gdb cannot access memory for program running in qemu-arm emulator

Question

I am trying to emulate a 'C' program by using qemu instruction level simulator.The 'C' program is compiled by issuing the following command

arm-none-linux-gnueabi-gcc -g ex_qsort.c -o ex_qsort_lin_work

I then start qemu with the following command

"qemu-arm -g 1234 -L /path/to/codesourcery/arm-none-linux-gnueabi/libc ./ex_qsort_lin_work"

Then I connect to the program using gdb. I am trying to access program memory location and change the assembly code. But when i try to access the memory I get the following error

   (gdb) x 0x00008510
   0x8510 <main+76>:    0xe3530004
   (gdb) set *(0x8510) = 0xe3530002
   Cannot access memory at address 0x8510

I am not sure why this error occurs . Gdb does not give any other warning . When i start gdb I have the sysroot pointed to the arm library. However when i check for the shared libarary I get the following message

   (gdb) info sharedlibrary 
   From        To          Syms Read   Shared Object Library
   0xf67d67d0  0xf67f0f58  Yes (*)     /path/to/codesourcery/arm-none-linux-gnueabi/libc/lib/ld-linux.so.3
   (*): Shared library is missing debugging information.

Not sure if this causes the problem. Statically linking the libraries also does not help My aim is to change the instruction at a given address

Chris Dodd · Accepted Answer · 2014-09-09 17:14:28Z

1

The issue is that the memory you are trying to modify is read-only. It looks like you're trying to modify code in the text segment of the executable, which is normally mapped read-only, so that is not unexpected. If you want to be able to modify it, it needs to be mapped as writable.

You can build an executable with a writable text segment (so it will load as writable by default) by linking with the -N flag -- use either -Xlinker -N or -Wl,-N on your gcc command line.

answered Sep 9, 2014 at 17:14

Chris Dodd

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3 Comments

balavins Over a year ago

I tried both the options you mentioned separately. I still get the same "Cannot access memory at address" error. Now there are no shared libraries in the code since it is statically linked

Chris Dodd Over a year ago

@user2955638: check the binary to make sure it's properly writable with objdump -p or -x. You can also modify an existing binary to make the text writeable with objcopy --writable-text, which should allow you to create a writeable text dynamically linked executable.

balavins Over a year ago

Thanks for the quick replies but it still does not work. The qemu-arm accepts ELF executable & Binary Flat file as input but not a writable one. The gdb 's input file is always a writable one. Still I am not able to write to a location. I am inclined to think it is more a problem with qemu-arm. Are there any other ways ?

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