genksyms: fix memory leak when the same symbol is added from source

[ Upstream commit 45c9c4101d3d2fdfa00852274bbebba65fcc3cf2 ]

When a symbol that is already registered is added again, __add_symbol()
returns without freeing the symbol definition, making it unreachable.

The following test cases demonstrate different memory leak points.

[Test Case 1]

Forward declaration with exactly the same definition

  $ cat foo.c
  #include <linux/export.h>
  void foo(void);
  void foo(void) {}
  EXPORT_SYMBOL(foo);

[Test Case 2]

Forward declaration with a different definition (e.g. attribute)

  $ cat foo.c
  #include <linux/export.h>
  void foo(void);
  __attribute__((__section__(".ref.text"))) void foo(void) {}
  EXPORT_SYMBOL(foo);

[Test Case 3]

Preserving an overridden symbol (compile with KBUILD_PRESERVE=1)

  $ cat foo.c
  #include <linux/export.h>
  void foo(void);
  void foo(void) { }
  EXPORT_SYMBOL(foo);

  $ cat foo.symref
  override foo void foo ( int )

The memory leaks in Test Case 1 and 2 have existed since the introduction
of genksyms into the kernel tree. [1]

The memory leak in Test Case 3 was introduced by commit 5dae9a55
("genksyms: allow to ignore symbol checksum changes").

When multiple init_declarators are reduced to an init_declarator_list,
the decl_spec must be duplicated. Otherwise, the following Test Case 4
would result in a double-free bug.

[Test Case 4]

  $ cat foo.c
  #include <linux/export.h>

  extern int foo, bar;

  int foo, bar;
  EXPORT_SYMBOL(foo);

In this case, 'foo' and 'bar' share the same decl_spec, 'int'. It must
be unshared before being passed to add_symbol().

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=46bd1da672d66ccd8a639d3c1f8a166048cca608



Fixes: 5dae9a55 ("genksyms: allow to ignore symbol checksum changes")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>