[Bug c/13182] New: -fstack-check probes too distant when allocating on stack

[Bug c/13182] New: -fstack-check probes too distant when allocating on stack

[schneck at math dot berkeley dot edu]

The stack probes generated by -fstack-check appear to be too distant
when something gets allocated on the stack.

Running "gcc -fstack-check -S tmp.c" where tmp.c is
<code>
f () {
  int a;
  int foo[2100];
}
int g() {
  int a;
  int foo[2100];
  f ();
}
</code>
produces the following output in tmp.s:
<code>
        .file   "tmp.c"
        .text
.globl _f
        .def    _f;     .scl    2;      .type   32;     .endef
_f:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        movl    %esp, -12(%ebp)
        leal    -12792(%esp), %edx
        movl    $0, (%edx)
        movl    $8400, -8(%ebp)
        movl    -8(%ebp), %eax
        call    __alloca
        movl    -12(%ebp), %esp
        leave
        ret
.globl _g
        .def    _g;     .scl    2;      .type   32;     .endef
_g:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        subl    $20, %esp
        leal    -4392(%esp), %eax
        movl    $0, (%eax)
        movl    %esp, %ebx
        leal    -12792(%esp), %eax
        movl    $0, (%eax)
        movl    $8400, -12(%ebp)
        movl    -12(%ebp), %eax
        call    __alloca
        call    _f
        movl    %ebx, %esp
        movl    -4(%ebp), %ebx
        leave
        ret
</code>

In f the first probe is at -12792, skipping over three whole pages!
In g, a non-leaf function, at least we first probe -4392, but there
are still two pages skipped between the two probes.

I think the culprit in the code (gcc-3.3.1-3 on cygwin)
is in the probe_stack_range function in explow.c:
<code>
      /* Start probing at FIRST + N * STACK_CHECK_PROBE_INTERVAL
         for values of N from 1 until it exceeds LAST.  If only one
         probe is needed, this will not generate any code.  Then probe
         at LAST.  */
      for (offset = first + STACK_CHECK_PROBE_INTERVAL;
           offset < INTVAL (size);
           offset = offset + STACK_CHECK_PROBE_INTERVAL)
        emit_stack_probe (gen_rtx_fmt_ee (STACK_GROW_OP, Pmode,
                                          stack_pointer_rtx,
                                          GEN_INT (offset)));

      emit_stack_probe (gen_rtx_fmt_ee (STACK_GROW_OP, Pmode,
                                        stack_pointer_rtx,
                                        plus_constant (size, first)));
</code>
It looks like there is a confusion about what 
probe_stack_range(first,size) means... is it probe from 
sp down to sp-size assuming that sp to sp-first is already probed?
Or is it to probe down to sp-(first+size)?

Additionally, it looks like it's being assumed that 4392 has already 
been probed, even in leaf functions where that has not been done.

Finally, can anyone tell me what the invariant guaranteed by 
-fstack-check is?  I think that it is "At entry to any function,
sp-4392 is guaranteed to be above the guard page."  (Then the probe in 
non-leaf functions is to preserve the guarantee in any functions that 
are called.)  Is that correct?

Robert

-- 
           Summary: -fstack-check probes too distant when allocating on
                    stack
           Product: gcc
           Version: 3.3.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: schneck at math dot berkeley dot edu
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-cygwin
  GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182

[Bug c/13182] -fstack-check probes too distant when allocating on stack

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-25 00:43 -------
This is documented in the GCC internals manual how stack-checking works: <http://gcc.gnu.org/
onlinedocs/gccint/Stack-Checking.html>.  Also -fstack-check is only usefully for threaded 
applications.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182

[Bug c/13182] -fstack-check probes too distant when allocating on stack

[schneck at math dot berkeley dot edu]

------- Additional Comments From schneck at math dot berkeley dot edu  2003-11-25 13:02 -------
(In reply to comment #1)

I fear you may have missed my point.

Stack probes are supposed to happen at STACK_CHECK_PROBE_INTERVAL,
i.e. 4096.

One function does one probe, at sp - 12792 (which is > 4096*3).
One function does two probes, one at sp - 4392,
the other at sp - 12792 (a difference of > 4096*2).

Even if the thread has TWO guard pages this could fail
to be safe!

Please forgive me if I misunderstand.  I had looked at the
internals manual, and I understand (I think) why it is
correct for the first check to be at sp - 4392.
But I am rather convinced that the interval had better
be 4096 (or less) thereafter.

Robert

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182

[Bug c/13182] -fstack-check probes too distant when allocating on stack

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-27 17:23 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-11-27 17:23:45
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182

[Bug middle-end/13182] -fstack-check probes too distant when allocating on stack

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-19 01:27 -------
This is related to bug 10127.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182

[Bug middle-end/13182] -fstack-check probes too distant when allocating on stack

[lothar at xcerla dot com]

------- Additional Comments From lothar at xcerla dot com  2003-12-21 00:59 -------
We also experience strange intermittent crashes (sometimes the program crashes, most times  
it doesn't) when using -fstack-check with GCC 3.3.2 on Linux. If the crash occurs it always  
shows a corrupt stack. It shows a function argument that is different from what the program  
had when it executed that function (we printed the argument). What puzzeled us, was that this  
kind of crash also occured with only one thread (the main thread).  
  

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13182