Re: [PATCH][gdb] fix unsigned overflow in charset.c

[John Baldwin <jhb@FreeBSD.org>]

On 10/16/18 8:58 AM, Pedro Alves wrote:
> On 10/11/2018 09:15 PM, John Baldwin wrote:
>> On 10/10/18 1:50 AM, Pedro Alves wrote:
>>> On 10/09/2018 08:58 PM, John Baldwin wrote:
>>>> On 10/9/18 11:10 AM, Paul Koning wrote:
>>>>>
>>>>>
>>>>>> On Oct 9, 2018, at 1:57 PM, John Baldwin <jhb@FreeBSD.org> wrote:
>>>>>>
>>>>>> On 10/9/18 10:40 AM, Paul Koning wrote:
>>>>>>>
>>>>>>>
>>>>>>>> On Oct 9, 2018, at 1:31 PM, Pedro Alves <palves@redhat.com> wrote:
>>>>>>>>
>>>
>>>>>> I also ran into the same failure using LLVM's ubsan on FreeBSD but in a different
>>>>>> use of obstack_blank_fast().  If we wanted to fix this, I wonder if we'd instead
>>>>>> want to fix it centrally in obstack_blank_fast (e.g. by using a ptrdiff_t cast)
>>>>>> rather than fixing various consumers of the API.  That would be a change to
>>>>>> libiberty though, not just gdb.
>>>>>
>>>>> I suppose.  But casts in macros scare me, they can hide mistakes.  It seems more reasonable to have the caller be responsible for creating a value of the correct type.  Since it's an adjustment, I suppose the cast should be for ptrdiff_t rather than ssize_t?
>>>>
>>>> So if obstack_blank_fast() were an inline function instead of a macro, I
>>>> suspect it's second argument would be of type ptrdiff_t in which case the
>>>> equivalent "hidden" cast would happen at the function call.  That said,
>>>> the obstack_blank() macro uses _OBSTACK_SIZE_T (which is an unsigned size_t)
>>>> when it declares a local variable to pass as the offset, so it seems obstack
>>>> really is relying on unsigned wrap around.
>>>
>>> The function is documented to take an int, at least:
>>>
>>>  void obstack_blank_fast (struct obstack *obstack-ptr, int size)
>>>
>>>  https://www.gnu.org/software/libc/manual/html_node/Summary-of-Obstacks.html
>>>
>>> Not sure what's best to do, but I think I leaning toward
>>> agreeing with Paul, in that passing down a signed negative
>>> integer rather than passing down a large unsigned integer
>>> expecting it to cast to a negative integer ends up
>>> being a little better.
>>
>> Ok.  Do you have a preference on the type to use (ssize_t vs ptrdiff_t vs
>> something else)?  Paul's original patch used ssize_t.  I'll probably patch
>> the one case I found in minsyms.c to match whatever we use here.
> 
> I don't really have much of a preference.
> 
> In practice, it probably doesn't make much of a difference nowadays.
> Likely ssize_t and ptrdiff_t have the same width on all supported
> hosts.
> 
> ssize_t is not standard C++ (it's standard POSIX), while ptrdiff_t is.
> OTOH, we already use ssize_t in gdb.  Pedantically incorrectly, I guess,
> if we follow the letter of the original ssize_t intention [1]:
> 
>   The type ssize_t shall be capable of storing values at least in the range [-1, {SSIZE_MAX}].
> 
> [1] - http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_types.h.html
> 
> From an aesthetic perspective, "ssize_t" seems better, as the "obvious
> signed version of size_t".  From a pedantic perspective, ptrdiff_t
> sounds better.

Ok, I think ssize_t is probably fine, so I Think Paul's original patch is
ok?

-- 
John Baldwin