On Thu, Jun 4, 2015 at 7:00 AM, Michael Niedermayer <[email protected]> wrote:
> On Wed, Jun 03, 2015 at 10:05:54PM -0400, Ganesh Ajjanagadde wrote:
>> While compiling ffmpeg, I noticed a bunch of -Warray-bounds warnings.
>> I think it would be great if ffmpeg could be built warning-free, and
>> -Warray-bounds seems to be one of the biggest culprits. Nevertheless,
>
>> -Warray-bounds is quite useful in most cases.
>
> how many of the warnings shown are real and how many are false
> positives (in some version not neccessarily the latest of FFmpeg)
> ?
So I checked out n2.5 release. There are too many warnings to quickly
audit all of them.
Nevertheless, I went through about 6 instances (randomly selected),
and found all of them to be false positives.
Basically, there is a basic pattern to all of these warnings:
struct foo {
int bar[MAX_BAR];
int num_bar;
}; // could have other declarations as well
// in usage
int set_foo(foo* my_foo) {
my_foo->num_bar = get_num_bar();
if (num_bar > MAX_BAR)
return -1;
}
int loop_over(const foo* my_foo) {
int i;
for(i = 0; i < foo->num_bar; i++) {
// do something with foo->bar[i]
// this triggers the warning: GCC tries to do a constant
propagation to verify array in bounds, since it knows MAX_BAR, i.e
foo->bar is an array.
// However, it can't do deep enough analysis to verify that in
set_foo, num_bar is sanitized to ensure proper in bounds access.
}
}
>
>
> [...]
>
> --
> Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
>
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