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

[Pedro Alves <palves@redhat.com>]

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.

Thanks,
Pedro Alves