When debugging a panic that occurred within a module, or using remote GDB against a machine that uses dynamic modules, you need to tell GDB how to obtain symbol information for those modules.
First, you need to build the module(s) with debugging information:
#
cd /sys/modules/linux
#
make clean; make COPTS=-g
If you are using remote GDB, you can run
kldstat
on the target machine to find out
where the module was loaded:
#
kldstat
Id Refs Address Size Name 1 4 0xc0100000 1c1678 kernel 2 1 0xc0a9e000 6000 linprocfs.ko 3 1 0xc0ad7000 2000 warp_saver.ko 4 1 0xc0adc000 11000 linux.ko
If you are debugging a crash dump, you will need to walk the
linker_files
list, starting at
linker_files->tqh_first
and following the
link.tqe_next
pointers until you find the
entry with the filename
you are looking for.
The address
member of that entry is the load
address of the module.
Next, you need to find out the offset of the text section within the module:
#
objdump --section-headers /sys/modules/linux/linux.ko | grep text
3 .rel.text 000016e0 000038e0 000038e0 000038e0 2**2 10 .text 00007f34 000062d0 000062d0 000062d0 2**2
The one you want is the .text
section,
section 10 in the above example. The fourth hexadecimal field
(sixth field overall) is the offset of the text section within
the file. Add this offset to the load address of the module to
obtain the relocation address for the module's code. In our
example, we get 0xc0adc000 + 0x62d0 = 0xc0ae22d0. Use the
add-symbol-file
command in GDB to tell the
debugger about the module:
(kgdb)
add-symbol-file /sys/modules/linux/linux.ko 0xc0ae22d0
add symbol table from file "/sys/modules/linux/linux.ko" at text_addr = 0xc0ae22d0? (y or n)y
Reading symbols from /sys/modules/linux/linux.ko...done.(kgdb)
You should now have access to all the symbols in the module.
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