debug_setcontext syscall

The following patch adds a syscall to the current linuxppc-2.5 tree. This syscall allows in-application debuggers to work; it allows debugging actions (currently setting the single-step bit and branch trace enable bits) to be done for a setcontext, like a return from a signal handler.

I have also attached a small program that demonstrates the usage of the single stepping and branch tracing.

I'd like to get this into the standard kernel, but this is (obviously) just for comment now.

-Corey

#include <signal.h>
#include <stdio.h>
#include <errno.h>
#include <sys/mman.h>
#include <asm/unistd.h>

struct my_sig_dbg_op {
	int dbg_type;
	unsigned long dbg_value;
};

/* Enable or disable single-stepping. The value sets the state. */
#define MY_SIG_DBG_SINGLE_STEPPING 1

/* Enable or disable branch tracing. The value sets the state. */
#define MY_SIG_DBG_BRANCH_TRACING 2

#ifndef __NR_dbg_sigreturn
#define __NR_dbg_sigreturn 256
#endif

/* Create the debug return syscall. */
_syscall3(int, dbg_sigreturn,

	  void *, ucontext,
	  int,    ndbg,
	  struct my_sig_dbg_op *, op);

volatile int called = 0;
volatile int called2 = 0;
volatile int called3 = 0;
volatile int trap_called = 0;

void sighand(int sig, siginfo_t *info, void *ucontext)

{
	struct my_sig_dbg_op op;

	called++;

	kill(getpid(), SIGUSR2);

	op.dbg_type = MY_SIG_DBG_SINGLE_STEPPING;
	op.dbg_value = 1;
	dbg_sigreturn(ucontext, 1, &op);
}

void sighand2(int sig, siginfo_t *info, void *ucontext)

{
	kill(getpid(), SIGPWR);
	called2++;
}

void sighand3(int sig, siginfo_t *info, void *ucontext)

{
	struct my_sig_dbg_op op;
	called3++;

	op.dbg_type = MY_SIG_DBG_SINGLE_STEPPING;
	op.dbg_value = 1;
	dbg_sigreturn(ucontext, 1, &op);
}

#define PAGE_SIZE 4096
#define TO_PAGEBASE(a) (((unsigned int) (a)) & (~(PAGE_SIZE-1))) #define TRAP_INSTRUCTION 0x0ce00097

static int
write_instruction(unsigned char *address,
unsigned long new_instr,
unsigned long *old_instr)

{
	char         *pagebase = (char *) TO_PAGEBASE(address);

	/* FIXME - currently assuming read-only executable memory, need a
	   way to fetch the old memory protection. */
	if (mprotect(pagebase,
		     PAGE_SIZE,
		     PROT_READ | PROT_WRITE | PROT_EXEC) != 0) {
		/* Couldn't change memory protection, return an error */
		return -1;
	}

	if (old_instr)
		*old_instr = *((volatile unsigned long *) address);

	*((volatile unsigned long *) address) = new_instr;

	mprotect(pagebase, PAGE_SIZE, PROT_READ | PROT_EXEC);

	/* Flush the cache for the instruction. */
	__asm__ ("dcbst 0,%0\n\ticbi 0,%0" : : "r" (address));

	return 0;
}

unsigned char *instr_addr;
unsigned long old_instr;
int restore = 0;
int tracing = 0;

/* The "old" ucontext. */

struct old_sigcontext_struct {
	unsigned long	_unused[4];
	int		signal;
	unsigned long	handler;
	unsigned long	oldmask;
	struct pt_regs 	*regs;
};
struct old_ucontext {
	unsigned long	  uc_flags;
	struct ucontext  *uc_link;
	stack_t		  uc_stack;
	struct sigcontext_struct uc_mcontext;
	sigset_t	  uc_sigmask;	/* mask last for extensibility */
};

unsigned long dbg_get_instruction_ptr_from_ucontext(void *ucontext)

{
    struct old_ucontext *uc = ucontext;
    struct pt_regs      *regs = uc->uc_mcontext.regs;
    return regs->nip;
}

void sigtrap(int sig, siginfo_t *info, void *ucontext)

{
	int old_errno = errno;
	trap_called++;
	if (restore) {
		write_instruction(instr_addr, old_instr, NULL);
		restore = 0;
	}
	if (tracing) {
		char buf[100];
		sprintf(buf, "Trap at %8.8x\n", dbg_get_instruction_ptr_from_ucontext(ucontext));
		__libc_write(2, buf, strlen(buf));
		if (trap_called > 20)
			tracing = 0;
		else {
			struct my_sig_dbg_op op;
			errno = old_errno;

			op.dbg_type = MY_SIG_DBG_BRANCH_TRACING;
			op.dbg_value = 1;
			dbg_sigreturn(ucontext, 1, &op);
		}
	}
	errno = old_errno;
}

void call_printf(void)

