[Rpm-maint] [rpm-software-management/rpm] debug edit build generation is endian dependent when cross-compiling? (#171)

[Mark Wielaard]

On Sun, 2017-03-05 at 09:48 -0800, Jeff Johnson wrote:
> For rpm portability with older versions of elfutils, this harmless sanity check might be useful:
> 
> (excuse the different hash functions in use in the patch snippet below)
> @@ -1533,40 +1493,12 @@
>    /* Now format the build ID bits in hex to print out.  */
>    {   
>      const uint8_t * id = (uint8_t *)build_id->d_buf + build_id_offset;
> -    char hex[build_id_size * 2 + 1];
> -    int n = snprintf (hex, 3, "%02" PRIx8, id[0]);
> -    assert (n == 2);
> -    for (i = 1; i < (int)build_id_size; ++i)
> -      {
> -       n = snprintf (&hex[i * 2], 3, "%02" PRIx8, id[i]);
> -       assert (n == 2);
> -      }
> +    char *hex = pgpHexStr(id, build_id_size);
>      puts (hex);
> +    free(hex);
>    }   
>  }

I don't understand this hunk. What is it attempting to do?

Also I assume you posted the reverse patch?

> -/* It avoided the segment fault while file's bss offset have a large number.
> -   See https://bugzilla.redhat.com/show_bug.cgi?id=1019707
> -       https://bugzilla.redhat.com/show_bug.cgi?id=1020842 for detail. */

This is indeed a bug in elfutils, fixed in elfutils 0.162.
But also only occurs for bogus/bad ELF files in the first place.

> -void valid_file(int fd)
> -{
> -  Elf *elf = elf_begin (fd, ELF_C_RDWR, NULL);
> -  if (elf == NULL)
> -  {
> -    error (1, 0, "elf_begin: %s", elf_errmsg (-1));
> -    return;
> -  }
> -
> -  elf_flagelf (elf, ELF_C_SET, ELF_F_LAYOUT);
> -    
> -  if (elf_update (elf, ELF_C_WRITE) < 0)
> -    error (1, 0, "elf_update: %s", elf_errmsg (-1));
> -
> -  elf_end (elf);
> -
> -  return;
> -}

So this opens and closes the ELF file and tries to write it out to disk,
but since nothing changed all this does is make libelf do a layout check
of all headers. Where it would notice nothing changed, so ELF_C_WRITE
would not actually write something out. Except if it does see a
discrepancy and might "correct" a header field. It sounds slightly safer
to do an ELF_C_UPDATE so that nothing might accidentally be written out.

Although it might indeed catch the issue with the bogus ELF file earlier
(and so prevent a crash with a buggy elfutils) I am not sure it is
really the task of rpm/debugedit to make sure the user doesn't have
bogus/bad ELF files and/or an old/buggy libelf implementation. But if
you don't mind opening/closing/reading the ELF file twice this might
indeed not be that expensive (compared to all the other work debugedit
does). It is somewhat bogus and obscure though.

Cheers,

Mark