readelf: Use unsigned loop variables in handle_verneed and handle_verdef

Prevent signed underflow by changing loop variables to unsigned and
doing count checks before decrementing. This isn't really a bug, but
prevents UB detected by ubsan on fuzzed input. The bad (fuzzed) input
data does get detected anyway.

	* src/readelf.c (handle_verneed): Use unsigned cnt, cnt2.
	(handle_verdef): Likewise.

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

diff --git a/src/readelf.c b/src/readelf.c
index 802f8edecc74ba33b32df2642d449e98cff61411..0e931184ed38e36f47cc790ab77f750b620d4b1e 100644 (file)
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -3159,7 +3159,7 @@ handle_verneed (Ebl *ebl, Elf_Scn *scn, GElf_Shdr *shdr)
          elf_strptr (ebl->elf, shstrndx, glink->sh_name));
 
   unsigned int offset = 0;
-  for (int cnt = shdr->sh_info; --cnt >= 0; )
+  for (unsigned int cnt = shdr->sh_info; cnt > 0; cnt--)
     {
       /* Get the data at the next offset.  */
       GElf_Verneed needmem;
@@ -3173,7 +3173,7 @@ handle_verneed (Ebl *ebl, Elf_Scn *scn, GElf_Shdr *shdr)
              (unsigned short int) need->vn_cnt);
 
       unsigned int auxoffset = offset + need->vn_aux;
-      for (int cnt2 = need->vn_cnt; --cnt2 >= 0; )
+      for (unsigned int cnt2 = need->vn_cnt; cnt2 > 0; cnt2--)
        {
          GElf_Vernaux auxmem;
          GElf_Vernaux *aux = gelf_getvernaux (data, auxoffset, &auxmem);
@@ -3236,7 +3236,7 @@ handle_verdef (Ebl *ebl, Elf_Scn *scn, GElf_Shdr *shdr)
          elf_strptr (ebl->elf, shstrndx, glink->sh_name));
 
   unsigned int offset = 0;
-  for (int cnt = shdr->sh_info; --cnt >= 0; )
+  for (unsigned int cnt = shdr->sh_info; cnt > 0; cnt--)
     {
       /* Get the data at the next offset.  */
       GElf_Verdef defmem;
@@ -3259,7 +3259,7 @@ handle_verdef (Ebl *ebl, Elf_Scn *scn, GElf_Shdr *shdr)
              elf_strptr (ebl->elf, shdr->sh_link, aux->vda_name));
 
       auxoffset += aux->vda_next;
-      for (int cnt2 = 1; cnt2 < def->vd_cnt; ++cnt2)
+      for (unsigned int cnt2 = 1; cnt2 < def->vd_cnt; ++cnt2)
        {
          aux = gelf_getverdaux (data, auxoffset, &auxmem);
          if (unlikely (aux == NULL))