Hi All,

Attaching the patch for adding signal handler support in AIX.

If gdb is debugging an application which has a signal handler and reaches 
the signal handler frame, then we need to read the back chain address from 
sigconext saved on the 
stack, similarly the LR.

As backchain at an offset 0 will be 0, because we will have sigconext 
saved after the minimum stack size. 
So, correct backchain will be at an offset after minimum stack and the LR 
at an offset 8 will be of the
signal millicode address. If the back chain pointer is NULL and the LR 
field is in the kernel 
segment(ex. 0x00004a14) then we can probably assume we are in a signal 
handler.

This can be demonstrated using the below sample program.

# cat aix-sighandle.c
#include <stdio.h>
#include <signal.h>
#include <string.h>

void sig_handle_aix(int signo)
{
  printf("signal is: %d\n", signo);
}

void foo()
{
  char *ptr; 
  signal(SIGSEGV, sig_handle_aix);
  strcpy(ptr, "signal");
}

int main()
{
  foo();
}


without patch
-----------------

Reading symbols from ./aix-sighandle...done.
(gdb) br sig_handle_aix
Breakpoint 1 at 0x100006bc: file aix-sighandle.c, line 7.
(gdb) r
Starting program: aix-sighandle 

Program received signal SIGSEGV, Segmentation fault.
0x0000000100000748 in foo () at /home/binutils-gdb/gdb/aix-sighandle.c:14
14        strcpy(ptr, "signal");
(gdb) c
Continuing.

Breakpoint 1, sig_handle_aix (signo=11) at 
/home/binutils-gdb/gdb/aix-sighandle.c:7
7         printf("signal is: %d\n", signo);
(gdb) bt
#0  sig_handle_aix (signo=11) at /home/binutils-gdb/gdb/aix-sighandle.c:7
#1  0x0000000000004a94 in ?? ()
(gdb) 

with patch
-------------

Reading symbols from ./aix-sighandle...done.
(gdb) br sig_handle_aix
Breakpoint 1 at 0x100006bc: file /home/binutils-gdb/gdb/aix-sighandle.c, 
line 7.
(gdb) r
Starting program: /home/binutils-gdb/gdb/aix-sighandle 

Program received signal SIGSEGV, Segmentation fault.
0x0000000100000748 in foo () at /home/binutils-gdb/gdb/aix-sighandle.c:14
14        strcpy(ptr, "signal");
(gdb) c
Continuing.

Breakpoint 1, sig_handle_aix (signo=11) at 
/home/binutils-gdb/gdb/aix-sighandle.c:7
7         printf("signal is: %d\n", signo);
(gdb) bt
#0  sig_handle_aix (signo=11) at/home/binutils-gdb/gdb/aix-sighandle.c:7
#1  <signal handler called>
#2  0x0000000100000748 in foo () at 
/home/binutils-gdb/gdb/aix-sighandle.c:14
#3  0x000000010000079c in main () at 
/home/binutils-gdb/gdb/aix-sighandle.c:19
warning: (Internal error: pc 0x1000004f3 in read in psymtab, but not in 
symtab.)

warning: (Internal error: pc 0x1000004f4 in read in psymtab, but not in 
symtab.)

(gdb) 


Summary of the gdb.base testsuites.
I saw assertion failure not related to the fix while running the testcases 
which i will be fixing it soon.

Wrote a small testcase to run it on only AIX to test the signal handler.

Without patch
-----------------
# of expected passes            16372
# of unexpected failures        2279
# of expected failures          14
# of unresolved testcases       30
# of untested testcases         68
# of unsupported tests          37

with patch
-------------
# of expected passes            16395
# of unexpected failures        2256
# of expected failures          14
# of unresolved testcases       30
# of untested testcases         68
# of unsupported tests          37


We already had some discussion on this before and here is the link.
https://sourceware.org/ml/gdb-patches/2018-01/msg00255.html


 

Thanks,
Sangamesh