objtool adds terminators to end of each .text section the
unwind information generated between a function+data sections build and
a normal build aren't directly comparable. Manual inspection suggests
that objtool is now generating the correct information, or at least
making more of an effort to do so than it did previously.

Signed-off-by: Allan Xavier <allan.x.xavier@oracle.com>
---
Changes since v1:
 - Cleaned up section symbol check.
 - Fixed brackets.
 - Moved mark_rodata to decode_sections().
 - Excluded *.str1.[18] sections from rodata sections.

 tools/objtool/check.c | 39 +++++++++++++++++++++++++++++++++------
 tools/objtool/check.h |  4 ++--
 tools/objtool/elf.c   |  1 +
 tools/objtool/elf.h   |  3 ++-
 4 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index f4a25bd1871fb..e3a5d53c4b83b 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -836,7 +836,7 @@ static int add_switch_table(struct objtool_file *file, struct instruction *insn,
 	struct symbol *pfunc = insn->func->pfunc;
 	unsigned int prev_offset = 0;
 
-	list_for_each_entry_from(rela, &file->rodata->rela->rela_list, list) {
+	list_for_each_entry_from(rela, &table->rela_sec->rela_list, list) {
 		if (rela == next_table)
 			break;
 
@@ -926,6 +926,7 @@ static struct rela *find_switch_table(struct objtool_file *file,
 {
 	struct rela *text_rela, *rodata_rela;
 	struct instruction *orig_insn = insn;
+	struct section *rodata_sec;
 	unsigned long table_offset;
 
 	/*
@@ -953,10 +954,13 @@ static struct rela *find_switch_table(struct objtool_file *file,
 		/* look for a relocation which references .rodata */
 		text_rela = find_rela_by_dest_range(insn->sec, insn->offset,
 						    insn->len);
-		if (!text_rela || text_rela->sym != file->rodata->sym)
+		if (!text_rela || text_rela->sym->type != STT_SECTION ||
+		    !text_rela->sym->sec->rodata)
 			continue;
 
 		table_offset = text_rela->addend;
+		rodata_sec = text_rela->sym->sec;
+
 		if (text_rela->type == R_X86_64_PC32)
 			table_offset += 4;
 
@@ -964,10 +968,10 @@ static struct rela *find_switch_table(struct objtool_file *file,
 		 * Make sure the .rodata address isn't associated with a
 		 * symbol.  gcc jump tables are anonymous data.
 		 */
-		if (find_symbol_containing(file->rodata, table_offset))
+		if (find_symbol_containing(rodata_sec, table_offset))
 			continue;
 
-		rodata_rela = find_rela_by_dest(file->rodata, table_offset);
+		rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
 		if (rodata_rela) {
 			/*
 			 * Use of RIP-relative switch jumps is quite rare, and
@@ -1052,7 +1056,7 @@ static int add_switch_table_alts(struct objtool_file *file)
 	struct symbol *func;
 	int ret;
 
-	if (!file->rodata || !file->rodata->rela)
+	if (!file->rodata)
 		return 0;
 
 	for_each_sec(file, sec) {
@@ -1197,10 +1201,34 @@ static int read_retpoline_hints(struct objtool_file *file)
 	return 0;
 }
 
+static void mark_rodata(struct objtool_file *file)
+{
+	struct section *sec;
+	bool found = false;
+	static const char *str1 = ".str1.";
+	const int str1len = strlen(str1) + 1;
+
+	for_each_sec(file, sec) {
+		if (strstr(sec->name, ".rodata") == sec->name) {
+			char *str1pos = sec->name + strlen(sec->name) - str1len;
+
+			/* Skips over .rodata.str1.1 and .rodata.str.1.8 */
+			if (strstr(sec->name, str1) != str1pos) {
+				sec->rodata = true;
+				found = true;
+			}
+		}
+	}
+
+	file->rodata = found;
+}
+
 static int decode_sections(struct objtool_file *file)
 {
 	int ret;
 
+	mark_rodata(file);
+
 	ret = decode_instructions(file);
 	if (ret)
 		return ret;
@@ -2170,7 +2198,6 @@ int check(const char *_objname, bool orc)
 	INIT_LIST_HEAD(&file.insn_list);
 	hash_init(file.insn_hash);
 	file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
-	file.rodata = find_section_by_name(file.elf, ".rodata");
 	file.c_file = find_section_by_name(file.elf, ".comment");
 	file.ignore_unreachables = no_unreachable;
 	file.hints = false;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index c6b68fcb926ff..a039521b67530 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -60,8 +60,8 @@ struct objtool_file {
 	struct elf *elf;
 	struct list_head insn_list;
 	DECLARE_HASHTABLE(insn_hash, 16);
-	struct section *rodata, *whitelist;
-	bool ignore_unreachables, c_file, hints;
+	struct section *whitelist;
+	bool ignore_unreachables, c_file, hints, rodata;
 };
 
