pahole: Add --inner_anon option

Allow making the inner struct/enum/union anonymous. This permits using
the header to inspect pointer values using -E, without having to care
about avoiding duplicate type definitions such as:

    struct foo { ... };
    struct bar {
        struct foo {
	     ....
	} a;
    };

With --inner_anon, the conflict between the two definitions of struct
foo is gone:

    struct foo { ... };
    struct bar {
        struct {
	     ....
	} a;
    };

Committer testing:

  $ cat inner_anon.c

  struct foo {
  	int  a;
  	char b;
  };

  struct bar {
          struct foo c;
  	int	   d;
  } bla;
  $ gcc -g -c inner_anon.c   -o inner_anon.o

No expansion:

  $ pahole inner_anon.o
  struct foo {
  	int                        a;                    /*     0     4 */
  	char                       b;                    /*     4     1 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* padding: 3 */
  	/* last cacheline: 8 bytes */
  };
  struct bar {
  	struct foo                 c;                    /*     0     8 */

  	/* XXX last struct has 3 bytes of padding */

  	int                        d;                    /*     8     4 */

  	/* size: 12, cachelines: 1, members: 2 */
  	/* paddings: 1, sum paddings: 3 */
  	/* last cacheline: 12 bytes */
  };

Expanding types:

  $ pahole -E inner_anon.o
  struct foo {
  	int                        a;                                                    /*     0     4 */
  	char                       b;                                                    /*     4     1 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* padding: 3 */
  	/* last cacheline: 8 bytes */
  };
  struct bar {
  	struct foo {
  		int                a;                                                    /*     0     4 */
  		char               b;                                                    /*     4     1 */
  	}c; /*     0     8 */

  	/* XXX last struct has 3 bytes of padding */

  	int                        d;                                                    /*     8     4 */

  	/* size: 12, cachelines: 1, members: 2 */
  	/* paddings: 1, sum paddings: 3 */
  	/* last cacheline: 12 bytes */
  };

Anonymising the inner struct:

  $ pahole -E --inner_anon inner_anon.o
  struct foo {
  	int                        a;                                                    /*     0     4 */
  	char                       b;                                                    /*     4     1 */

  	/* size: 8, cachelines: 1, members: 2 */
  	/* padding: 3 */
  	/* last cacheline: 8 bytes */
  };
  struct bar {
  	struct /* foo */ {
  		int                a;                                                    /*     0     4 */
  		char               b;                                                    /*     4     1 */
  	}c; /*     0     8 */

  	/* XXX last struct has 3 bytes of padding */

  	int                        d;                                                    /*     8     4 */

  	/* size: 12, cachelines: 1, members: 2 */
  	/* paddings: 1, sum paddings: 3 */
  	/* last cacheline: 12 bytes */
  };

Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
[ Added man page entry for --inner_anon, refreshed the patch to cope with the btf_tag series ]
Link: https://lore.kernel.org/all/20211019100724.325570-3-douglas.raillard@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

2 files changed, 15 insertions, 0 deletions

diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index c1ec63e4..f81a2965 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -123,6 +123,13 @@ Expand class members. Useful to find in what member of inner structs where an
 offset from the beginning of a struct is.
 
 .TP
+.B \-\-inner_anon
+Allow making inner structs, enums and unions anonymous, so that when using -E
+to expand types we don't end up with multiple definitions for the expanded
+inner structs/enums/unions, allowing the resulting expanded struct to be
+compilable.
+
+.TP
 .B \-F, \-\-format_path
 Allows specifying a list of debugging formats to try, in order. Right now this
 includes "ctf" and "dwarf". The default format path used is equivalent to
diff --git a/pahole.c b/pahole.c
index f3a51cb2..c89155f2 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1127,6 +1127,7 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARGP_skip_encoding_btf_decl_tag 331
 #define ARGP_skip_missing          332
 #define ARGP_skip_encoding_btf_type_tag 333
+#define ARGP_inner_anon		   334
 
 static const struct argp_option pahole__options[] = {
 	{
@@ -1235,6 +1236,11 @@ static const struct argp_option pahole__options[] = {
 		.doc  = "expand class members",
 	},
 	{
+		.name = "inner_anon",
+		.key  = ARGP_inner_anon,
+		.doc  = "expanded class members are anonymous",
+	},
+	{
 		.name = "nr_members",
 		.key  = 'n',
 		.doc  = "show number of members",
@@ -1666,6 +1672,8 @@ static error_t pahole__options_parser(int key, char *arg,
 		conf_load.skip_missing = true;          break;
 	case ARGP_skip_encoding_btf_type_tag:
 		conf_load.skip_encoding_btf_type_tag = true;	break;
+	case ARGP_inner_anon:
+		conf.inner_anonymous = true;            break;
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}