bug#20614: alignment problem with recent Emacs unexelf.c changes

[Paul Eggert <eggert@cs.ucla.edu>]

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I ran into a porting problem when using Emacs's recent unexelf.c changes in a 
picky build using GCC’s -fsanitize=undefined option. The attached patch worked 
for me, so I installed it into emacs-25. Alan, could you please check and/or 
improve this patch, and/or think about whether there might be similar problems 
elsewhere? Thanks.

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From 0b3f6f7f182f3363fc499e52b27649e05c1ad381 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 16 Nov 2015 17:48:08 -0800
Subject: [PATCH] ELF unexec: align section header
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

This ports the recent unexelf.c changes to Fedora x86-64
when configured with GCC’s -fsanitize=undefined option.
* src/unexelf.c (unexec): Align new_data2_size to a multiple
of ElfW (Shdr)’s alignment, so that NEW_SECTION_H returns a
pointer aligned appropriately for its type.
---
 src/unexelf.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/unexelf.c b/src/unexelf.c
index c10c7f2..03e6daf 100644
--- a/src/unexelf.c
+++ b/src/unexelf.c
@@ -247,7 +247,7 @@ unexec (const char *new_name, const char *old_name)
 
   ElfW (Phdr) *old_bss_seg, *new_bss_seg;
   ElfW (Addr) old_bss_addr, new_bss_addr;
-  ElfW (Word) old_bss_size, new_data2_size;
+  ElfW (Word) old_bss_size, bss_size_growth, new_data2_size;
   ElfW (Off) old_bss_offset, new_data2_offset;
 
   ptrdiff_t n;
@@ -331,7 +331,11 @@ unexec (const char *new_name, const char *old_name)
 
   new_break = sbrk (0);
   new_bss_addr = (ElfW (Addr)) new_break;
-  new_data2_size = new_bss_addr - old_bss_addr;
+  bss_size_growth = new_bss_addr - old_bss_addr;
+  new_data2_size = bss_size_growth;
+  new_data2_size += alignof (ElfW (Shdr)) - 1;
+  new_data2_size -= new_data2_size % alignof (ElfW (Shdr));
+
   new_data2_offset = old_bss_offset;
 
 #ifdef UNEXELF_DEBUG
@@ -399,7 +403,8 @@ unexec (const char *new_name, const char *old_name)
   new_bss_seg->p_memsz = new_bss_seg->p_filesz;
 
   /* Copy over what we have in memory now for the bss area. */
-  memcpy (new_base + new_data2_offset, (caddr_t) old_bss_addr, new_data2_size);
+  memcpy (new_base + new_data2_offset, (caddr_t) old_bss_addr,
+	  bss_size_growth);
 
   /* Walk through all section headers, copying data and updating.  */
   for (n = 1; n < old_file_h->e_shnum; n++)
-- 
2.1.0