gdb and mmap

gdb and mmap

[David Stroupe]

I am mmap()ing a memory range proovided by a driver.  In gdb if I do a 
/x address I get an Cannot access memory at address 0x4001b000 error. 
If on the other hand I do somethink like
str = (char*) address;
printf("%s\n", str);

I see the string that is stored in the address.

Is there a way for me to tell gdb that the mmap()ped address is a valid one?

TIA
David

Re: gdb and mmap

[Daniel Jacobowitz]

On Mon, May 13, 2002 at 02:19:31PM -0500, David Stroupe wrote:
> I am mmap()ing a memory range proovided by a driver.  In gdb if I do a 
> /x address I get an Cannot access memory at address 0x4001b000 error. 
> If on the other hand I do somethink like
> str = (char*) address;
> printf("%s\n", str);
> 
> I see the string that is stored in the address.
> 
> Is there a way for me to tell gdb that the mmap()ped address is a valid one?

This is not a bug in GDB but a limitation of your operating system. 
For instance, GNU/Linux chooses not to expose I/O mappings to ptrace(). 
I believe it can't generically be done safely, without some significant
kernel rewrite.

You can just write a function to peek at the address and call it via
'call' or 'print' at the GDB prompt.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer

Re: GDB and mmap

[Rodrigo Rubira Branco]

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On Fri, 2007-07-13 at 10:15 +0200, Jan Somr wrote:
> Hi all, please help. 
> I have an application which makes cores and uses mapped memory (mmap).  How can I access this memory in GDB? 
> When I try print it I get this message: "Cannot access memory at address xxxxxxxxxx". Is the shared memory dumped in the core?
> Thank you.

If the vm_area_struct of the mapped area have the vm_flags attribute set
to VM_IO, this area will not be dumped...


cya,


Rodrigo (BSDaemon).

-- 
Rodrigo Rubira Branco
Software Engineer 
Advanced Linux Response Team (ALRT) / Linux on Power Toolchain
IBM Linux Technology Center (IBM/LTC)
rrbranco@br.ibm.com

GPG KeyID: 1FCEDEA1

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Re: GDB and mmap

[Daniel Jacobowitz]

On Fri, Jul 13, 2007 at 10:15:06AM +0200, Jan Somr wrote:
> Hi all, please help. 
> I have an application which makes cores and uses mapped memory (mmap).  How can I access this memory in GDB? 
> When I try print it I get this message: "Cannot access memory at address xxxxxxxxxx". Is the shared memory dumped in the core?

No, it usually is not dumped.  There's nothing GDB can do about that.

Some operating systems have configuration options to control what is
dumped.

-- 
Daniel Jacobowitz
CodeSourcery

GDB and mmap

[Jan Somr]

Hi all, please help. 
I have an application which makes cores and uses mapped memory (mmap).  How can I access this memory in GDB? 
When I try print it I get this message: "Cannot access memory at address xxxxxxxxxx". Is the shared memory dumped in the core?
Thank you.