libelf: Use offsetof to get field of unaligned
gcc undefined sanitizer flags:
elf_begin.c:230:18: runtime error: member access within misaligned
address 0xf796400a for type 'struct Elf64_Shdr', which requires 4 byte
alignment struct.
We aren't actually accessing the field member of the struct, but are
taking the address of it. Which the compiler can take as a hint that
the address is correctly aligned. But we can do the same by adding
the field offsetof to the base address. Which doesn't trigger a
runtime error.
Signed-off-by: Mark Wielaard <mark@klomp.org>
diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 041da9b..96059ef 100644
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2021-12-15 Mark Wielaard <mark@klomp.org>
+
+ * elf_begin.c (get_shnum): Use offsetof to get field of unaligned
+ struct.
+
2021-09-06 Dmitry V. Levin <ldv@altlinux.org>
* common.h (allocate_elf): Remove cast of calloc return value.
diff --git a/libelf/elf_begin.c b/libelf/elf_begin.c
index 93d1e12..bd3399d 100644
--- a/libelf/elf_begin.c
+++ b/libelf/elf_begin.c
@@ -1,5 +1,6 @@
/* Create descriptor for processing file.
Copyright (C) 1998-2010, 2012, 2014, 2015, 2016 Red Hat, Inc.
+ Copyright (C) 2021 Mark J. Wielaard <mark@klomp.org>
This file is part of elfutils.
Written by Ulrich Drepper <drepper@redhat.com>, 1998.
@@ -170,9 +171,10 @@
if (likely (map_address != NULL))
/* gcc will optimize the memcpy to a simple memory
access while taking care of alignment issues. */
- memcpy (&size, &((Elf32_Shdr *) ((char *) map_address
- + ehdr.e32->e_shoff
- + offset))->sh_size,
+ memcpy (&size, ((char *) map_address
+ + ehdr.e32->e_shoff
+ + offset
+ + offsetof (Elf32_Shdr, sh_size)),
sizeof (Elf32_Word));
else
if (unlikely ((r = pread_retry (fildes, &size,
@@ -227,9 +229,10 @@
if (likely (map_address != NULL))
/* gcc will optimize the memcpy to a simple memory
access while taking care of alignment issues. */
- memcpy (&size, &((Elf64_Shdr *) ((char *) map_address
- + ehdr.e64->e_shoff
- + offset))->sh_size,
+ memcpy (&size, ((char *) map_address
+ + ehdr.e64->e_shoff
+ + offset
+ + offsetof (Elf64_Shdr, sh_size)),
sizeof (Elf64_Xword));
else
if (unlikely ((r = pread_retry (fildes, &size,
