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SubjectRe: sigaction with SA_SIGINFO does not pass the correct return context
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Thanks for the pointers, Mikael. I appreciate it.

Alin


On Thu, 2008-09-04 at 09:33 +0200, Mikael Pettersson wrote:
> Alin Jula writes:
> > Summary: sigaction with SG_SIGINFO passes the incorrect context on
> > OpenSuse (kernels 2.6.16.21-0.8-smp and 2.6.25.11-0.1-default).
> > However, it passes the correct one on Ubuntu (kernel
> > 2.6.24-19-generic).
> >
> > I'm trying to catch the SIGSEGV signal, do something, and then return
> > control back to the application. I use "sigaction" with the SA_SIGINFO
> > flag. This flags allows a handler to be called upon a signal, and it
> > should (POSIX defined) also provide the context of the thread when the
> > signal was issued. The problem is that the right context does _not_ work
> > on OpenSuse, but it works on Ubuntu.
> >
> > So I was wondering if there is a specific action that one needs to take
> > on OpenSuse when dealing with this issue, or if there is a bug in the
> > kernel/opensuse ?
> >
> > On Ubuntu (kernel 2.6.24-19-generic), sigaction correctly provides the
> > context of the thread that raised the signal. However, on two other
> > installations of OpenSuse (kernels 2.6.16.21-0.8-smp and
> > 2.6.25.11-0.1-default), sigaction provides a different context.
> >
> > Below is the code example, with its summary. 4KB get allocated with
> > mmap, with "not to be accessed" protection. A write to this memory chunk
> > raises a SIGSEGV signal. This signal is then caught, and the memory
> > protection is changed to read+write. Then, the control is given back to
> > the instruction that raised this signal, and the application should
> > proceed from there. The code works on Ubuntu, but on the both OpenSuse
> > installations that I tried (see above for kernels), the context passed
> > by sigaction is the context of the handler instead of the instruction
> > that raised the signal.
> >
> > Here is the code:
> > #include <unistd.h>
> > #include <signal.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <sys/mman.h>
> > #include <ucontext.h>
> >
> > #define handle_error(msg) \
> > do { perror(msg); exit(EXIT_FAILURE); } while (0)
> >
> >
> > ucontext_t context_trigger, context_sig;
> > struct sigaction sa_new, sa_old;
> >
> > static void
> > handler(int sig, siginfo_t *si, void *context)
> > {
> > if ((sig == SIGSEGV) || (sig == SIGBUS) )
> > {
> > printf("\nGot SIGSEGV or SIGBUS at address: 0x%lx and context
> > =%lx\n",(long) si->si_addr, context);
> >
> > if (mprotect(si->si_addr, 4096 ,PROT_READ | PROT_WRITE) == -1)
> > handle_error("mprotect failed from the handler");
> > else
> > printf("\nmprotect set the protection to RW\n");
> >
> > #if 1
> > // The intention here is to switch back to the context that signaled
> > the SIGSEGV. Unfortunately, it does not
> > // do that. Instead, it switches the context back to the handler
> > function, in a recursive call.
> > swapcontext(&context_sig, (ucontext_t*)context);
> > #else
> > // Manually set the context to the point previous to accessing the
> > memory. This works !
> > swapcontext(&context_sig, &context_trigger);
> > #endif
> > }
> >
> > }
> >
> > int
> > main(int argc, char *argv[])
> > {
> > /* Set the handler */
> > sa_new.sa_flags = SA_SIGINFO;
> > sigemptyset(&sa_new.sa_mask);
> > sa_new.sa_sigaction = handler;
> > if (sigaction(SIGSEGV, &sa_new, &sa_old) == -1)
> > handle_error("sigaction");
> >
> >
> > /* Allocate 4096 bytes with initial protection PROT_NONE */
> >
> > char* p = (char*)mmap(0,4096,PROT_NONE, MAP_ANONYMOUS |
> > MAP_PRIVATE, -1, 0);
> > if (p == MAP_FAILED)
> > handle_error("mmap");
> >
> > getcontext(&context_trigger);
> >
> > /* Access the protected memory, which will raise a SIGSEGV or SIGBUS
> > */
> > *(p) = 'a';
> >
> > printf("\nSuccess\n");
> > exit(EXIT_SUCCESS);
> > }
> >
> >
> > Any help/pointer would be greatly appreciated.
>
> Your're doing this all wrong.
>
> An SA_SIGINFO signal handler gets a pointer to the faulting context
> in its third parameter. To modify it you assign fields in it, e.g.
> the program counter. Then you return from the signal handler. Upon
> return the kernel reloads this possibly modified context.
>
> If you don't modify it the faulting instruction will usually be
> rexecuted (though x87 fp exceptions are delayed so the original
> faulting instruction will be skipped). In your simple example all
> you need to do is to mprotect the page and return without changing
> the context.
>
> Last time I checked getcontext/swapcontext weren't system calls,
> so what they do is up to whetever libc your code is linked to.
>
> For examples of FP signal handlers look in the runtime system of
> the Erlang/OTP system, and I'm sure lots of garbage collectors and
> distributed shared memory systems out there use sigsegv handlers.
>
> Finally, a distro specific problem should be taken up with that
> distro.
>
> /Mikael
> ( mikpe at acumem dot com )

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