 int check(const char *objname, bool orc);
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 7ec85d567598c..f7082de1ee829 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -379,6 +379,7 @@ static int read_relas(struct elf *elf)
 			rela->offset = rela->rela.r_offset;
 			symndx = GELF_R_SYM(rela->rela.r_info);
 			rela->sym = find_symbol_by_index(elf, symndx);
+			rela->rela_sec = sec;
 			if (!rela->sym) {
 				WARN("can't find rela entry symbol %d for %s",
 				     symndx, sec->name);
diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
index de5cd2ddded98..bc97ed86b9cd8 100644
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -48,7 +48,7 @@ struct section {
 	char *name;
 	int idx;
 	unsigned int len;
-	bool changed, text;
+	bool changed, text, rodata;
 };
 
 struct symbol {
@@ -68,6 +68,7 @@ struct rela {
 	struct list_head list;
 	struct hlist_node hash;
 	GElf_Rela rela;
+	struct section *rela_sec;
 	struct symbol *sym;
 	unsigned int type;
 	unsigned long offset;
-- 
2.18.0

Re: [PATCH v2] objtool: Support multiple rodata sections.

[Allan Xavier]

Ping... are there any comments on this?

On 03/08/18 19:40, Allan Xavier wrote:
> This commit adds support for processing switch jump tables in objects
> with multiple .rodata sections, such as those created when using
> -ffunction-sections and -fdata-sections.  Currently, objtool always
> looks in .rodata for jump table information which results in many
> "sibling call from callable instruction with modified stack frame"
> warnings with objects compiled using those flags.
> 
> The fix is comprised of three parts:
> 
> 1. Flagging all .rodata sections when importing elf information for
> easier checking later.
> 
> 2. Keeping a reference to the section each relocation is from in order
> to get the list_head for the other relocations in that section.
> 
> 3. Finding jump tables by following relocations to .rodata sections,
> rather than always referencing a single global .rodata section.
> 
> The patch has been tested without data sections enabled and no
> differences in the resulting orc unwind information were seen.
> 
> Note that as objtool adds terminators to end of each .text section the
> unwind information generated between a function+data sections build and
> a normal build aren't directly comparable. Manual inspection suggests
> that objtool is now generating the correct information, or at least
> making more of an effort to do so than it did previously.
> 
> Signed-off-by: Allan Xavier <allan.x.xavier@oracle.com>
> ---
> Changes since v1:
>  - Cleaned up section symbol check.
>  - Fixed brackets.
>  - Moved mark_rodata to decode_sections().
>  - Excluded *.str1.[18] sections from rodata sections.
> 
>  tools/objtool/check.c | 39 +++++++++++++++++++++++++++++++++------
>  tools/objtool/check.h |  4 ++--
>  tools/objtool/elf.c   |  1 +
>  tools/objtool/elf.h   |  3 ++-
>  4 files changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index f4a25bd1871fb..e3a5d53c4b83b 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -836,7 +836,7 @@ static int add_switch_table(struct objtool_file *file, struct instruction *insn,
>  	struct symbol *pfunc = insn->func->pfunc;
>  	unsigned int prev_offset = 0;
>  
> -	list_for_each_entry_from(rela, &file->rodata->rela->rela_list, list) {
> +	list_for_each_entry_from(rela, &table->rela_sec->rela_list, list) {
>  		if (rela == next_table)
>  			break;
>  
> @@ -926,6 +926,7 @@ static struct rela *find_switch_table(struct objtool_file *file,
>  {
>  	struct rela *text_rela, *rodata_rela;
>  	struct instruction *orig_insn = insn;
> +	struct section *rodata_sec;
>  	unsigned long table_offset;
>  
>  	/*
> @@ -953,10 +954,13 @@ static struct rela *find_switch_table(struct objtool_file *file,
>  		/* look for a relocation which references .rodata */
>  		text_rela = find_rela_by_dest_range(insn->sec, insn->offset,
>  						    insn->len);
> -		if (!text_rela || text_rela->sym != file->rodata->sym)
> +		if (!text_rela || text_rela->sym->type != STT_SECTION ||
> +		    !text_rela->sym->sec->rodata)
>  			continue;
>  
>  		table_offset = text_rela->addend;
> +		rodata_sec = text_rela->sym->sec;
> +
>  		if (text_rela->type == R_X86_64_PC32)
>  			table_offset += 4;
>  
> @@ -964,10 +968,10 @@ static struct rela *find_switch_table(struct objtool_file *file,
>  		 * Make sure the .rodata address isn't associated with a