{
	printf("test\n");
}

int main(int argc, char *argv)

{
	struct sigaction act;
	int rv;

	act.sa_sigaction = sighand;
	act.sa_flags = SA_SIGINFO;
	sigemptyset(&act.sa_mask);
	sigaddset(&act.sa_mask, SIGUSR2);
	rv = sigaction(SIGUSR1, &act, NULL);
	if (rv == -1) {
		perror("sigaction");
		exit(1);
	}

	act.sa_sigaction = sighand2;
	rv = sigaction(SIGUSR2, &act, NULL);
	if (rv == -1) {
		perror("sigaction");
		exit(1);
	}

	act.sa_sigaction = sighand3;
	rv = sigaction(SIGPWR, &act, NULL);
	if (rv == -1) {
		perror("sigaction");
		exit(1);
	}

	act.sa_sigaction = sigtrap;
	rv = sigaction(SIGTRAP, &act, NULL);
	if (rv == -1) {
		perror("sigaction");
		exit(1);
	}

	kill(getpid(), SIGUSR1);

	if (!called)
		printf("Didn't get called\n");
	else
		printf("Got called %d times\n", called);

	if (!called2)
		printf("Didn't get called 2\n");
	else
		printf("Got called 2 %d times\n", called2);

	if (!called3)
		printf("Didn't get called 3\n");
	else
		printf("Got called 3 %d times\n", called3);

	if (!trap_called) {
		printf("ERROR: Didn't get trapped\n");
		exit(1);
	} else
		printf("Got trapped %d times\n", trap_called);

	instr_addr = (unsigned char *) call_printf;

	write_instruction(instr_addr, TRAP_INSTRUCTION, &old_instr);
	restore = 1;
	tracing = 1;
	call_printf();

	exit(0);
}

diff -ur 2.5.old/arch/ppc/kernel/misc.S 2.5/arch/ppc/kernel/misc.S
@@ -1385,3 +1385,5 @@
 	.long sys_statfs64
 	.long sys_fstatfs64
 	.long ppc_fadvise64_64
+	.long sys_ni_syscall	/* 255 */
+	.long sys_debug_setcontext

diff -ur 2.5.old/arch/ppc/kernel/traps.c 2.5/arch/ppc/kernel/traps.c
@@ -462,7 +462,7 @@
 void
 SingleStepException(struct pt_regs *regs)
 {
-	regs->msr &= ~MSR_SE;  /* Turn off 'trace' bit */
+	regs->msr &= ~(MSR_SE | MSR_BE);  /* Turn off 'trace' bits */
 	if (debugger_sstep(regs))
 		return;
 	_exception(SIGTRAP, regs, TRAP_TRACE, 0);

diff -ur 2.5.old/include/asm-ppc/signal.h 2.5/include/asm-ppc/signal.h
@@ -153,4 +153,23 @@
 #define ptrace_signal_deliver(regs, cookie) do { } while (0)
 #endif /* __KERNEL__ */
 
+/*
+ * These are parameters to dbg_sigreturn syscall.  They enable or
+ * disable certain debugging things that can be done from signal
+ * handlers.  The dbg_sigreturn syscall *must* be called from a
+ * SA_SIGINFO signal so the ucontext can be passed to it.  It takes an
+ * array of struct sig_dbg_op, which has the debug operations to
+ * perform before returning from the signal.
+ */
+struct sig_dbg_op {
+	int dbg_type;
+	unsigned long dbg_value;
+};
+
+/* Enable or disable single-stepping.  The value sets the state. */
+#define SIG_DBG_SINGLE_STEPPING		1
+
+/* Enable or disable branch tracing.  The value sets the state. */
+#define SIG_DBG_BRANCH_TRACING		2
+
 #endif

diff -ur 2.5.old/include/asm-ppc/unistd.h 2.5/include/asm-ppc/unistd.h
@@ -259,8 +259,9 @@
 #define __NR_statfs64		252
 #define __NR_fstatfs64		253
 #define __NR_fadvise64_64	254
+#define __NR_debug_setcontext	256
 
-#define __NR_syscalls		255
+#define __NR_syscalls		257
 
 #define __NR(n)	#n