Re: [PATCH] libstdc++: Check [ptr,end) and [ptr,ptr+n) ranges with _GLIBCXX_ASSERTIONS

["François Dumont" <frs.dumont@gmail.com>]

On 14/10/21 7:43 pm, Jonathan Wakely wrote:
> On Thu, 14 Oct 2021 at 18:11, François Dumont <frs.dumont@gmail.com> wrote:
>> Hi
>>
>>       On a related subject I am waiting for some feedback on:
>>
>> https://gcc.gnu.org/pipermail/libstdc++/2021-August/053005.html
> I'm concerned that this adds too much overhead for the
> _GLIBCXX_ASSERTIONS case. It adds function calls which are not
> necessarily inlined, and which perform arithmetic and comparisons on
> the arguments. That has a runtime cost which is non-zero.

I thought that limiting the checks to __valid_range would be fine for 
_GLIBCXX_ASSERTIONS. If you do not want any overhead you just don't 
define it.

>
> The patches I sent in this thread have zero runtime cost, because they
> use the compiler built-in which compiles away to nothing if the sizes
> aren't known.
I'll try to find out if it can help for the test case on std::copy which 
I was adding with my proposal.
>
>> On 11/10/21 6:49 pm, Jonathan Wakely wrote:
>>> This enables lightweight checks for the __glibcxx_requires_valid_range
>>> and __glibcxx_requires_string_len macros  when _GLIBCXX_ASSERTIONS is
>>> defined.  By using __builtin_object_size we can check whether the end of
>>> the range is part of the same object as the start of the range, and
>>> detect problems like in PR 89927.
>>>
>>> libstdc++-v3/ChangeLog:
>>>
>>>        * include/debug/debug.h (__valid_range_p, __valid_range_n): New
>>>        inline functions using __builtin_object_size to check ranges
>>>        delimited by pointers.
>>>        [_GLIBCXX_ASSERTIONS] (__glibcxx_requires_valid_range): Use
>>>        __valid_range_p.
>>>        [_GLIBCXX_ASSERTIONS] (__glibcxx_requires_string_len): Use
>>>        __valid_range_n.
>>>
>>>
>>> The first patch allows us to detect bugs like string("foo", "bar"),
>>> like in PR 89927. Debug mode cannot currently detect this. The new
>>> check uses the compiler built-in to detect when the two arguments are
>>> not part of the same object. This assumes we're optimizing and the
>>> compiler knows the values of the pointers. If it doesn't, then the
>>> function just returns true and should inline to nothing.
>> I see, it does not detect that input pointers are unrelated but as they
>> are the computed size is >= __sz.
>>
>> Isn't it UB to compare unrelated pointers ?
> Yes, and my patch doesn't compare any pointers, does it?
>
+      __UINTPTR_TYPE__ __f = (__UINTPTR_TYPE__)__first;
+      __UINTPTR_TYPE__ __l = (__UINTPTR_TYPE__)__last;
+      if (const std::size_t __sz = __builtin_object_size(__first, 3))
+    return __f <= __l && (__l - __f) <= __sz;

Isn't it a comparison ?

But maybe this is what the previous cast is for, I never understood it.

Note that those cast could be moved within the if branch, even if I 
guess that the compiler does it.