Expand comments in elf/ldconfig.c (search_dir)

Developers creating development packages must take care
to have their static linker DSO link point to the actual
SONAME file. This allows ldconfig to correctly create
the required links for the SONAME. The behaviour is now
more clearly documented in a code comment added by this
patch.

2 files changed, 28 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 8a6e89e..39e70e1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2014-11-29  Carlos O'Donell  <carlos@redhat.com>
+
+	* elf/ldconfig.c (search_dir): Expand comment.
+
 2014-11-29  Joseph Myers  <joseph@codesourcery.com>
 
 	* conform/Makefile (linknamespace-symlist-stdlibs-base): New
diff --git a/elf/ldconfig.c b/elf/ldconfig.c
index 4211f4c..2d9c780 100644
--- a/elf/ldconfig.c
+++ b/elf/ldconfig.c
@@ -893,8 +893,30 @@ search_dir (const struct dir_entry *entry)
       /* A link may just point to itself.  */
       if (is_link)
 	{
-	  /* If the path the link points to isn't its soname and it is not
-	     .so symlink for ld(1) only, we treat it as a normal file.  */
+	  /* If the path the link points to isn't its soname or it is not
+	     the .so symlink for ld(1), we treat it as a normal file.
+
+	     You should always do this:
+
+		libfoo.so -> SONAME -> Arbitrary package-chosen name.
+
+	     e.g. libfoo.so -> libfoo.so.1 -> libfooimp.so.9.99.
+	     Given a SONAME of libfoo.so.1.
+
+	     You should *never* do this:
+
+		libfoo.so -> libfooimp.so.9.99
+
+	     If you do, and your SONAME is libfoo.so.1, then libfoo.so
+	     fails to point at the SONAME. In that case ldconfig may consider
+	     libfoo.so as another implementation of SONAME and will create
+	     symlinks against it causing problems when you try to upgrade
+	     or downgrade. The problems will arise because ldconfig will,
+	     depending on directory ordering, creat symlinks against libfoo.so
+	     e.g. libfoo.so.1.2 -> libfoo.so, but when libfoo.so is removed
+	     (typically by the removal of a development pacakge not required
+	     for the runtime) it will break the libfoo.so.1.2 symlink and the
+	     application will fail to start.  */
 	  const char *real_base_name = basename (real_file_name);
 
 	  if (strcmp (real_base_name, soname) != 0)