On Tue, Jul 14, 2015 at 5:53 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > Why wouldn't you just use an ld preload library to intercept stat() > calls in userspace and manipulate them there? No kernel changes > required, no app changes required.... In theory I love this approach but, sadly, using LD_PRELOADed libraries in practice has a lot of problems: - You can't intercept intra-libc calls, so other libc functions that might call stat() potentially need to be overridden. - You can't intercept calls from static binaries, which are common on Sandstorm. - You can't intercept syscalls made directly via syscall() or inline assembly, which happen from time to time. - The library file needs to be mapped into the container somewhere. Normally the application controls the entire container layout, so this is awkward. - Lots of other issues. Some of the above can theoretically be solved by using seccomp to raise SIGSYS on every stat() call and perhaps using a userspace ELF loader, but this brings its own set of complications and performance issues and would take a long time to build. Note that we would *love* the ability to more cleanly and robustly filter / rewrite syscalls in userspace, for exactly this sort of thing. But it seems the available options (ptrace and LD_PRELOAD) are far too difficult and quirky to use effectively as-is. -Kenton -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
