Bug#806667: emacs24: FTBFS on sparc64, Bus Error during lucid build

[John Paul Adrian Glaubitz]

Hi David!

On 11/29/2015 09:47 PM, David Matthew Mattli wrote:
> I've attached a patch that corrects an unaligned pointer dereference
> in src/unexelf.c. But my fix is more of a hack and probably not suitable
> for submitting to upstream.

Thanks, great work! I'll upload a patched version to 'unreleased'
which will allow packages which build-depend on emacs24 to be
built.

> I'm going to continue looking at this issue to try to create a 
> more robust fix to offer upstream.

Maybe try contacting upstream right away with a bug report. They
might be able to find the proper fix faster.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaub...@debian.org
`. `'   Freie Universitaet Berlin - glaub...@physik.fu-berlin.de
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Bug#806667: emacs24: FTBFS on sparc64, Bus Error during lucid build

[David Matthew Mattli]

Source: emacs24
Severity: normal
Tags: upstream patch
User: debian-sp...@lists.debian.org
Usertags: sparc64

Dear Maintainer,

The emacs24 build is currently failing on sparc64 with a Bus
Error. You can see an example build log here:

https://buildd.debian.org/status/fetch.php?pkg=emacs24=sparc64=24.5%2B1-5=1448688813

I've attached a patch that corrects an unaligned pointer dereference
in src/unexelf.c. But my fix is more of a hack and probably not suitable
for submitting to upstream.

I'm going to continue looking at this issue to try to create a 
more robust fix to offer upstream.

Below is a short explaination of what I've found looking into this
build failure.


The lucid build is failing with this error:

Loading /«BUILDDIR»/emacs24-24.5+1/debian/build-lucid/lisp/electric.el 
(source)...
Loading /«BUILDDIR»/emacs24-24.5+1/debian/build-lucid/lisp/tooltip.el 
(source)...
Finding pointers to doc strings...
Finding pointers to doc strings...done
Dumping under the name emacs
/bin/bash: line 7: 19090 Bus error   ./temacs --batch --load loadup 
bootstrap
make[3]: *** [bootstrap-emacs] Error 1
Makefile:815: recipe for target 'bootstrap-emacs' failed
make[3]: Leaving directory '/«BUILDDIR»/emacs24-24.5+1/debian/build-lucid/src'
make[2]: *** [src] Error 2
Makefile:389: recipe for target 'src' failed

Loading 'temacs' in gdb the bus error occurs in the file
src/unexelf.c, around line 980:

  */

  if (NEW_SECTION_H (nn).sh_offset >= old_bss_offset
  || (NEW_SECTION_H (nn).sh_offset + NEW_SECTION_H (nn).sh_size
  > new_data2_offset))
NEW_SECTION_H (nn).sh_offset += new_data2_incr;

With this instruction:

0x294c20   ldx  [ %g4 + 0x18 ], %g2

   // info reg g4 => 0x80011458013b -140732852076229

So it's trying to load an extended word (64bits, 8 bytes) from an
unaligned address.

NEW_SECTION_H is a macro that generates a pointer offset from
'new_section_h'.

static void * 
   entry_address (void *section_h, ptrdiff_t idx, ptrdiff_t entsize)
{
  char *h = section_h;
  return h + idx * entsize;
}

#define NEW_SECTION_H(n) \
  (*(ElfW (Shdr) *) entry_address (new_section_h, n, 
new_file_h->e_shentsize))

If 'new_section_h' is properly aligned this should be ok because it's
adding multiples of 'e_shentsize' which is 8. But new_section_h is
/not/ aligned so the resulting pointer is also not aligned.

Here is how 'new_section_h' is computed:

new_section_h = (ElfW (Shdr) *)
((byte *) new_base + old_file_h->e_shoff + new_data2_incr);

GDB says the components of this sum have these values:

(gdb) print new_base
$6 = (caddr_t) 0x8001126aa000 "\177ELF\002\002\001"
(gdb) print old_file_h->e_shoff
$7 = 15751616
(gdb) print new_data2_incr
$8 = 16582459


'new_base' and 'old_file_h->e_shoff' are both 8 byte aligned but
'new_data2_incr' is not.

'new_data2_incr' is calculated this way:

new_data2_incr = new_data2_size + (new_data2_offset - old_bss_offset);

It seems like 'new_data2_incr' is just an offset so it would be ok to
round it up to the nearest multiple of 8, using a function already
used in the file:

new_data2_incr = round_up(new_data2_incr, (ElfW (Addr))8);

Adding this line to src/unexelf.c allows the build to finish normally. 

-- System Information:
Debian Release: stretch/sid
  APT prefers unreleased
  APT policy: (500, 'unreleased'), (500, 'unstable')
Architecture: sparc64

Kernel: Linux 4.3.0-gentoo (SMP w/1 CPU core)
Locale: LANG=en_SG.UTF-8, LC_CTYPE=en_SG.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: unable to detect
--- a/src/unexelf.c
+++ b/src/unexelf.c
@@ -779,6 +779,9 @@
  the end of the old .data section (and thus the offset of the .bss
  section) was unaligned.  */
   new_data2_incr = new_data2_size + (new_data2_offset - old_bss_offset);
+#ifdef __sparc_v9__
+  new_data2_incr = round_up(new_data2_incr, (ElfW (Addr))8);
+#endif
 
 #ifdef UNEXELF_DEBUG
   fprintf (stderr, "old_bss_index %td\n", old_bss_index);