objtool: Warn on functions with ambiguous -ffunction-sections section names

When compiled with -ffunction-sections, a function named startup() will
be placed in .text.startup.  However, .text.startup is also used by the
compiler for functions with __attribute__((constructor)).

That creates an ambiguity for the vmlinux linker script, which needs to
differentiate those two cases.

Similar naming conflicts exist for functions named exit(), split(),
unlikely(), hot() and unknown().

One potential solution would be to use '#ifdef CC_USING_FUNCTION_SECTIONS'
to create two distinct implementations of the TEXT_MAIN macro.  However,
-ffunction-sections can be (and is) enabled or disabled on a per-object
basis (for example via ccflags-y or AUTOFDO_PROFILE).

So the recently unified TEXT_MAIN macro (commit 1ba9f8979426
("vmlinux.lds: Unify TEXT_MAIN, DATA_MAIN, and related macros")) is
necessary.  This means there's no way for the linker script to
disambiguate things.

Instead, use objtool to warn on any function names whose resulting
section names might create ambiguity when the kernel is compiled (in
whole or in part) with -ffunction-sections.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: live-patching@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/65fedea974fe14be487c8867a0b8d0e4a294ce1e.1762991150.git.jpoimboe@kernel.org

2 files changed, 40 insertions, 0 deletions

diff --git a/tools/objtool/Documentation/objtool.txt b/tools/objtool/Documentation/objtool.txt
index 9e97fc25b2d8..f88f8d28513a 100644
--- a/tools/objtool/Documentation/objtool.txt
+++ b/tools/objtool/Documentation/objtool.txt
@@ -456,6 +456,13 @@ the objtool maintainers.
     these special names and does not use module_init() / module_exit()
     macros to create them.
 
+13. file.o: warning: func() function name creates ambiguity with -ffunctions-sections
+
+    Functions named startup(), exit(), split(), unlikely(), hot(), and
+    unknown() are not allowed due to the ambiguity of their section
+    names when compiled with -ffunction-sections.  For more information,
+    see the comment above TEXT_MAIN in include/asm-generic/vmlinux.lds.h.
+
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask objtool maintainers for help.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 57fac6ce3454..72c7f6f03350 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2663,6 +2663,37 @@ static int decode_sections(struct objtool_file *file)
 	return 0;
 }
 
+/*
+ * Certain function names are disallowed due to section name ambiguities
+ * introduced by -ffunction-sections.
+ *
+ * See the comment above TEXT_MAIN in include/asm-generic/vmlinux.lds.h.
+ */
+static int validate_function_names(struct objtool_file *file)
+{
+	struct symbol *func;
+	int warnings = 0;
+
+	for_each_sym(file->elf, func) {
+		if (!is_func_sym(func))
+			continue;
+
+		if (!strcmp(func->name, "startup")	|| strstarts(func->name, "startup.")	||
+		    !strcmp(func->name, "exit")		|| strstarts(func->name, "exit.")	||
+		    !strcmp(func->name, "split")	|| strstarts(func->name, "split.")	||
+		    !strcmp(func->name, "unlikely")	|| strstarts(func->name, "unlikely.")	||
+		    !strcmp(func->name, "hot")		|| strstarts(func->name, "hot.")	||
+		    !strcmp(func->name, "unknown")	|| strstarts(func->name, "unknown.")) {
+
+			WARN("%s() function name creates ambiguity with -ffunction-sections",
+			     func->name);
+			warnings++;
+		}
+	}
+
+	return warnings;
+}
+
 static bool is_special_call(struct instruction *insn)
 {
 	if (insn->type == INSN_CALL) {
@@ -4932,6 +4963,8 @@ int check(struct objtool_file *file)
 	if (!nr_insns)
 		goto out;
 
+	warnings += validate_function_names(file);
+
 	if (opts.retpoline)
 		warnings += validate_retpoline(file);