Re: [Valgrind-users] can memchecker mark memory as read-only ?

[John R. <jr...@bi...>]

> Currently, my understanding is that valgrind memchecker can mark a
> memory area as non accessible via the VALGRIND_MAKE_MEM_NOACCESS()
> macro, but there is currently no way to mark it as read-only.

Yes. READ_ONLY is not a recognized state.

> if yes, is there any plan to develop such a feature ?

No.  The accounting bits are organized into two phases: is the address legal
at all, and is the byte writable and initialized.  Adding a new state
READ_ONLY would be a large task with significant runtime costs.
It might be best applied to the first phase, but complications
regarding overlap [or splitting] between existing regions and new READ_ONLY
regions, plus granularity assumptions (pages now, bytes for READ_ONLY)
probably would be messy.

>>From an OpenMPI developer point of view, it would be nice to mark the send
> buffer as read-only between MPI_Isend() and MPI_Wait() so valgrind can trap
> incorrect user code that modifies the send buffer when it is not allowed.

Use LD_PRELOAD to intercept MPI_Isend() [etc.].  Allocate a new buffer,
copy the message into it, mark the original buffer as NOACCESS, call the
original MPI_Isend on the new buffer. Reverse the effects after
the original MPI_Isend() is complete, waited, and checked.
See the manual page for "man dlsym" and the RTLD_NEXT functionality.
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