Re: [Xen-devel] [PATCH 2/2] xen/livepatch: Don't crash on encountering STN_UNDEF relocations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.06.17 at 12:13, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 14/06/17 11:11, Jan Beulich wrote:
>>>>> On 13.06.17 at 22:51, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/livepatch.c
>>> +++ b/xen/arch/x86/livepatch.c
>>> @@ -170,14 +170,22 @@ int arch_livepatch_perform_rela(struct livepatch_elf 
> *elf,
>>>          uint8_t *dest = base->load_addr + r->r_offset;
>>>          uint64_t val;
>>>  
>>> -        if ( symndx > elf->nsym )
>>> +        if ( symndx == STN_UNDEF )
>>> +            val = 0;
>>> +        else if ( symndx > elf->nsym )
>>>          {
>>>              dprintk(XENLOG_ERR, LIVEPATCH "%s: Relative relocation wants 
> symbol@%u which is past end!\n",
>>>                      elf->name, symndx);
>>>              return -EINVAL;
>>>          }
>>> -
>>> -        val = r->r_addend + elf->sym[symndx].sym->st_value;
>>> +        else if ( !elf->sym[symndx].sym )
>>> +        {
>>> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: No symbol@%u\n",
>>> +                    elf->name, symndx);
>>> +            return -EINVAL;
>>> +        }
>>> +        else
>>> +            val = r->r_addend + elf->sym[symndx].sym->st_value;
>> I don't understand this: st_value for STN_UNDEF is going to be zero
>> (so far there's also no extension defined for the first entry, afaict),
>> so there should be no difference between hard-coding the zero and
>> reading the symbol table entry. Furthermore r_addend would still
>> need applying. And finally "val" is never being cast to a pointer, and
>> hence I miss the connection to whatever crash you've been
>> observing.
> 
> elf->sym[0].sym is the NULL pointer.
> 
> ->st_value dereferences it.

Ah, but that is then what you want to change (unless we decide
to outright refuse STN_UNDEF, which still depends on why it's
there in the first place).

Jan


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