Re: [PATCH] MIPS: Provide correct siginfo_t.si_stime

[Petr Malat <oss@xxxxxxxxx>]

On Fri, Dec 12, 2014 at 09:11:36AM -0800, David Daney wrote:
> Please state how this patch effects binary compatibility with
> previous releases of the kernel.

Hi David,
the kernel returns a random value in the field si_stime. With the patch
applied, the correct value is present in the field. This is the only 
change visible in userspace, because copy_siginfo() is used just for 
coping done in kernel. To the userspace data are copried by a different
function - copy_siginfo_to_user(), which copies field by field, so 
information leakage caused by this change is not possible.

Here is an output from a program (attached), which illustrates the 
issue:

X86_64:
usage.ru_stime 1000 ms
info->si_stime 1000 ms (64)

MIPS (Octeon) with the patch applied:
usage.ru_stime 1000 ms
info->si_stime 1000 ms (64)

MIPS (Octeon) without the patch (3 executions): 
usage.ru_stime 1000 ms
info->si_stime 5532471680 ms (20f9e1c0)
usage.ru_stime 1000 ms
info->si_stime 5532484000 ms (20f9e690)
usage.ru_stime 1000 ms
info->si_stime 5532484640 ms (20f9e6d0)

Regards,
  Petr  

//
// Fork a child, which spends 1 second in system and print
// stime obtained from getrusage and stime received in siginfo
// of the SIGCHLD
//

#include <sys/resource.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

void handler(int sig, siginfo_t *info, void *context)
{
	printf("info->si_stime %ld ms (%lx)\n", 
			1000 * info->si_stime / sysconf(_SC_CLK_TCK),
			info->si_stime);
}

int main(int argc, char *argv[])
{
	struct sigaction act = { .sa_sigaction = handler, .sa_flags = SA_SIGINFO };
	sigaction(SIGCHLD, &act, NULL);

	if (fork()) {
		wait(NULL);
	} else {
		struct rusage usage;
		do {
			int fd = open("/proc/self/maps", O_RDONLY);
			char buf[4096];

			read(fd, buf, sizeof buf);
			close(fd);
			getrusage(RUSAGE_SELF, &usage);
		} while (usage.ru_stime.tv_sec < 1);
		printf("usage.ru_stime %ld ms\n", 
			1000 * usage.ru_stime.tv_sec + 
			usage.ru_stime.tv_usec / 1000);
	}
	return 0;
}