Re: [Valgrind-users] memcheck question

[Philippe W. <phi...@sk...>]

On Mon, 2016-08-29 at 07:28 -0700, Mark Roberts wrote:
> The C/C++ front end to our tool Daikon includes most of Valgrind’s
> memcheck code.  We monitor the execution of a user’s program and
> record the values seen for various program variables.  As we follow
> pointer variables, we need to make sure they point to valid memory
> before we attempt to read the contents.  We have seen very
> intermittent failures in the code, but I think I have finally got a
> repeatable one.  Now to my question.
> 
>  
> 
> We are using “is_mem_defined” (in mc_main.) to test for “is it safe to
> read this location?” and that does not appear to be quite right.  It
> looks as though an address in a code segment is defined, but not
> readable.  We get a SIGSEGV with a bad permissions message when we
> try.  Whether or not that is the correct analysis is not as important
> as the primary question:
> 
>  
> 
> What is the proper way to check to see if an address is readable?
> And, similarly, is writable?

Memcheck VA bits are only maintaining addressibility and, when
addressable the definedness.
The memcheck bits do not allow to make the distinction based
on protection (r and/or w and/or x).

Checking the protection can be done with VG_(am_is_valid_for_client).
This check is implemented by the address space manager, that
maintains the address space segments and protections.

However, in case very strange things/bugs are done by the application,
possibly, the address space manager state is desynchronised
from the real state.

You might be interested to read mc_leakcheck.c heuristic
and memory scan, that are protecting themselves against
dereferencing 'invalid pointers'.
You might also read the test memcheck/tests/leak-segv-jmp.c
which verifies the behaviour when such strange things happens.


Philippe