gendwarfksyms: Add support for kABI rules

Distributions that want to maintain a stable kABI need the ability
to make ABI compatible changes to kernel without affecting symbol
versions, either because of LTS updates or backports.

With genksyms, developers would typically hide these changes from
version calculation with #ifndef __GENKSYMS__, which would result
in the symbol version not changing even though the actual type has
changed.  When we process precompiled object files, this isn't an
option.

To support this use case, add a --stable command line flag that
gates kABI stability features that are not needed in mainline
kernels, but can be useful for distributions, and add support for
kABI rules, which can be used to restrict gendwarfksyms output.

The rules are specified as a set of null-terminated strings stored
in the .discard.gendwarfksyms.kabi_rules section. Each rule consists
of four strings as follows:

  "version\0type\0target\0value"

The version string ensures the structure can be changed in a
backwards compatible way. The type string indicates the type of the
rule, and target and value strings contain rule-specific data.

Initially support two simple rules:

  1. Declaration-only types

     A type declaration can change into a full definition when
     additional includes are pulled in to the TU, which changes the
     versions of any symbol that references the type. Add support
     for defining declaration-only types whose definition is not
     expanded during versioning.

  2. Ignored enumerators

     It's possible to add new enum fields without changing the ABI,
     but as the fields are included in symbol versioning, this would
     change the versions. Add support for ignoring specific fields.

  3. Overridden enumerator values

     Add support for overriding enumerator values when calculating
     versions. This may be needed when the last field of the enum
     is used as a sentinel and new fields must be added before it.

Add examples for using the rules under the examples/ directory.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

1 files changed, 22 insertions, 3 deletions

diff --git a/scripts/gendwarfksyms/dwarf.c b/scripts/gendwarfksyms/dwarf.c
index bdf899d60707..17f7e6b9a7ff 100644
--- a/scripts/gendwarfksyms/dwarf.c
+++ b/scripts/gendwarfksyms/dwarf.c
@@ -120,13 +120,16 @@ static bool is_definition_private(Dwarf_Die *die)
 	return !!res;
 }
 
-static bool is_kabi_definition(Dwarf_Die *die)
+static bool is_kabi_definition(struct die *cache, Dwarf_Die *die)
 {
 	bool value;
 
 	if (get_flag_attr(die, DW_AT_declaration, &value) && value)
 		return false;
 
+	if (kabi_is_declonly(cache->fqn))
+		return false;
+
 	return !is_definition_private(die);
 }
 
@@ -515,9 +518,10 @@ static void __process_structure_type(struct state *state, struct die *cache,
 	process(cache, " {");
 	process_linebreak(cache, 1);
 
-	expand = state->expand.expand && is_kabi_definition(die);
+	expand = state->expand.expand && is_kabi_definition(cache, die);
 
 	if (expand) {
+		state->expand.current_fqn = cache->fqn;
 		check(process_die_container(state, cache, die, process_func,
 					    match_func));
 	}
@@ -548,13 +552,26 @@ DEFINE_PROCESS_STRUCTURE_TYPE(union)
 static void process_enumerator_type(struct state *state, struct die *cache,
 				    Dwarf_Die *die)
 {
+	bool overridden = false;
 	Dwarf_Word value;
 
+	if (stable) {
+		/* Get the fqn before we process anything */
+		update_fqn(cache, die);
+
+		if (kabi_is_enumerator_ignored(state->expand.current_fqn,
+					       cache->fqn))
+			return;
+
+		overridden = kabi_get_enumerator_value(
+			state->expand.current_fqn, cache->fqn, &value);
+	}
+
 	process_list_comma(state, cache);
 	process(cache, "enumerator");
 	process_fqn(cache, die);
 
-	if (get_udata_attr(die, DW_AT_const_value, &value)) {
+	if (overridden || get_udata_attr(die, DW_AT_const_value, &value)) {
 		process(cache, " = ");
 		process_fmt(cache, "%" PRIu64, value);
 	}
@@ -620,6 +637,7 @@ static void process_cached(struct state *state, struct die *cache,
 static void state_init(struct state *state)
 {
 	state->expand.expand = true;
+	state->expand.current_fqn = NULL;
 	cache_init(&state->expansion_cache);
 }
 
@@ -627,6 +645,7 @@ static void expansion_state_restore(struct expansion_state *state,
 				    struct expansion_state *saved)
 {
 	state->expand = saved->expand;
+	state->current_fqn = saved->current_fqn;
 }
 
 static void expansion_state_save(struct expansion_state *state,