[PATCH] libelf: Set dst to zero on failure in __elf_getphdrnum_rdlock

[Mark Wielaard]

GCC with -flto finds some (very) unlikely error paths. It flags callers
of __elf_getphdrnum_chk_rdlock with *dst not yet set because an internal
call to __elf_getphdrnum_rdlock might not initialize *dst either in one
particular failure path. The sanity check that __elf_getphdrnum_chk_rdlock
then performs might happen against uninitialized data. So just set *dst
to zero on failure in __elf_getphdrnum_rdlock so any caller can simply
check either the result/error code or whether *dst is zero or not.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libelf/ChangeLog        | 5 +++++
 libelf/elf_getphdrnum.c | 1 +
 2 files changed, 6 insertions(+)

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 7bd15a28..aee1ffff 100644
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2020-11-01  Mark Wielaard  <mark@klomp.org>
+
+	* elf_getphdrnum.c (__elf_getphdrnum_rdlock): Set *dst to zero on
+	error.
+
 2020-11-01  Mark Wielaard  <mark@klomp.org>
 
 	* libelfP.h (__libelf_data_type): Take an GElf_Ehdr instead of an
diff --git a/libelf/elf_getphdrnum.c b/libelf/elf_getphdrnum.c
index 53617dfc..60ebec58 100644
--- a/libelf/elf_getphdrnum.c
+++ b/libelf/elf_getphdrnum.c
@@ -44,6 +44,7 @@ __elf_getphdrnum_rdlock (Elf *elf, size_t *dst)
  if (unlikely (elf->state.elf64.ehdr == NULL))
    {
      /* Maybe no ELF header was created yet.  */
+     *dst = 0;
      __libelf_seterrno (ELF_E_WRONG_ORDER_EHDR);
      return -1;
    }
-- 
2.18.4