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[Qemu-devel] GDB function call failing due to memory protection of stack

From: Abid, Hafiz
Subject: [Qemu-devel] GDB function call failing due to memory protection of stack page in QEMU
Date: Thu, 11 Apr 2013 12:20:57 +0100

Hi All,
I have faced a problem using GDB with user-mode qemu. Although problem was observed for MIPS, I think it is a generic problem. I would appreciate an advice from experts on how best to deal with. Here is the problem description.

GDB has ability to call function from the program being debugged (http://sourceware.org/gdb/onlinedocs/gdb/Calling.html#Calling). For MIPS (and on other architectures), it put a breakpoint on the stack that is used as return address of the function. When that breakpoint is hit, GDB knows that function is complete and it can return control to user. This breakpoint on stack causes problem for QEMU. When it runs that instruction from stack, it add write-protection to that page. So after the function call, GDB is not able to write to stack. So any future function call or other operation that need to write to stack will fail. I show an example session below taken mostly from call-sc.exp of GDB testsuite.

There are 2 possible solution in my mind. One was to allow writing memory if this page originally had write access. This is a single line fix and a patch is below.

Second option was to not add write protection to the page in the first place if
i) Page currently has write access.
ii) First instruction is a breakpoint.

If 2nd looks a better option then I can prepare a patch for it. If there some other approach that will better solve this problem then please let me know.


GDB session:
GNU gdb (GDB)
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "--host=i686-pc-linux-gnu --target=mips-linux-gnu".
For bug reporting instructions, please see:
Reading symbols from /home/abidh/work/mips-demo/call_static...done.
(gdb) target remote :8000
Remote debugging using :8000
[New Remote target]
[Switching to Remote target]
__start () at ../ports/sysdeps/mips/start.S:84
84      ../ports/sysdeps/mips/start.S: No such file or directory.
(gdb) break main
Breakpoint 1 at 0x4011e8: file call.c, line 63.
(gdb) c

Breakpoint 1, main () at call.c:63
63        Fun(foo);     
(gdb) p /c fun()
$1 = 49 '1'
(gdb) p /c fun()
Cannot access memory at address 0x40800258

Signed-off-by: Hafiz Abid Qadeer <address@hidden>
 exec.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/exec.c b/exec.c
index fa1e0c3..af5610b 100644
--- a/exec.c
+++ b/exec.c
@@ -1844,8 +1844,9 @@ int cpu_memory_rw_debug(CPUArchState *env, target_ulong addr,
         if (!(flags & PAGE_VALID))
             return -1;
         if (is_write) {
-            if (!(flags & PAGE_WRITE))
+ if ((!(flags & PAGE_WRITE)) && (!(flags & PAGE_WRITE_ORG))) {
                 return -1;
+            }
             /* XXX: this code should not depend on lock_user */
             if (!(p = lock_user(VERIFY_WRITE, addr, l, 0)))
                 return -1;

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