[FFmpeg-devel] [PATCH]lavu/mem: Make alloc array functions more similar to av_malloc

[Carl Eugen Hoyos]

Am So., 12. Apr. 2020 um 23:58 Uhr schrieb James Almer <jamrial at gmail.com>:
>
> On 4/12/2020 6:53 PM, Carl Eugen Hoyos wrote:
> > Am So., 12. Apr. 2020 um 23:52 Uhr schrieb James Almer <jamrial at gmail.com>:
> >>
> >> On 4/12/2020 5:55 PM, Carl Eugen Hoyos wrote:
> >>> Am So., 12. Apr. 2020 um 22:48 Uhr schrieb James Almer <jamrial at gmail.com>:
> >>>>
> >>>> On 4/11/2020 8:53 PM, Carl Eugen Hoyos wrote:
> >>>>> Am So., 12. Apr. 2020 um 00:44 Uhr schrieb Carl Eugen Hoyos
> >>>>> <ceffmpeg at gmail.com>:
> >>>>>>
> >>>>>> Am So., 5. Apr. 2020 um 14:03 Uhr schrieb Michael Niedermayer
> >>>>>> <michael at niedermayer.cc>:
> >>>>>>>
> >>>>>>> On Sat, Apr 04, 2020 at 12:46:36AM +0200, Carl Eugen Hoyos wrote:
> >>>>>>>> Hi!
> >>>>>>>>
> >>>>>>>> Attached patch makes the alloc array functions more similar to
> >>>>>>>> av_malloc, depending on max_alloc_size instead of INT_MAX.
> >>>>>>>>
> >>>>>>>> Allows a work-around for ticket #7140
> >>>>>>>>
> >>>>>>>> Please comment, Carl Eugen
> >>>>>>>
> >>>>>>>>  mem.c |    8 ++++----
> >>>>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
> >>>>>>>> 507531ed6f0932834d005bc1dd7d18e762f158b2  0001-lavu-mem-Make-alloc-array-functions-more-similar-to-.patch
> >>>>>>>> From 7ae240a9f7885130251031aba5d0764b11947fec Mon Sep 17 00:00:00 2001
> >>>>>>>> From: Carl Eugen Hoyos <ceffmpeg at gmail.com>
> >>>>>>>> Date: Sat, 4 Apr 2020 00:37:03 +0200
> >>>>>>>> Subject: [PATCH] lavu/mem: Make alloc array functions more similar to
> >>>>>>>>  av_malloc().
> >>>>>>>>
> >>>>>>>> Do not limit the array allocation functions to allocations of INT_MAX,
> >>>>>>>> instead depend on max_alloc_size like av_malloc().
> >>>>>>>>
> >>>>>>>> Allows a workaround for ticket #7140.
> >>>>>>>> ---
> >>>>>>>>  libavutil/mem.c | 8 ++++----
> >>>>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
> >>>>>>>
> >>>>>>> av_size_mult() may be faster
> >>>>>>
> >>>>>> New patch attached.
> >>>>>
> >>>>> And an actually working variant.
> >>>>>
> >>>>> Please comment, Carl Eugen
> >>>>
> >>>>> From 643c501d6698d7d17e47a9f907165649f1446fa6 Mon Sep 17 00:00:00 2001
> >>>>> From: Carl Eugen Hoyos <ceffmpeg at gmail.com>
> >>>>> Date: Sun, 12 Apr 2020 00:36:30 +0200
> >>>>> Subject: [PATCH] lavu/mem: Make other alloc functions more similar to av_malloc().
> >>>>>
> >>>>> Do not limit the array allocation functions and av_calloc() to allocations
> >>>>> of INT_MAX, instead depend on max_alloc_size like av_malloc().
> >>>>>
> >>>>> Allows a workaround for ticket #7140.
> >>>>> ---
> >>>>>  libavutil/mem.c | 20 ++++++++++++--------
> >>>>>  1 file changed, 12 insertions(+), 8 deletions(-)
> >>>>>
> >>>>> diff --git a/libavutil/mem.c b/libavutil/mem.c
> >>>>> index 88fe09b179..e044374c62 100644
> >>>>> --- a/libavutil/mem.c
> >>>>> +++ b/libavutil/mem.c
> >>>>> @@ -183,23 +183,26 @@ int av_reallocp(void *ptr, size_t size)
> >>>>>
> >>>>>  void *av_malloc_array(size_t nmemb, size_t size)
> >>>>>  {
> >>>>> -    if (!size || nmemb >= INT_MAX / size)
> >>>>> +    size_t result;
> >>>>> +    if (av_size_mult(nmemb, size, &result) < 0)
> >>>>>          return NULL;
> >>>>> -    return av_malloc(nmemb * size);
> >>>>> +    return av_malloc(result);
> >>>>
> >>>> If I'm reading this right, when size is 0, instead of NULL this will now
> >>>> return av_malloc(0), which looks like it may end up being a pointer to a
> >>>> 1 byte big buffer. Is that intended?
> >>>>
> >>>> The previous version you sent apparently considered that scenario.
> >>>
> >>> But it did not pass fate because the behaviour before the patch
> >>> was not to return NULL for alloc(0).
> >>
> >> Before this patch it would return NULL when size was 0 and alloc(0) when
> >> nmemb was 0. Now it will return alloc(0) when either of the two
> >> arguments is 0.
> >>
> >> The check should be (!size || av_size_mult(nmemb, size, &result) < 0) or
> >> similar instead, if we want to keep the original behavior.
> >
> > How did the original behaviour make any sense?
>
> Not saying it made sense, i'm saying we changed that behavior when the
> patch, at least based on the description, only tried to replace the
> INT_MAX limit with max_alloc_size.
>
> If making size 0 return malloc(0) was intended, or ultimately preferred,
> then I'll not oppose to it.

To me, the old behaviour (returning NULL for some argument being 0
but not the other) made less sense than the new behaviour (not
special-casing 0 for any argument).
The fact that returning NULL broke fate surprised me but I failed
to find the reason.

Carl Eugen