[PATCH v2 04/19] gendwarfksyms: Add support for type pointers

[Sami Tolvanen <samitolvanen@google.com>]

The compiler may choose not to emit type information in DWARF for
external symbols. Clang, for example, does this for symbols not
defined in the current TU.

To provide a way to work around this issue, add support for
__gendwarfksyms_ptr_<symbol> pointers that force the compiler to emit
the necessary type information in DWARF also for the missing symbols.

Example usage:

  #define GENDWARFKSYMS_PTR(sym) \
      static typeof(sym) *__gendwarfksyms_ptr_##sym __used  \
          __section(".discard.gendwarfksyms") = &sym;

  extern int external_symbol(void);
  GENDWARFKSYMS_PTR(external_symbol);

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 scripts/gendwarfksyms/dwarf.c         | 26 +++++++++++++++++++++++++-
 scripts/gendwarfksyms/gendwarfksyms.h |  6 ++++++
 scripts/gendwarfksyms/symbols.c       | 16 ++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/scripts/gendwarfksyms/dwarf.c b/scripts/gendwarfksyms/dwarf.c
index 71cfab0553da..956b30224316 100644
--- a/scripts/gendwarfksyms/dwarf.c
+++ b/scripts/gendwarfksyms/dwarf.c
@@ -94,6 +94,28 @@ static int process_variable(struct state *state, Dwarf_Die *die)
 	return check(process(state, "variable;\n"));
 }
 
+static int process_symbol_ptr(struct state *state, Dwarf_Die *die)
+{
+	Dwarf_Die ptr_type;
+	Dwarf_Die type;
+
+	if (!get_ref_die_attr(die, DW_AT_type, &ptr_type) ||
+	    dwarf_tag(&ptr_type) != DW_TAG_pointer_type) {
+		error("%s must be a pointer type!", get_name(die));
+		return -1;
+	}
+
+	if (!get_ref_die_attr(&ptr_type, DW_AT_type, &type)) {
+		error("%s pointer missing a type attribute?", get_name(die));
+		return -1;
+	}
+
+	if (dwarf_tag(&type) == DW_TAG_subroutine_type)
+		return check(process_subprogram(state, &type));
+	else
+		return check(process_variable(state, &ptr_type));
+}
+
 static int process_exported_symbols(struct state *state, Dwarf_Die *die)
 {
 	int tag = dwarf_tag(die);
@@ -114,7 +136,9 @@ static int process_exported_symbols(struct state *state, Dwarf_Die *die)
 
 		debug("%s", state->sym->name);
 
-		if (tag == DW_TAG_subprogram)
+		if (is_symbol_ptr(get_name(&state->die)))
+			check(process_symbol_ptr(state, &state->die));
+		else if (tag == DW_TAG_subprogram)
 			check(process_subprogram(state, &state->die));
 		else
 			check(process_variable(state, &state->die));
diff --git a/scripts/gendwarfksyms/gendwarfksyms.h b/scripts/gendwarfksyms/gendwarfksyms.h
index cb9106dfddb9..8f6acd1b8f8f 100644
--- a/scripts/gendwarfksyms/gendwarfksyms.h
+++ b/scripts/gendwarfksyms/gendwarfksyms.h
@@ -61,6 +61,11 @@ extern bool debug;
 /*
  * symbols.c
  */
+
+/* See symbols.c:is_symbol_ptr */
+#define SYMBOL_PTR_PREFIX "__gendwarfksyms_ptr_"
+#define SYMBOL_PTR_PREFIX_LEN (sizeof(SYMBOL_PTR_PREFIX) - 1)
+
 struct symbol_addr {
 	uint32_t section;
 	Elf64_Addr address;
@@ -78,6 +83,7 @@ struct symbol {
 	struct hlist_node name_hash;
 };
 
+extern bool is_symbol_ptr(const char *name);
 extern int symbol_read_exports(FILE *file);
 extern int symbol_read_symtab(int fd);
 extern struct symbol *symbol_get(const char *name);
diff --git a/scripts/gendwarfksyms/symbols.c b/scripts/gendwarfksyms/symbols.c
index f96acb941196..d6d016458ae1 100644
--- a/scripts/gendwarfksyms/symbols.c
+++ b/scripts/gendwarfksyms/symbols.c
@@ -41,6 +41,20 @@ static int __for_each_addr(struct symbol *sym, symbol_callback_t func,
 	return processed;
 }
 
+/*
+ * For symbols without debugging information (e.g. symbols defined in other
+ * TUs), we also match __gendwarfksyms_ptr_<symbol_name> symbols, which the
+ * kernel uses to ensure type information is present in the TU that exports
+ * the symbol. A __gendwarfksyms_ptr pointer must have the same type as the
+ * exported symbol, e.g.:
+ *
+ *   typeof(symname) *__gendwarf_ptr_symname = &symname;
+ */
+bool is_symbol_ptr(const char *name)
+{
+	return name && !strncmp(name, SYMBOL_PTR_PREFIX, SYMBOL_PTR_PREFIX_LEN);
+}
+
 static int for_each(const char *name, bool name_only, symbol_callback_t func,
 		    void *data)
 {
@@ -49,6 +63,8 @@ static int for_each(const char *name, bool name_only, symbol_callback_t func,
 
 	if (!name || !*name)
 		return 0;
+	if (is_symbol_ptr(name))
+		name += SYMBOL_PTR_PREFIX_LEN;
 
 	hash_for_each_possible_safe(symbol_names, match, tmp, name_hash,
 				    name_hash(name)) {
-- 
2.46.0.184.g6999bdac58-goog