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Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference
- From: Jeff Law <law at redhat dot com>
- To: Jonathan Wakely <jwakely dot gcc at gmail dot com>, Florian Weimer <fweimer at redhat dot com>
- Cc: Sandra Loosemore <sandra at codesourcery dot com>, Jakub Jelinek <jakub at redhat dot com>, Jeff Prothero <jprother at altera dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Fri, 20 Feb 2015 10:01:36 -0700
- Subject: Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference
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- References: <pdf61azt48b dot fsf at sj-interactive3 dot altera dot com> <20150218192943 dot GR1746 at tucnak dot redhat dot com> <54E64DFF dot 8030100 at codesourcery dot com> <54E71534 dot 8070805 at redhat dot com> <CAH6eHdT3jPVY-5n009r9xyRkhXUQkPkAN5cPGJEL+DREREO_+A at mail dot gmail dot com>
On 02/20/15 04:43, Jonathan Wakely wrote:
But that's always true -- this isn't any different than aliasing,
arithmetic overflow, etc. The standards define the contract between
the compiler/library implementors and the developers. Once the contract
is broken, all bets are off.
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.
const unsigned char *source;
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.