[lldb-dev] LLDB blocks progress of program that handles SIGSEGV

[Eugene Birukov]

 Hello,
 
I am running Linux Ubuntu 14.04 and I tried both LLDB-3.6 (installed binaries using apt-get) and LLDB-3.7 (built from sources sync'ed to trunk), the result is the same.
 
I have a simple program (the source code is at the bottom of the message) that maps a page of memory as not accessible, installs a SIGSEGV handler that remaps this page as read-write, and then tries to read from it. So, the expected result is that program initially receives SIGSEGV but happily continues after the handler fixes the problem.
 
The program runs as expected, both standalone and under GDB:
 
$ ./mm
signal 11 received
success 777

$ gdb --quiet ./mm
Reading symbols from ./mm...done.
(gdb) r
Starting program: /home/eugene/tmp/mm
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400acc in main () at mm.cpp:27
27          int x = *(int*)address;
(gdb) c
Continuing.
signal 11 received
success 777
[Inferior 1 (process 14155) exited normally]
(gdb) q

But under LLDB it spins forever on failing instruction without invoking the signal handler. Also, setting "process handle" does not have any effect at all:
 
$ ~/llvm/bin/lldb ./mm
(lldb) target create "./mm"
Current executable set to './mm' (x86_64).
(lldb) br se -b main
Breakpoint 1: where = mm`main + 30 at mm.cpp:24, address = 0x0000000000400a7e
(lldb) pr lau
Process 14194 launched: './mm' (x86_64)
Process 14194 stopped
* thread #1: tid = 14194, 0x0000000000400a7e mm`main + 30 at mm.cpp:24, name = 'mm', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400a7e mm`main + 30 at mm.cpp:24
   21
   22   int main()
   23   {
-> 24       sigset(SIGSEGV, handler);
   25
   26       address = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
   27       int x = *(int*)address;
(lldb) pr ha SIGSEGV -s false -p true -n false
NAME        PASS   STOP   NOTIFY
==========  =====  =====  ======
SIGSEGV     true   false  false
(lldb) c
Process 14194 resuming
Process 14194 stopped
* thread #1: tid = 14194, 0x0000000000400acc mm`main + 108 at mm.cpp:27, name = 'mm', stop reason = address access protected (fault address: 0x7ffff7ff7000)
    frame #0: 0x0000000000400acc mm`main + 108 at mm.cpp:27
   24       sigset(SIGSEGV, handler);
   25
   26       address = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
-> 27       int x = *(int*)address;
   28       std::cout << (signaled ? "success " : "failure ") << x << "\n";
   29   }
(lldb) c
Process 14194 resuming
Process 14194 stopped
* thread #1: tid = 14194, 0x0000000000400acc mm`main + 108 at mm.cpp:27, name = 'mm', stop reason = address access protected (fault address: 0x7ffff7ff7000)
    frame #0: 0x0000000000400acc mm`main + 108 at mm.cpp:27
   24       sigset(SIGSEGV, handler);
   25
   26       address = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
-> 27       int x = *(int*)address;
   28       std::cout << (signaled ? "success " : "failure ") << x << "\n";
   29   }
(lldb) c
Process 14194 resuming
Process 14194 stopped
* thread #1: tid = 14194, 0x0000000000400acc mm`main + 108 at mm.cpp:27, name = 'mm', stop reason = address access protected (fault address: 0x7ffff7ff7000)
    frame #0: 0x0000000000400acc mm`main + 108 at mm.cpp:27
   24       sigset(SIGSEGV, handler);
   25
   26       address = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
-> 27       int x = *(int*)address;
   28       std::cout << (signaled ? "success " : "failure ") << x << "\n";
   29   }
(lldb)

So, do I miss some LLDB setting?
Here is the program code:
 
$ cat mm.cpp
#include <iostream>
#include <sys/mman.h>
#include <signal.h>
#include <assert.h>
#include <sys/types.h>
#include <unistd.h>
 
void* address;
size_t size = 0x1000;
bool signaled = false;
 
void handler(int sig)
{
    std::cout << "signal " << sig << " received\n";
    signaled = true;
    munmap(address, size);
    void* newaddr = mmap(address, size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_FIXED | MAP_PRIVATE, -1, 0);
    assert(newaddr == address);
    *(int*)newaddr = 777;
}
 
int main()
{
    sigset(SIGSEGV, handler);
    address = mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    int x = *(int*)address;
    std::cout << (signaled ? "success " : "failure ") << x << "\n";
}
$

Thanks,
Eugene
 
 
 		 	   		  
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