Re: [RFC PATCH 3/3] fuzz: Add callbacks for dma-access functions

[Alexander Bulekov <alxndr@bu.edu>]

On 200623 1514, Stefan Hajnoczi wrote:
> On Thu, Jun 11, 2020 at 01:56:51AM -0400, Alexander Bulekov wrote:
> > Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> > ---
> >  exec.c                                | 17 ++++++++++++++++-
> >  include/exec/memory.h                 |  8 ++++++++
> >  include/exec/memory_ldst_cached.inc.h |  9 +++++++++
> >  include/sysemu/dma.h                  |  5 ++++-
> >  memory_ldst.inc.c                     | 12 ++++++++++++
> >  5 files changed, 49 insertions(+), 2 deletions(-)
> 
> Please rename dma_read_cb() to fuzz_dma_read_cb() so the purpose of the
> function is clear.
> 
> The ifdefs can be avoided by defining an empty function when CONFIG_FUZZ
> is undefined. In a header file:
> 
>   #ifdef CONFIG_FUZZ
>   void fuzz_dma_read_cb(size_t addr, size_t len);
>   #else
>   static inline void fuzz_dma_read_cb(size_t addr, size_t len)
>   {
>       /* Do nothing */
>   }
>   #endif
> 
> Now the compiler should eliminate the deadcode:
> 
>   #ifdef CONFIG_FUZZ
>   if (as->root == get_system_memory()) {
>       dma_read_cb(addr, len);
>   }
>   #endif
> 
> becomes:
> 
>   if (as->root == get_system_memory()) {
>       fuzz_dma_read_cb(addr, len);
>   }
> 
> Hopefully gcc and clang will eliminate this and emit no instructions
> when CONFIG_FUZZ is undefined. If not, you can simply pass in 'as' and
> 'is_write' too:
> 
>   void fuzz_dma_read_cb(AddressSpace *as, bool is_write, size_t addr, size_t len)
> 
> This way the conditional is moved inside fuzz_dma_read_cb() and deadcode
> elimination becomes trivial for the compiler:
> 
>   fuzz_read_cb(as, is_write, addr, len);

Do you think it would be better to have a "trace_dma_read" or
"trace_device_read_from_guest_memory"? I haven't looked under the hood
too much for the tracepoint api, but these would just be standard
tracepoints(disabled for the majority of builds). When we build the
fuzzer, we could compile with --wrap="trace_dma_read" and implement
a __wrap_trace_dma_read in the generic fuzzer. I looked at the symbols
for a qemu build and it looks like trace_* are actual functions, rather
than preprocessor magic, so maybe this could work?
-Alex