Friday, June 08, 2007

Worse is worse

Last week I presented my work on the ZeroBUGS debugger at I started my talk by saying that the debugging support in Linux is in line with the worse is better principle: the building blocks are rudimentary for the sake of keeping the implementation simple.

For example, there is no native BreakpointEvent notification. Rather, the debugger implementer needs to keep track of all breakpoints; if a SIGTRAP occurs at the address where an active breakpoint exists, then it is most likely because the said breakpoint was hit.

Another solution is to use PTRACE_GETSIGINFO (available since kernel 2.3.99) and inspect the siginfo_t structure:

struct siginfo_t {
int si_signo; /* Signal number */
int si_errno; /* An errno value */
int si_code; /* Signal code */
pid_t si_pid; /* Sending process ID */
uid_t si_uid; /* Real user ID of sending process */
int si_status; /* Exit value or signal */
clock_t si_utime; /* User time consumed */
clock_t si_stime; /* System time consumed */
sigval_t si_value; /* Signal value */
int si_int; /* POSIX.1b signal */
void *si_ptr; /* POSIX.1b signal */
void *si_addr; /* Memory location which caused fault */
int si_band; /* Band event */
int si_fd; /* File descriptor */

For a SIGTRAP signal, the si_code field may be TRAP_BRKPT, in which case we know that the program hit a breakpoint.

At any rate, it is up to the debugger application to create higher-level abstractions, based on signals and ptrace notifications.

Linux is not the easiest nor most pleasant system to program on, but the implementation is so simple a child could understand it. Ahem.

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