libdwfl: Declare possible zero sized arrays only when non-zero

The gcc undefined sanitizer complains when seeing a zero sized array
declaration. Move the declaration to the point in the code where we
know they aren't zero sized.

https://sourceware.org/bugzilla/show_bug.cgi?id=28720

Signed-off-by: Mark Wielaard <mark@klomp.org>

2 files changed, 7 insertions, 2 deletions

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index aace969f..b2588b12 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,5 +1,10 @@
 2022-01-03  Mark Wielaard  <mark@klomp.org>
 
+        * link_map.c (dwfl_link_map_report): Only declare d32 and d64 before
+        actual use.
+
+2022-01-03  Mark Wielaard  <mark@klomp.org>
+
         * dwfl_segment_report_module.c (dwfl_segment_report_module): Copy
         dyn_data and set xlatefrom.d_buf to dyns when dyns is not aligned.
 
diff --git a/libdwfl/link_map.c b/libdwfl/link_map.c
index 99222bb9..c0207cd3 100644
--- a/libdwfl/link_map.c
+++ b/libdwfl/link_map.c
@@ -1037,8 +1037,6 @@ dwfl_link_map_report (Dwfl *dwfl, const void *auxv, size_t auxv_size,
                   return false;
                 }
               void *buf = malloc (dyn_filesz);
-              Elf32_Dyn (*d32)[dyn_filesz / sizeof (Elf32_Dyn)] = buf;
-              Elf64_Dyn (*d64)[dyn_filesz / sizeof (Elf64_Dyn)] = buf;
               if (unlikely (buf == NULL))
                 {
                   __libdwfl_seterrno (DWFL_E_NOMEM);
@@ -1068,6 +1066,7 @@ dwfl_link_map_report (Dwfl *dwfl, const void *auxv, size_t auxv_size,
                   /* We are looking for DT_DEBUG.  */
                   if (elfclass == ELFCLASS32)
                     {
+                      Elf32_Dyn (*d32)[dyn_filesz / sizeof (Elf32_Dyn)] = buf;
                       size_t n = dyn_filesz / sizeof (Elf32_Dyn);
                       for (size_t i = 0; i < n; ++i)
                         if ((*d32)[i].d_tag == DT_DEBUG)
@@ -1078,6 +1077,7 @@ dwfl_link_map_report (Dwfl *dwfl, const void *auxv, size_t auxv_size,
                     }
                   else
                     {
+                      Elf64_Dyn (*d64)[dyn_filesz / sizeof (Elf64_Dyn)] = buf;
                       size_t n = dyn_filesz / sizeof (Elf64_Dyn);
                       for (size_t i = 0; i < n; ++i)
                         if ((*d64)[i].d_tag == DT_DEBUG)