[FFmpeg-cvslog] r29509 - trunk/libswscale/swscale.c

[Måns Rullgård]

Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:

> On Fri, Aug 14, 2009 at 09:40:30AM +0100, M?ns Rullg?rd wrote:
>> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
>> 
>> > On Fri, Aug 14, 2009 at 02:01:12AM +0100, M?ns Rullg?rd wrote:
>> >> Ramiro Polla <ramiro.polla at gmail.com> writes:
>> >> 
>> >> > On Thu, Aug 13, 2009 at 4:24 PM, Reimar
>> >> > D?ffinger<Reimar.Doeffinger at gmx.de> wrote:
>> >> >> On Thu, Aug 13, 2009 at 08:29:54PM +0200, ramiro wrote:
>> >> >>> +#ifdef MAP_ANONYMOUS
>> >> >>> + ? ? ? ? ? ?mprotect(c->lumMmx2FilterCode, MAX_MMX2_FILTER_CODE_SIZE, PROT_EXEC | PROT_READ);
>> >> >>> + ? ? ? ? ? ?mprotect(c->chrMmx2FilterCode, MAX_MMX2_FILTER_CODE_SIZE, PROT_EXEC | PROT_READ);
>> >> >>
>> >> >> Both for mmap and mprotect: Why PROT_READ though? I don't think
>> >> >> EXEC without read is possible on most architectures, but still we
>> >> >> do not need read access in any case I think...
>> >> >
>> >> > Mans, or anyone else that knows an extensive amount of platforms, can
>> >> > you comment on this?
>> >> 
>> >> On ARM read permission is required to execute.  I see no harm in
>> >> always requesting both.
>> >
>> > mmap/mprotect can and will automatically "upgrade" the permissions.
>> 
>> On some systems perhaps, but these are arguably not strictly
>> conforming to spec.  This is from the spec page for mmap():
>> 
>>   If an implementation cannot support the combination of access types
>>   specified by prot, the call to mmap() shall fail.
>> 
>> It says the same thing about mprotect().  It is of course unreasonable
>> to expect the caller to know the precise combinations possible on
>> every bit of hardware, so adjusting to something it can do is a good
>> idea.  It's probably not compliant to the letter though.
>
> Uh, you forgot to quote the following sentence, which clarifies what
> the thing you quoted meant:
>> An implementation may permit accesses other than those specified by
>> prot;
>
> The case where it "shall fail" is e.g. a system where there is no memory
> that would allow both executing and reading the data in principle,
> then PROT_READ | PROT_EXEC mmap shall fail (instead of not giving one of
> the requested permissions).

That is probably the intent, but that's not what it says.

-- 
M?ns Rullg?rd
mans at mansr.com