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Re: Force software breakpoint on read-only memory
- From: Andrew Burgess <andrew dot burgess at embecosm dot com>
- To: Mathieu Tarral <mathieu dot tarral at protonmail dot com>
- Cc: "gdb at sourceware dot org" <gdb at sourceware dot org>
- Date: Wed, 21 Nov 2018 15:02:38 +0000
- Subject: Re: Force software breakpoint on read-only memory
- References: <UpeoDeMdWlsVJSDXnbjmxHp8zp3Qx_idZKmlMb5AElmjUCmP7cMVlZJSRhhgpoKUqTPFSVGBAuiyYnlu8PkQnA_1cjC6gGwELiT5pQqXPGY=@protonmail.com>
* Mathieu Tarral <mathieu.tarral@protonmail.com> [2018-11-21 09:40:15 +0000]:
> Hi,
>
> I'm trying to set a software breakpoint on a read-only memory page.
> GDB automatically sets the breakpoint as hardware, but I would like to
> keep using software breakpoints.
>
> I found the `set breakpoint auto-hw off option in` in the
> documentation:
> "If the target provides a memory map, GDB will warn when
> trying to set software breakpoint at a read-only address."
>
> However, even when I set it, GDB gives me a warning and my command is aborted:
>
> $ gdb -x gdbinit ~/tmp/ntoskrnl.exe
> 0x80545c9f in ?? ()
> (gdb) set breakpoint auto-hw off
> (gdb) b NtOpenFile
> Breakpoint 1 at 0x4a30b1
> (gdb) continue
> Continuing.
> Warning:
> Cannot insert breakpoint 1.
> Cannot set software breakpoint at read-only address 0x4a30b1
>
> Command aborted.
> (gdb)
>
>
>
> My stub interacts directly with the guest hardware, from the
> hypervisor, so I can bypass the read-only protections.
> That's why i wanted to force the software breakpoint
> and ignore the memory map.
>
> Is this a bug in GDB ?
Not in the sense that this is the designed behaviour. Once GDB has a
memory map it assumes this is accurate. When it spots you trying to
place a breakpoint in read-only memory if checks the 'auto-hw'
setting. If this is on then it converts to a h/w breakpoint,
otherwise, it fails, after all you can't write to read-only memory.
The place to look in the GDB source is gdb/breakpoint.c:insert_bp_location.
My question would be, do you allow _all_ writes to read-only memory,
or only breakpoint writes? If it's all, then could you "fix" the
memory map?
If you only want to allow s/w breakpoint writes then you'll need to
hack GDB, possibly adding a switch to ignore memory maps for s/w
breakpoints.
Thanks,
Andrew
>
> Thanks !
> --
> Mathieu Tarral
>
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