Process Level Redundancy (PLR)

An Implementation of Software Transient Fault Tolerance (Detection & Recovery) through Process Level Redundancy (PLR)

More complete description TBD

Limitations

• Only supports 3 redundant processes right now. 2 process (i.e. detection w/o recovery) mode will be forthcoming.
• Only supports single-threaded programs.
• Probably doesn't work right on programs that spawn children (i.e. fork()).
• Programs which make system calls directly (using 'int 0x80' or 'syscall') rather than passing through glibc will likely work incorrectly, or at best have incomplete protection. This is because syscalls are intercepted at the glibc level using LD_PRELOAD rather than hooking them in the kernel.
• Signals are not currently forwarded from the figurehead to the redundant processes.
• Redundant processes receiving signals at different times can lead to nondeterminism and issues, especially if a signal interrupts a syscall.

Todo List

• STDIN & STDOUT redirection between figurehead to/from redundant processes
• Clean up fault cases like multiple faulted processes to exit PLR gracefully

Known Issues

• File offset will sometimes end up at the wrong position (2x where it should be) for a slave process after fwrite or fseek
• Trying to force EOF indicator in fwrite emulation using fgetc after fseek sometimes reads data??