[RFC PATCH v3 09/15] objtool: Warn about non __ro_after_init static key usage in .noinstr

[Valentin Schneider <vschneid@redhat.com>]

Later commits will disallow runtime-mutable text in .noinstr sections in
order to safely defer instruction patching IPIs.

All static keys used in .noinstr sections have now been checked as being
either flagged as __ro_after_init, or as forceful static keys. Any
occurrence of this new warning would be the result of a code change that
will need looking at.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
offset_of(static_key.type) and JUMP_TYPE_FORCEFUL would need to be shoved
into a somewhat standalone header file that could be included by objtool
itself.
---
 tools/objtool/Documentation/objtool.txt | 13 ++++++++
 tools/objtool/check.c                   | 41 +++++++++++++++++++++++++
 tools/objtool/include/objtool/check.h   |  1 +
 tools/objtool/include/objtool/special.h |  2 ++
 tools/objtool/special.c                 |  3 ++
 5 files changed, 60 insertions(+)

diff --git a/tools/objtool/Documentation/objtool.txt b/tools/objtool/Documentation/objtool.txt
index 7c3ee959b63c7..06fa285873387 100644
--- a/tools/objtool/Documentation/objtool.txt
+++ b/tools/objtool/Documentation/objtool.txt
@@ -447,6 +447,19 @@ the objtool maintainers.
    names and does not use module_init() / module_exit() macros to create
    them.
 
+13. file.o: warning: func()+0x2a: non __ro_after_init static key "key" in .noinstr section
+
+  This means that the noinstr function func() uses a static key that can be
+  modified at runtime. This is not allowed as noinstr functions rely on
+  containing stable instructions after init.
+
+  Check whether the static key in question can really be modified at runtime,
+  and if it is only enabled during init then mark it as __ro_after_init. If it
+  genuinely needs to be modified at runtime:
+
+  1) Directly rely on the underlying atomic count of they key in the noinstr
+     functions.
+  2) Mark the static key as forceful.
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask the objtool maintainer for help.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 00e25492f5065..c1fb02c326839 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2056,6 +2056,9 @@ static int add_special_section_alts(struct objtool_file *file)
 		alt->next = orig_insn->alts;
 		orig_insn->alts = alt;
 
+		if (special_alt->key_sym)
+			orig_insn->key_sym = special_alt->key_sym;
+
 		list_del(&special_alt->list);
 		free(special_alt);
 	}
@@ -3605,6 +3608,41 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct
 	return 0;
 }
 
+static bool static_key_is_forceful(struct symbol *key)
+{
+	if (!strcmp(key->sec->name, ".data")) {
+		unsigned long data_offset = key->sec->data->d_off;
+		unsigned long key_offset = key->sym.st_value;
+		char* data = key->sec->data->d_buf;
+
+		/*
+		 * offset_of(static_key.type)
+		 *                        v
+		 *                        v            JUMP_TYPE_FORCEFUL
+		 *                        v                    v
+		 */
+		return data[(key_offset + 8) - data_offset] & 0x4;
+	}
+
+	return false;
+}
+
+static int validate_static_key(struct instruction *insn, struct insn_state *state)
+{
+	if (state->noinstr && state->instr <= 0) {
+		if (static_key_is_forceful(insn->key_sym))
+			return 0;
+
+		if ((strcmp(insn->key_sym->sec->name, ".data..ro_after_init"))) {
+			WARN_INSN(insn, "non __ro_after_init static key \"%s\" in .noinstr section",
+				  insn->key_sym->name);
+			return 1;
+		}
+	}
+
+	return 0;
+}
+
 static struct instruction *next_insn_to_validate(struct objtool_file *file,
 						 struct instruction *insn)
 {
@@ -3766,6 +3804,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 		if (handle_insn_ops(insn, next_insn, &state))
 			return 1;
 
+		if (insn->key_sym)
+			validate_static_key(insn, &state);
+
 		switch (insn->type) {
 
 		case INSN_RETURN:
diff --git a/tools/objtool/include/objtool/check.h b/tools/objtool/include/objtool/check.h
index daa46f1f0965a..35dd21f8f41e1 100644
--- a/tools/objtool/include/objtool/check.h
+++ b/tools/objtool/include/objtool/check.h
@@ -77,6 +77,7 @@ struct instruction {
 	struct symbol *sym;
 	struct stack_op *stack_ops;
 	struct cfi_state *cfi;
+	struct symbol *key_sym;
 };
 
 static inline struct symbol *insn_func(struct instruction *insn)
diff --git a/tools/objtool/include/objtool/special.h b/tools/objtool/include/objtool/special.h
index 86d4af9c5aa9d..0e61f34fe3a28 100644
--- a/tools/objtool/include/objtool/special.h
+++ b/tools/objtool/include/objtool/special.h
@@ -27,6 +27,8 @@ struct special_alt {
 	struct section *new_sec;
 	unsigned long new_off;
 
+	struct symbol *key_sym;
+
 	unsigned int orig_len, new_len; /* group only */
 };
 
diff --git a/tools/objtool/special.c b/tools/objtool/special.c
index 097a69db82a0e..fefab2b471bf8 100644
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -127,6 +127,9 @@ static int get_alt_entry(struct elf *elf, const struct special_entry *entry,
 			return -1;
 		}
 		alt->key_addend = reloc_addend(key_reloc);
+
+		reloc_to_sec_off(key_reloc, &sec, &offset);
+		alt->key_sym = find_symbol_by_offset(sec, offset & ~3);
 	}
 
 	return 0;
-- 
2.43.0