Hi Gábor,
On Mon, Mar 16, 2009 at 3:36 AM, Gábor Melis <[email protected]> wrote:
> On Domingo 15 Marzo 2009, Oleg Nesterov wrote:
> > On 03/15, Gábor Melis wrote:
> > > On Domingo 15 Marzo 2009, Oleg Nesterov wrote:
> > > > If test_signal (SIGUSR1) is blocked, this means it is already
> > > > delivered, and the handler will be invoked when we return from
> > > > sigsegv_handler(), please see below.
> > >
> > > SIGUSR1 is delivered, its sigmask is added to the current mask but
> > > the handler is not yet invoked and in this instant synchronous
> > > sigsegv is delivered, its handler invoked?
> >
> > Can't understand the question. Could you reiterate?
>
> No need, my question was answered below.
>
> > > > When sigprocmask(SIG_UNBLOCK) returns, both signals are
> > > > delivered. The kernel deques 1 first, then 2. This means that the
> > > > handler for "2" will be called first.
> > >
> > > My mental model that matches what I quickly glean from the sources
> > > (from kernel/signal.c, arch/x86/kernel/signal_32.c) goes like this:
> > >
> > > - signal 1 and signal 2 are generated and made pending
> > > - they are unblocked by sigprocmask
> > > - signal 1 is delivered: signals in its mask (only itself here) are
> > > blocked
> >
> > yes.
> >
> > the kernel changes ip (instruction pointer) to sig_1.
> >
> > > its handler is invoked
> >
> > no.
> >
> > We never return to user-space with a pending signal. We dequeue
> > signal 2 too, and change ip to sig_2.
> >
> > Now, since there are no more pending signals, we return to the user
> > space, and start sig_2().
>
> I see. I guess in addition to changing the ip, the stack frobbing magic
> arranges that sig_2 returns to sig_1 or some code that calls sig_1.
>
> From the point of view of sig_2 it seems that sig_1 is already invoked
> because it has its sigmask in effect and the ip in the ucontext of
> sig_2 points to sig_1 as the attached signal-test.c shows:
>
> sig_1=80485a7
> sig_2=80485ed
> 2 1
> eip: 80485a7
> 1 0
> eip: b7fab424
>
> The revised signal-delivery-order.c (also attached) outputs:
Our Project does Kernel Testing (http://ltp.sourceforge.net/). I was
watching this thread and found the discussion interesting. Could you please
tell us, whether these test programs are dependent on each other, or they
need to be executed separately to produce 2 different outputs, and then
compare these outputs for conclusion. With your permission we would like to
add these tests(under GPL) in to our automated test environment. Could you
help us here ?
Regards--
Subrata
>
> test_handler=8048727
> sigsegv_handler=804872c
> eip: 8048727
> esp: b7d94cb8
>
> which shows that sigsegv_handler also has incorrect eip in the context.
>
--
Regards & Thanks--
Subrata
#include <signal.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/ucontext.h>
#include <unistd.h>
int test_signal;
int *page_address;
pthread_t tid;
int is_signal_blocked(int signum)
{
sigset_t set;
pthread_sigmask(SIG_BLOCK, 0, &set);
return (sigismember(&set, signum));
}
void *eip(struct ucontext *context)
{
return (void *)context->uc_mcontext.gregs[14];
}
void test_handler(int signal, siginfo_t *info, void *context)
{
}
void sigsegv_handler(int signal, siginfo_t *info, void *context)
{
/* The test signal is blocked only in test_handler. */
if (is_signal_blocked(test_signal)) {
printf("eip: %x\n", eip(context));
_exit(27);
}
mprotect(page_address, 4096, PROT_READ | PROT_WRITE);
}
void *make_faults(void *arg)
{
while (1) {
mprotect(page_address, 4096, PROT_NONE);
*page_address = 1;
}
}
void *reserve_page(void)
{
int flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE;
void *actual = mmap(0, 4096, PROT_NONE, flags, -1, 0);
if (actual == MAP_FAILED) {
perror("mmap");
return 0;
}
return actual;
}
void install_handlers(void)
{
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sa.sa_sigaction = test_handler;
sigaction(test_signal, &sa, 0);
sa.sa_sigaction = sigsegv_handler;
sigaction(SIGSEGV, &sa, 0);
}
void test_with_pthread_kill()
{
page_address = (int *)reserve_page();
install_handlers();
if (pthread_create(&tid, 0, make_faults, 0) < 0)
perror("pthread_create");
while(1) {
pthread_kill(tid, test_signal);
}
}
void test_with_kill()
{
pid_t pid = fork();
if (pid == 0) {
page_address = (int *)reserve_page();
install_handlers();
make_faults(0);
} else {
while (1) {
kill(pid, test_signal);
}
}
}
int main(void)
{
test_signal = SIGUSR1;
printf("test_handler=%x\n", test_handler);
printf("sigsegv_handler=%x\n", sigsegv_handler);
test_with_pthread_kill();
/* Forking and kill()ing works as expected: */
/* test_with_kill(); */
}
#include <syscall.h>
#include <signal.h>
#include <stdio.h>
#include <ucontext.h>
int is_blocked(int sig)
{
sigset_t set;
sigprocmask(SIG_BLOCK, NULL, &set);
return sigismember(&set, sig);
}
void *eip(struct ucontext *context)
{
return (void *)context->uc_mcontext.gregs[14];
}
void sig_1(int sig, siginfo_t *info, struct ucontext *context)
{
printf("%d %d\n", sig, is_blocked(2));
printf("eip: %x\n", eip(context));
}
void sig_2(int sig, siginfo_t *info, struct ucontext *context)
{
printf("%d %d\n", sig, is_blocked(1));
printf("eip: %x\n", eip(context));
}
int tkill(int tid, int sig)
{
syscall(SYS_tkill, tid, sig);
}
int main(void)
{
sigset_t set;
struct sigaction sa;
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
sa.sa_sigaction = sig_1;
sigaction(1, &sa, 0);
sa.sa_sigaction = sig_2;
sigaction(2, &sa, 0);
sigemptyset(&set);
sigaddset(&set, 1);
sigaddset(&set, 2);
printf("sig_1=%x\n", sig_1);
printf("sig_2=%x\n", sig_2);
sigprocmask(SIG_BLOCK, &set, NULL);
kill(getpid(), 1);
kill(getpid(), 2);
sigprocmask(SIG_UNBLOCK, &set, NULL);
return 0;
}
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