[glibc] dlfcn: Avoid one-element flexible array in Dl_serinfo [BZ #24166]

[glibc] dlfcn: Avoid one-element flexible array in Dl_serinfo [BZ #24166]

[Florian Weimer]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=fabf5e49ddd61312027de8e92cc1b8528c2a929d

commit fabf5e49ddd61312027de8e92cc1b8528c2a929d
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Jun 19 10:13:50 2019 +0200

    dlfcn: Avoid one-element flexible array in Dl_serinfo [BZ #24166]
    
    The dls_serpath path field, as an array of length 1, introduces
    unexpected array subscript checks with some compilers.
    
    GCC versions before 3.0 treat the nested anonymous union as a
    declaration of an unnamed type, and not as a member declaration,
    so this construct cannot be used for these compilers.

Diff:
---
 ChangeLog     |  6 ++++++
 dlfcn/dlfcn.h | 12 ++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index ac06595..f32b734 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2019-06-19  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #24166]
+	* dlfcn/dlfcn.h (Dl_serinfo): Do not use array of length 1 for
+	dls_serpath field.
+
 2019-06-18  Florian Weimer  <fweimer@redhat.com>
 
 	[BZ #24323]
diff --git a/dlfcn/dlfcn.h b/dlfcn/dlfcn.h
index 896ad6f..c550371 100644
--- a/dlfcn/dlfcn.h
+++ b/dlfcn/dlfcn.h
@@ -180,7 +180,19 @@ typedef struct
 {
   size_t dls_size;		/* Size in bytes of the whole buffer.  */
   unsigned int dls_cnt;		/* Number of elements in `dls_serpath'.  */
+# if __GNUC_PREREQ (3, 0)
+  /* The zero-length array avoids an unwanted array subscript check by
+     the compiler, while the surrounding anonymous union preserves the
+     historic size of the type.  At the time of writing, GNU C does
+     not support structs with flexible array members in unions.  */
+  __extension__ union
+  {
+    Dl_serpath dls_serpath[0]; /* Actually longer, dls_cnt elements.  */
+    Dl_serpath __dls_serpath_pad[1];
+  };
+# else
   Dl_serpath dls_serpath[1];	/* Actually longer, dls_cnt elements.  */
+# endif
 } Dl_serinfo;
 #endif /* __USE_GNU */