linux-kernel - [RFC 27/31] objtool: Fix weak symbol detection

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Message-ID: <bcedaf8559e7e276e4d9ba511dab038ed70ebd6c.1725334260.git.jpoimboe@kernel.org>
Date: Mon,  2 Sep 2024 21:00:10 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: live-patching@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	x86@...nel.org,
	Miroslav Benes <mbenes@...e.cz>,
	Petr Mladek <pmladek@...e.com>,
	Joe Lawrence <joe.lawrence@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Marcos Paulo de Souza <mpdesouza@...e.com>,
	Song Liu <song@...nel.org>
Subject: [RFC 27/31] objtool: Fix weak symbol detection

find_symbol_hole_containing() fails to find a symbol hole (aka stripped
weak symbol) if its section has no symbols before the hole.  This breaks
weak symbol detection if -ffunction-sections is enabled.

Fix it by allowing the interval tree to contain section symbols, which
are always at offset zero for a given section.

Fixes a bunch of (-ffunction-sections) warnings like:

  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x10: unreachable instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x14: unreachable instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x19: unreachable instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x20: unreachable instruction

Fixes: 4adb23686795 ("objtool: Ignore extra-symbol code")
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 tools/objtool/elf.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 471df0336aa7..3109277804cc 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -110,7 +110,7 @@ struct symbol_hole {
 };
 
 /*
- * Find !section symbol where @offset is after it.
+ * Find the last symbol before @offset.
  */
 static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
 {
@@ -121,8 +121,7 @@ static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
 		return -1;
 
 	if (sh->offset >= s->offset + s->len) {
-		if (s->type != STT_SECTION)
-			sh->sym = s;
+		sh->sym = s;
 		return 1;
 	}
 
@@ -416,7 +415,8 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym)
 	sym->len = sym->sym.st_size;
 
 	__sym_for_each(s, &sym->sec->symbol_tree, sym->offset, sym->offset) {
-		if (s->offset == sym->offset && s->type == sym->type)
+		if (s->type == sym->type && s->offset == sym->offset &&
+		    s->len == sym->len)
 			s->alias = sym;
 	}
 
@@ -433,9 +433,13 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym)
 	/*
 	 * Don't store empty STT_NOTYPE symbols in the rbtree.  They
 	 * can exist within a function, confusing the sorting.
+	 *
+	 * TODO: is this still true?
 	 */
-	if (!sym->len)
+#if 0
+	if (sym->type == STT_NOTYPE && !sym->len)
 		__sym_remove(sym, &sym->sec->symbol_tree);
+#endif
 }
 
 static void read_symbols(struct elf *elf)
-- 
2.45.2