‘struct sigcontext’ On using Linux’s signaling mechanism for debugqing application programs

Installing a signal-handler #include <signal.h> void handler( int signo, siginfo_t *si, void *sc ); int main( void ) { struct sigaction oa, sa = {0}; sa.sa_flags = SA_SIGINFO; sa.sa_sigaction = handler; sigaction( SIGSEGV, &sa, &oa ); }

Layout of kernel-stack (by cpu) Upon entry to kernel’s exception-handler: SS saved pointer to user-stack ESP EFLAGS CS saved pointer to user-opcode EIP error code SS:ESP ‘error code’ = segment-selector for the illegal memory-segment access

Layout of kernel-stack (by Linux) ss esp eflags cs eip error gs fs es ds eax ebp edi esi edx ecx SS:ESP ebx

command-line arguments command-line arguments Layout of user-stack Upon entering ‘main()’ Upon entering ‘handler()’ environment strings environment strings command-line arguments command-line arguments envp envp argv argv argc argc tos return-address return-address sigstub sigcontext siginfo sc si signum tos return-address

‘segvtrap.cpp’ We have constructed this demo-program to show how you could utilize the context information that Linux can provide to your signal-handler (if you use SA_SIGINFO) You can use our ‘run.cpp’ tool to examine an application-program’s exit-status

In-class exercises Try commenting out the ‘exit(1)’ statement Then add statement: sc ->eip += 1; Try replacing ‘asm(“ hlt “)’ statement with this privileged instruction: inb( 0x1F7 ); Try replacing ‘asm(“ hlt “)’ statement with this illegal assignment: *(char*)0 = 0; Try replacing ‘asm(“ hlt” )’ statement with an instruction that reads a kernel-address