Re: Removing old symbols when debugging code that relocates itself

Hi Shahab,

On 26.05.22 00:13, Shahab Vahedi wrote:
 > Hi Wolfgang,
 >
 > On Wed, May 25, 2022 at 11:14:28PM +0200, Wolfgang Wallner via Gdb wrote:
 >> It seems I can only remove the symbols that I have added with
 >> add-symbol-file, but not the initial ones ...
 >
 > Maybe you could extract the symbol file [1] and then strip it away from
 > the binary [2]. This way, you have to use "add-symbol-file" to add it
 > in different stages and according to what you say you should be able
 > to remove it after each adding.
 >
 > [1]
 > $ objcopy --only-keep-debug uboot.bin uboot.sym
 >
 > [2]
 > $ objcopy --strip-all uboot.bin uboot.strp

Thanks for the feedback!

I tried your recommendations, and while it seems to help somewhat,
unfortunately it does not fully solve my troubles.

Here is what I did:

1) Create the files as you described:

    arm-linux-gnueabihf-objcopy --strip-all u-boot u-boot.strp
    arm-linux-gnueabihf-objcopy --only-keep-debug u-boot my_u-boot.sym

Remark: The U-Boot build already creates a file called 'u-boot.sym', so
I gave my file another name (my_u-boot.sym).
The already existing u-boot.sym file is the symbol table as text file
created by calling 'objdump -t'.

2) I start QEMU as described in my first mail
3) I start GDB using the 'u-boot.strp' ELF file
4) As first command, I add 'my_u-boot.sym' via add-symbol-file

I then step through the code as described in my last mail, but
somehow in this configuration I cannot use GDB's 'until' command.
Using 'until' continues execution, and the temporary breakpoint is
never hit. It is interesting to note that this is *only in assembly*,
using 'until' works fine in C-files!

As a workaround for this issue I set explicit breakpoints.

5) I continue to step through the code, until I reach the relocated code. I
then drop all symbols via 'symbol-file', and add them back for the relocated
location via 'add-symbol-file'.

Showing information about breakpoints shows that there are no duplicate
breakpoints any more --> woohoo :)

    (gdb) info breakpoints
    Num     Type           Disp Enb Address    What
    1       breakpoint     keep y   0x47f3c668 arch/arm/lib/crt0.S:156
            breakpoint already hit 1 time
    2       breakpoint     keep y   0x47f3c754 arch/arm/lib/relocate.S:134
            breakpoint already hit 1 time

While these breakpoints were set at the old locations, now they already
point
to the symbols at the relocated locations.

But when I try to set a new breakpoint, it is again a duplicate:

    (gdb) break board_init_r
    Breakpoint 3 at 0x1dc28 (2 locations)

    (gdb) info breakpoints
    Num     Type           Disp Enb Address    What
    1       breakpoint     keep y   0x47f3c668 arch/arm/lib/crt0.S:156
            breakpoint already hit 1 time
    2       breakpoint     keep y   0x47f3c754 arch/arm/lib/relocate.S:134
            breakpoint already hit 1 time
    3       breakpoint     keep y   <MULTIPLE>
    3.1                         y   0x0001dc28 <board_init_r+4>
    3.2                         y   0x47f58c24 in board_init_r at
common/board_r.c:817

So this leaves me with the following questions:

    *) Does anyone know why the 'until' command stopped working with this
       approach in assembly files? Is this expected, or is it a bug?

    *) Why are there still duplicate breakpoints?

    *) Are there other approaches to deal with relocated code?
       While the objcopy steps have helped me, for a more permanent
       workflow I would like to avoid them (e.g. it would be nice to
       directly set up a debug environment in an IDE, and any required
       extra steps between compilation and debugging would make that more
       complicated)

regards, Wolfgang

Remark: I think there was a typo in my initial mail. The breakpoint in
relocate.S should be in line 134, not line 118.