Subject: Re: bug#20614: Segmentation fault when building on Power8 Little Endian

From: Petr Hracek <[email-masked]>
Date: Mon, 5 Oct 2015 13:23:42 +0200

On 10/05/2015 11:39 AM, Eli Zaretskii wrote:
>> Cc: [email-masked]
>> From: Petr Hracek <[email-masked]>
>> Date: Mon, 5 Oct 2015 11:16:32 +0200
>>
>> (gdb) p old_section_names + NEW_SECTION_H (nn).sh_name
>> $2 = 0x4000128ceb51 <Address 0x4000128ceb51 out of bounds>
> We have a function valid_pointer_p in alloc.c.  What happens if you
> use the same code in unexec, before the series of calls to strcmp that
> crash, and only proceed to call strcmp if valid_pointer_p returns a
> positive value?
I have no problem to do it.
I have made a this patch against emacs-24.3
diff --git a/src/unexelf.c b/src/unexelf.c
index d365940..677a1af 100644
--- a/src/unexelf.c
+++ b/src/unexelf.c
@@ -389,6 +389,7 @@ temacs:
  #include <unexec.h>

  extern void fatal (const char *msgid, ...);
+extern int valid_pointer_p (void *);

  #include <sys/types.h>
  #include <stdint.h>
@@ -845,8 +846,7 @@ unexec (const char *new_name, const char *old_name)
        ElfW (Word) alignment = (NEW_PROGRAM_H (n)).p_align;
        if ((OLD_SECTION_H (old_bss_index)).sh_addralign > alignment)
      alignment = OLD_SECTION_H (old_bss_index).sh_addralign;
-
-#ifdef __sgi
+#if defined __sgi || defined __powerpc64__
        /* According to [email-masked] (Karsten Kuenne)
           and [email-masked] (Alexandre Oliva), on IRIX 5.2, we
           always get "Program segment above .bss" when dumping
@@ -866,8 +866,10 @@ unexec (const char *new_name, const char *old_name)
            == round_up (old_bss_addr, alignment)))
      break;
      }
+#ifndef __powerpc64__
    if (n < 0)
      fatal ("Couldn't find segment next to .bss in %s\n", old_name, 0);
+#endif

    /* Make sure that the size includes any padding before the old .bss
       section.  */
@@ -1251,6 +1253,12 @@ temacs:
           be no harm in that provided that r_offset is always the first
           member.  */
        nn = section.sh_info;
+      valid_pointer_p (old_section_names + NEW_SECTION_H (nn).sh_name);
        if (!strcmp (old_section_names + NEW_SECTION_H (nn).sh_name, 
".data")
            || !strcmp ((old_section_names + NEW_SECTION_H (nn).sh_name),
                ".sdata")

But if fails with
/home/phracek/rpmbuild/BUILD/emacs-24.3/src/unexelf.c:1263: undefined 
reference to `valid_pointer_p'
collect2: error: ld returned 1 exit status
make[2]: *** [temacs] Error 1
make[2]: Leaving directory `/home/phracek/rpmbuild/BUILD/emacs-24.3/src'
make[1]: *** [src] Error 2
make[1]: Leaving directory `/home/phracek/rpmbuild/BUILD/emacs-24.3'
make: *** [bootstrap] Error 2
error: Bad exit status from /var/tmp/rpm-tmp.QoDw34 (%build)

-- 
Petr Hracek
Software Engineer
Developer Experience
Red Hat, Inc
Mob: +420777056169
email: [email-masked]