[PATCH] Handle loading improper core files gracefully in the mips backend.

[Luis Machado]

On 01/12/2016 10:46 AM, Pedro Alves wrote:
> On 01/11/2016 03:47 PM, Luis Machado wrote:
>> diff --git a/gdb/mips-tdep.c b/gdb/mips-tdep.c
>> index ca17864..cdfd80e 100644
>> --- a/gdb/mips-tdep.c
>> +++ b/gdb/mips-tdep.c
>> @@ -8208,6 +8208,12 @@ mips_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
>>     int dspacc;
>>     int dspctl;
>>
>> +  /* Sanity check the e_machine field.  */
>> +  if (info.abfd
>> +      && bfd_get_flavour (info.abfd) == bfd_target_elf_flavour
>> +      && elf_elfheader (info.abfd)->e_machine != EM_MIPS)
>> +    return NULL;
>
> This callback is registered for bfd_arch_mips:
>
>    gdbarch_register (bfd_arch_mips, mips_gdbarch_init, mips_dump_tdep);
>
> Does bfd think this a bfd_arch_mips binary?  How so?

In the second time we call gdbarch_info_fill, when opening the core file 
alone, we have this:

p *info
$8 = {bfd_arch_info = 0x0, byte_order = BFD_ENDIAN_UNKNOWN, 
byte_order_for_code = BFD_ENDIAN_UNKNOWN, abfd = 0xe1ce80, tdep_info = 
0x0, osabi = GDB_OSABI_UNINITIALIZED, target_desc = 0x0}

p *info->abfd->arch_info
$10 = {bits_per_word = 32, bits_per_address = 32, bits_per_byte = 8, 
arch = bfd_arch_unknown, mach = 0, arch_name = 0x9b799f "unknown", 
printable_name = 0x9b799f "unknown", section_align_power = 2, 
the_default = 1, compatible = 0x78a592 <bfd_default_compatible>,
   scan = 0x78a60a <bfd_default_scan>, fill = 0x78acc6 
<bfd_arch_default_fill>, next = 0x0}

p *default_bfd_arch
$12 = {bits_per_word = 32, bits_per_address = 32, bits_per_byte = 8, 
arch = bfd_arch_mips, mach = 0, arch_name = 0x9d98e0 "mips", 
printable_name = 0x9d98e0 "mips", section_align_power = 3, the_default = 
1, compatible = 0x832b40 <mips_compatible>,
   scan = 0x78a60a <bfd_default_scan>, fill = 0x78acc6 
<bfd_arch_default_fill>, next = 0x9d9b00 <arch_info_struct>}

The data above leads gdbarch_info_fill to assign default_bfd_arch to 
info->bfd_arch_info here:

   /* From the default.  */
   if (info->bfd_arch_info == NULL)
     info->bfd_arch_info = default_bfd_arch;

So the core file essentially turns into a mips-compatible core file. 
This also happens with a powerpc-targeted gdb and likely any other 
architecture.

For powerpc we get lucky and end up "passing" this test because it has 
no fatal failing conditions. It ends up displaying frame -1 for me, like so:

PC not available^M
#-1 <unavailable> in ?? ()