linux-kernel - [PATCH v5 09/15] objtool: Find end of switch table directly

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Message-ID: <3e31a9910c471ad9c17b76a96cc910e7a8aa2a10.1736955567.git.christophe.leroy@csgroup.eu>
Date: Wed, 15 Jan 2025 23:42:49 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Julien Thierry <jthierry@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Raphael Gault <raphael.gault@....com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	llvm@...ts.linux.dev
Subject: [PATCH v5 09/15] objtool: Find end of switch table directly

At the time being, the end of a switch table can only be known
once the start of the following switch table has ben located.

This is a problem when switch tables are nested because until the first
switch table is properly added, the second one cannot be located as a
the backward walk will abut on the dynamic switch of the previous one.

So perform a first forward walk in the code in order to locate all
possible relocations to switch tables and build a local table with
those relocations. Later on once one switch table is found, go through
this local table to know where next switch table starts.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 tools/objtool/check.c | 63 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 72b977f81dd6..0ad2bdd92232 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2058,14 +2058,30 @@ static void find_jump_table(struct objtool_file *file, struct symbol *func,
 	}
 }
 
+static struct reloc *find_next_table(struct instruction *insn,
+				     struct reloc **table, unsigned int size)
+{
+	unsigned long offset = reloc_offset(insn_jump_table(insn));
+	int i;
+	struct reloc *reloc = NULL;
+
+	for (i = 0; i < size; i++) {
+		if (reloc_offset(table[i]) > offset &&
+		    (!reloc || reloc_offset(table[i]) < reloc_offset(reloc)))
+			reloc = table[i];
+	}
+	return reloc;
+}
+
 /*
  * First pass: Mark the head of each jump table so that in the next pass,
  * we know when a given jump table ends and the next one starts.
  */
 static int mark_add_func_jump_tables(struct objtool_file *file,
-				     struct symbol *func)
+				     struct symbol *func,
+				     struct reloc **table, unsigned int size)
 {
-	struct instruction *insn, *last = NULL, *insn_t1 = NULL, *insn_t2;
+	struct instruction *insn, *last = NULL;
 	int ret = 0;
 
 	func_for_each_insn(file, func, insn) {
@@ -2094,23 +2110,11 @@ static int mark_add_func_jump_tables(struct objtool_file *file,
 		if (!insn_jump_table(insn))
 			continue;
 
-		if (!insn_t1) {
-			insn_t1 = insn;
-			continue;
-		}
-
-		insn_t2 = insn;
-
-		ret = add_jump_table(file, insn_t1, insn_jump_table(insn_t2));
+		ret = add_jump_table(file, insn, find_next_table(insn, table, size));
 		if (ret)
 			return ret;
-
-		insn_t1 = insn_t2;
 	}
 
-	if (insn_t1)
-		ret = add_jump_table(file, insn_t1, NULL);
-
 	return ret;
 }
 
@@ -2123,15 +2127,42 @@ static int add_jump_table_alts(struct objtool_file *file)
 {
 	struct symbol *func;
 	int ret;
+	struct instruction *insn;
+	unsigned int size = 0, i = 0;
+	struct reloc **table = NULL;
 
 	if (!file->rodata)
 		return 0;
 
+	for_each_insn(file, insn) {
+		struct instruction *dest_insn;
+		struct reloc *reloc;
+		unsigned long table_size;
+
+		func = insn_func(insn) ? insn_func(insn)->pfunc : NULL;
+		reloc = arch_find_switch_table(file, insn, &table_size, NULL);
+		/*
+		 * Each table entry has a rela associated with it.  The rela
+		 * should reference text in the same function as the original
+		 * instruction.
+		 */
+		if (!reloc)
+			continue;
+		dest_insn = find_insn(file, reloc->sym->sec, reloc_addend(reloc));
+		if (!dest_insn || !insn_func(dest_insn) || insn_func(dest_insn)->pfunc != func)
+			continue;
+		if (i == size) {
+			size += 1024;
+			table = realloc(table, size * sizeof(*table));
+		}
+		table[i++] = reloc;
+	}
+
 	for_each_sym(file, func) {
 		if (func->type != STT_FUNC)
 			continue;
 
-		ret = mark_add_func_jump_tables(file, func);
+		ret = mark_add_func_jump_tables(file, func, table, i);
 		if (ret)
 			return ret;
 	}
-- 
2.47.0