Re: [PATCH] tools/lib/perf: Fix -Werror=alloc-size-larger-than in cpumap.c

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Fri, May 02, 2025 at 01:14:32PM +0530, Mukesh Kumar Chaurasiya wrote:
> On Fri, Apr 25, 2025 at 02:46:43PM -0300, Arnaldo Carvalho de Melo wrote:
> > Maybe that max() call in perf_cpu_map__intersect() somehow makes the
> > compiler happy.

> > And in perf_cpu_map__alloc() all calls seems to validate it.
 
> > Like:

> > +++ b/tools/lib/perf/cpumap.c
> > @@ -411,7 +411,7 @@ int perf_cpu_map__merge(struct perf_cpu_map **orig, struct perf_cpu_map *other)
> >         }
> >  
> >         tmp_len = __perf_cpu_map__nr(*orig) + __perf_cpu_map__nr(other);
> > -       tmp_cpus = malloc(tmp_len * sizeof(struct perf_cpu));
> > +       tmp_cpus = calloc(tmp_len, sizeof(struct perf_cpu));
> >         if (!tmp_cpus)
> >                 return -ENOMEM;

> > ⬢ [acme@toolbx perf-tools-next]$

> > And better, do the max size that the compiler is trying to help us
> > catch?

> Isn't it better to use perf_cpu_map__nr. That should fix this problem.

Maybe, have you tried it?
 
> One question I have, in perf_cpu_map__nr, the function is returning
> 1 in case *cpus is NULL. Is it ok to do that? wouldn't it cause problems?

Indeed this better be documented, as by just looking at:

int perf_cpu_map__nr(const struct perf_cpu_map *cpus)
{
        return cpus ? __perf_cpu_map__nr(cpus) : 1;
}

It really doesn't make much sense to say that a NULL map has one entry.

But the next functions are:

bool perf_cpu_map__has_any_cpu_or_is_empty(const struct perf_cpu_map *map)
{
        return map ? __perf_cpu_map__cpu(map, 0).cpu == -1 : true;
}

bool perf_cpu_map__is_any_cpu_or_is_empty(const struct perf_cpu_map *map)
{
        if (!map)
                return true;

        return __perf_cpu_map__nr(map) == 1 && __perf_cpu_map__cpu(map, 0).cpu == -1;
}

bool perf_cpu_map__is_empty(const struct perf_cpu_map *map)
{
        return map == NULL;
}

So it seems that a NULL cpu map means "any/all CPU) and a map with just
one entry would have as its content "-1" that would mean "any/all CPU".

Ian did work on trying to simplify/clarify this, so maybe he can chime
in :-)

- Arnaldo