On Thu, 2017-10-19 at 12:06 -0400, Zack Weinberg wrote: > I can work around this by forcibly adjusting valgrind's state for the > "data" object (compile with -DDO_MAKE_MEM_WRITABLE to see) but I > would > like to understand *why* this memory has become unwritable, why the > STACK_REGISTER/DEREGISTER was insufficient, and whether there's a > more > appropriate work-around.
STACK_REGISTER (STACK_DEREGISTER) is just marking a piece of memory as being a stack (not being a stack anymore). This is a.o. used to help unwind logic, provide better description of an address in error messages, etc ... These do not change the Addressable or Validity-bits of the piece of memory. So, after a piece of memory has been used as a stack, depending on how much of this stack was used, the 'normal' valgrind memcheck code has marked some memory unaccessible Typically, when a function returns, the space that was used by the frame of the returning function is marked as unaccessible. When you deregister the stack, these V-bits/A-bits are not changed, and so part of this memory is now marked as not addressable. IMO, before switching to a new stack, it would be better to mark the stack as not addressable (maybe a small part of the stack must be kept addressable, to just allow switching to this stack). Marking the stack as unaddressable would help memcheck to detect e.g. a dereference to a 'not valid' part of the stack. And when a stack is not used anymore, it might be needed (like in your case) to mark the memory as addressable again. So, I think that what you see is all normal : STACK_REGISTER (STACK_DEREGISTER) are low level features, they do not do the whole valgrind magic that might be needed e.g. for your memset after usage. And I think that your usage of VALGRIND_MAKE_MEM_UNDEFINED is the correct thing to do. Philippe ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Valgrind-users mailing list Valgrind-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/valgrind-users
