From: Florian Weimer <fweimer@redhat.com>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: Sandra Loosemore <sandra@codesourcery.com>,
Jakub Jelinek <jakub@redhat.com>,
Jeff Prothero <jprother@altera.com>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference
Date: Fri, 20 Feb 2015 12:05:00 -0000 [thread overview]
Message-ID: <54E7230D.8090102@redhat.com> (raw)
In-Reply-To: <CAH6eHdT3jPVY-5n009r9xyRkhXUQkPkAN5cPGJEL+DREREO_+A@mail.gmail.com>
On 02/20/2015 12:43 PM, Jonathan Wakely wrote:
> On 20 February 2015 at 11:06, Florian Weimer wrote:
>> On 02/19/2015 09:56 PM, Sandra Loosemore wrote:
>>> Hmmmm, Passing the additional option in user code would be one thing,
>>> but what about library code? E.g., using memcpy (either explicitly or
>>> implicitly for a structure copy)?
>>
>> The memcpy problem isn't restricted to embedded architectures.
>>
>> size_t size;
>> const unsigned char *source;
>> std::vector<char> vec;
>> â¦
>> vec.resize(size);
>> memcpy(vec.data(), source, size);
>>
>> std::vector<T>::data() can return a null pointer if the vector is empty,
>> which means that this code is invalid for empty inputs.
>>
>> I think the C standard is wrong here. We should extend it, as a QoI
>> matter, and support null pointers for variable-length inputs and outputs
>> if the size is 0. But I suspect this is still a minority view.
>
> I'm inclined to agree.
>
> Most developers aren't aware of the preconditions on memcpy, but GCC
> optimizes aggressively based on those preconditions, so we have a
> large and potentially dangerous gap between what developers expect and
> what actually happens.
Maybe we can add, as a compromise, an always-inline wrapper like this?
void *memcpy(void *dst, const *void src, size_t size)
{
if (__builtin_constant_p(size > 0) && size > 0) {
// Or whatever else is needed as non-null assertions.
*(char *)dst;
*(const char *)src;
}
return memcpy_real(dst, src, size); // Without non-null assertion.
}
Then we'll still get the non-NULL optimization for the common positive
size case.
--
Florian Weimer / Red Hat Product Security
next prev parent reply other threads:[~2015-02-20 12:05 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-18 19:24 Jeff Prothero
2015-02-18 19:29 ` Jakub Jelinek
2015-02-19 20:56 ` Sandra Loosemore
2015-02-19 21:16 ` Daniel Gutson
2015-02-19 22:10 ` Jakub Jelinek
2015-02-19 22:26 ` Sandra Loosemore
2015-02-19 21:23 ` Joel Sherrill
2015-02-19 21:56 ` Chris Johns
2015-02-20 17:30 ` Jeff Law
2015-02-26 16:23 ` David Malcolm
2015-02-27 20:55 ` [RFC/patch for stage1] Embed compiler dumps into generated .o files (was Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference) David Malcolm
2015-02-20 11:06 ` Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference Florian Weimer
2015-02-20 11:43 ` Jonathan Wakely
2015-02-20 12:05 ` Florian Weimer [this message]
2015-02-20 17:01 ` Jeff Law
2015-02-20 17:09 ` Florian Weimer
2015-02-20 17:32 ` Jeff Law
2015-02-20 18:01 ` Paul_Koning
2015-02-20 12:11 ` Jakub Jelinek
2015-02-20 17:03 ` Jeff Law
2015-03-03 19:57 ` Martin Sebor
2015-03-03 23:38 ` Jeff Law
2015-03-04 12:36 ` Richard Biener
2015-02-20 12:13 ` Andrew Haley
2015-02-20 17:03 ` Jeff Law
2015-02-18 19:30 ` Andrew Pinski
2015-02-20 9:30 ` Andrew Haley
2015-02-20 11:45 ` Florian Weimer
2015-02-20 17:01 ` Jeff Law
2015-02-20 18:07 ` Paul_Koning
2015-02-20 1:04 Jeff Prothero
2015-02-27 22:13 Manuel López-Ibáñez
2015-03-03 8:41 ` Chris Johns
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