Re: [Qemu-devel] [PATCH] hw/arm/realview.c: Fix memory leak in realview_init()

[Nikita Belov]

On 2014-10-29 19:03, Peter Maydell wrote:
> On 29 October 2014 14:03, Nikita Belov <address@hidden> wrote:
> > Variable 'ram_lo' is allocated unconditionally, but used only in some 
> > cases.
> > When it is unused pointer will be lost at function exit, resulting in 
> > a
> > memory leak. Free memory in this case.
> >
> > Valgrind output:
> > ==16879== 240 bytes in 1 blocks are definitely lost in loss record 
> > 6,033 of 7,018
> > ==16879==    at 0x4C2AB80: malloc (in 
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==16879==    by 0x33D2CE: malloc_and_trace (vl.c:2804)
> > ==16879==    by 0x509E610: g_malloc (in 
> > /lib/x86_64-linux-gnu/libglib-2.0.so.0.4000.0)
> > ==16879==    by 0x288836: realview_init (realview.c:55)
> > ==16879==    by 0x28988C: realview_pb_a8_init (realview.c:375)
> > ==16879==    by 0x341426: main (vl.c:4413)
> >
> > Signed-off-by: Nikita Belov <address@hidden>
> > ---
> >  hw/arm/realview.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/hw/arm/realview.c b/hw/arm/realview.c
> > index af65aa4..673a540 100644
> > --- a/hw/arm/realview.c
> > +++ b/hw/arm/realview.c
> > @@ -141,6 +141,8 @@ static void realview_init(MachineState *machine,
> >                                 &error_abort);
> >          vmstate_register_ram_global(ram_lo);
> >          memory_region_add_subregion(sysmem, 0x20000000, ram_lo);
> > +    } else {
> > +        g_free(ram_lo);
> >      }
> >
> >      memory_region_init_ram(ram_hi, NULL, "realview.highmem", 
> > ram_size,
>
> We leak all of the MemoryRegions we allocate here, because we
> don't have a persistent state struct to keep them in. This
> doesn't really matter much because they're generally needed
> for the lifetime of the QEMU process anyway, and we only call
> board init functions once. So why worry about ram_lo in
> particular (and why this board in particular)?
>
> thanks
> -- PMM

Indeed, generally we need memory regions for the lifetime of QEMU, but 
'mem_lo'
is different. It may not be used at all. We use 'ram_lo' only when a 
condition is
true, in other case we will lose this pointer. Because of that if the 
condition is
false we have memory leak immediately (not when QEMU exits).

Semantically the touched code looks like this:
  if (/* condition */) {
      /* ... */
      memory_region_init_ram(ram_lo, NULL, "realview.lowmem", 
low_ram_size);
      vmstate_register_ram_global(ram_lo);
      memory_region_add_subregion(sysmem, 0x20000000, ram_lo);
+ } else {
+     g_free(ram_lo);
  }
  /* ram_lo is not used any more */

> and why this board in particular?
I just memory leak where I found it.