On Tue, Aug 12, 2003 at 06:24:47PM -0400, Camm Maguire wrote: > GCL stores addresses of its functions in data structures allocated in > the normal way in its .data section. It then runs an unexec routine > which saves the memory image including these addresses to a file. > When the file is then later run against ostensibly compatible but > different shared library versions, occasionally the stored function > addresses will no longer point to the correct function. > > One mechanism I've identified leading to this behavior is that on > ia64, for example, passing a function address as an argument to > another function which stores it in a saved variable does not actually > pass the address, but rather an address to some region governed > directly by ld.so, at which location the desired function address is > to be found.
Right. ia64, parisc and ppc64 all use function descriptors (aka fat function pointers). > Somehow the compiler has instructed the code to look in this ld.so > space for the function address, and adopts the convention that when > one calls the function via (*fn)(...), one really calls the address > pointed to by fn as opposed to fn itself. > > So when for example I run the program against two different but > compatible versions of shared libraries, ldd reports a different > mapping on ia64 in the 0x20000... range, and while fLformat has the > same address 0x40000000001a9390 in both instances, and while the > stored address in the calling variable has the same value of > 0x200000000003c358 in both instances, the address stored at the latter > location is not reliably the former unless ld.so makes the same > mapping for the libs, which presumably can only be guaranteed by > using identical versions. > > Is there a gcc compiler option which can disable this behavior, and > pass the function addresses as they are? Mmm.. don't think so. It's part of the ABI. -- "It's not Hollywood. War is real, war is primarily not about defeat or victory, it is about death. I've seen thousands and thousands of dead bodies. Do you think I want to have an academic debate on this subject?" -- Robert Fisk
