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

2. 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 ); }

3. 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

4. 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

5. 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

6. ‘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

7. 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