>  		 * symbol.  gcc jump tables are anonymous data.
>  		 */
> -		if (find_symbol_containing(file->rodata, table_offset))
> +		if (find_symbol_containing(rodata_sec, table_offset))
>  			continue;
>  
> -		rodata_rela = find_rela_by_dest(file->rodata, table_offset);
> +		rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
>  		if (rodata_rela) {
>  			/*
>  			 * Use of RIP-relative switch jumps is quite rare, and
> @@ -1052,7 +1056,7 @@ static int add_switch_table_alts(struct objtool_file *file)
>  	struct symbol *func;
>  	int ret;
>  
> -	if (!file->rodata || !file->rodata->rela)
> +	if (!file->rodata)
>  		return 0;
>  
>  	for_each_sec(file, sec) {
> @@ -1197,10 +1201,34 @@ static int read_retpoline_hints(struct objtool_file *file)
>  	return 0;
>  }
>  
> +static void mark_rodata(struct objtool_file *file)
> +{
> +	struct section *sec;
> +	bool found = false;
> +	static const char *str1 = ".str1.";
> +	const int str1len = strlen(str1) + 1;
> +
> +	for_each_sec(file, sec) {
> +		if (strstr(sec->name, ".rodata") == sec->name) {
> +			char *str1pos = sec->name + strlen(sec->name) - str1len;
> +
> +			/* Skips over .rodata.str1.1 and .rodata.str.1.8 */
> +			if (strstr(sec->name, str1) != str1pos) {
> +				sec->rodata = true;
> +				found = true;
> +			}
> +		}
> +	}
> +
> +	file->rodata = found;
> +}
> +
>  static int decode_sections(struct objtool_file *file)
>  {
>  	int ret;
>  
> +	mark_rodata(file);
> +
>  	ret = decode_instructions(file);
>  	if (ret)
>  		return ret;
> @@ -2170,7 +2198,6 @@ int check(const char *_objname, bool orc)
>  	INIT_LIST_HEAD(&file.insn_list);
>  	hash_init(file.insn_hash);
>  	file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
> -	file.rodata = find_section_by_name(file.elf, ".rodata");
>  	file.c_file = find_section_by_name(file.elf, ".comment");
>  	file.ignore_unreachables = no_unreachable;
>  	file.hints = false;
> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> index c6b68fcb926ff..a039521b67530 100644
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -60,8 +60,8 @@ struct objtool_file {
>  	struct elf *elf;
>  	struct list_head insn_list;
>  	DECLARE_HASHTABLE(insn_hash, 16);
> -	struct section *rodata, *whitelist;
> -	bool ignore_unreachables, c_file, hints;
> +	struct section *whitelist;
> +	bool ignore_unreachables, c_file, hints, rodata;
>  };
>  
>  int check(const char *objname, bool orc);
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 7ec85d567598c..f7082de1ee829 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -379,6 +379,7 @@ static int read_relas(struct elf *elf)
>  			rela->offset = rela->rela.r_offset;
>  			symndx = GELF_R_SYM(rela->rela.r_info);
>  			rela->sym = find_symbol_by_index(elf, symndx);
> +			rela->rela_sec = sec;
>  			if (!rela->sym) {
>  				WARN("can't find rela entry symbol %d for %s",
>  				     symndx, sec->name);
> diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
> index de5cd2ddded98..bc97ed86b9cd8 100644
> --- a/tools/objtool/elf.h
> +++ b/tools/objtool/elf.h
> @@ -48,7 +48,7 @@ struct section {
>  	char *name;
>  	int idx;
>  	unsigned int len;
> -	bool changed, text;
> +	bool changed, text, rodata;
>  };
>  
>  struct symbol {
> @@ -68,6 +68,7 @@ struct rela {
>  	struct list_head list;
>  	struct hlist_node hash;
>  	GElf_Rela rela;
> +	struct section *rela_sec;
>  	struct symbol *sym;
>  	unsigned int type;
>  	unsigned long offset;
> 

Re: [PATCH v2] objtool: Support multiple rodata sections.

[Josh Poimboeuf]

On Fri, Aug 03, 2018 at 07:40:40PM +0100, Allan Xavier wrote:
> +static void mark_rodata(struct objtool_file *file)
> +{
> +	struct section *sec;
> +	bool found = false;
> +	static const char *str1 = ".str1.";
> +	const int str1len = strlen(str1) + 1;
> +

A comment here would help, explaining that this looks for both .rodata
and .rodata.func_name sections (when using -fdata-sections).

> +	for_each_sec(file, sec) {
> +		if (strstr(sec->name, ".rodata") == sec->name) {
> +			char *str1pos = sec->name + strlen(sec->name) - str1len;
> +
> +			/* Skips over .rodata.str1.1 and .rodata.str.1.8 */
> +			if (strstr(sec->name, str1) != str1pos) {
> +				sec->rodata = true;
> +				found = true;
> +			}
> +		}
> +	}


This could be simplified and made more readable with something like:

	for_each_sec(file, sec) {
		if (!strncmp(sec->name, ".rodata", 7) &&
		    !strstr(sec->name, ".str1.") {
		    	sec->rodata = true;
			found = true;
		}
	}

-- 
Josh