arm: global variables in kernel module not loaded correctly in gdb
Arend van Spriel
arend.vanspriel at broadcom.com
Tue Apr 16 02:13:15 PDT 2024
This issue should probably be reported to GDB bugzilla, but wanted to
put it out here as well. Maybe someone else stumbled upon this or simply
knows what is going on. Google searches did not provide much clues nor
searching GDB bugzilla.
So what is my issue? When I load a .ko file into GDB using
add-symbol-file the (static) global variables do not have the correct
memory address. When I use the .o file iso the .ko file things work as
expected.
I created a dummy kernel module and compiled it against 6.9-rc4. I did
have my suspicion about PLTS which is enabled in my kernel config. When
CONFIG_ARM_MODULE_PLTS is disabled the symbol address is correct. From
my limited understanding this involves an additional link table. AFAICT
the BFD library which is used by GDB supports this, but can not rule out
there is a bug in there. I rebuild GDB and added some debug print and
also changed a debug variable which makes symbol loading a bit more verbose.
Attached are the two kernel config files and the dummy module source
file. Any help would be appreciated.
Regards,
Arend
with CONFIG_ARM_MODULE_PLTS
---------------------------
Reading symbols from vmlinux...
(gdb) add-symbol-file dummy.ko -s .text 0xbf607000 -s .bss 0xbf6091c0
add symbol table from file "dummy.ko" at
.text_addr = 0xbf607000
.bss_addr = 0xbf6091c0
(y or n) y
Reading symbols from dummy.ko...
using ABFD: name=.text idx=2 addr=0 lower=0
addrs[0]: name=.text idx=2 addr=bf607000
using ABFD: name=.bss idx=19 addr=0 lower=bf607000
addrs[1]: name=.bss idx=19 addr=bf6091c0
Reading minimal symbols of objfile
/home/av935118/sandbox/mod_dummy/dummy.ko ...
Recording minsym: mst_file_data 0x45 12
__UNIQUE_ID_depends126
Recording minsym: mst_file_data 0x4e 12
__UNIQUE_ID_name125
Recording minsym: mst_file_data 0x59 12
__UNIQUE_ID_vermagic124
Recording minsym: mst_file_data 0x0 1 _note_15
Recording minsym: mst_file_data 0x18 1 _note_14
Recording minsym: mst_file_data 0x0 5 .LC0
Recording minsym: mst_file_text 0x0 3
dummy_exit
Recording minsym: mst_file_data 0xc 5 .LC1
Recording minsym: mst_file_data 0x20 5 .LC2
Recording minsym: mst_file_text 0x0 4
dummy_init
Recording minsym: mst_file_bss 0x0 19 .LANCHOR0
Recording minsym: mst_file_bss *0x0* 19
dummy_value
Recording minsym: mst_file_data 0x0 15
__UNIQUE_ID___addressable_cleanup_module125
Recording minsym: mst_file_data 0x0 16
__UNIQUE_ID___addressable_init_module124
Recording minsym: mst_file_data 0x0 12
__UNIQUE_ID_license127
Recording minsym: mst_file_data 0xe 12
__UNIQUE_ID_author126
Recording minsym: mst_data 0x0 17
__this_module
Recording minsym: mst_text 0x0 3
cleanup_module
Recording minsym: mst_text 0x0 4
init_module
Recording minsym: mst_text 0x0 2
dummy_set_value
Recording minsym: mst_text 0x10 2
dummy_get_value
(gdb) add-symbol-file dummy.o -s .text 0xbf607000 -s .bss 0xbf6091c0
add symbol table from file "dummy.o" at
.text_addr = 0xbf607000
.bss_addr = 0xbf6091c0
(y or n) y
Reading symbols from dummy.o...
using ABFD: name=.text idx=0 addr=0 lower=0
addrs[0]: name=.text idx=0 addr=bf607000
using ABFD: name=.bss idx=2 addr=0 lower=bf607000
addrs[1]: name=.bss idx=2 addr=bf6091c0
Reading minimal symbols of objfile
/home/av935118/sandbox/mod_dummy/dummy.o ...
Recording minsym: mst_file_data 0x0 3 .LC0
Recording minsym: mst_file_text 0x38 4
dummy_exit
Recording minsym: mst_file_data 0xc 3 .LC1
Recording minsym: mst_file_data 0x20 3 .LC2
Recording minsym: mst_file_text 0x4c 7
dummy_init
Recording minsym: mst_file_bss 0xbf6091c0 2 .LANCHOR0
Recording minsym: mst_file_bss *0xbf6091c0* 2
dummy_value
Recording minsym: mst_file_data 0xa8 12
__UNIQUE_ID___addressable_cleanup_module125
Recording minsym: mst_file_data 0xac 13
__UNIQUE_ID___addressable_init_module124
Recording minsym: mst_file_data 0xb0 14
__UNIQUE_ID_license127
Recording minsym: mst_file_data 0xbe 14
__UNIQUE_ID_author126
Recording minsym: mst_text 0x38 4
cleanup_module
Recording minsym: mst_text 0x4c 7
init_module
Recording minsym: mst_text 0xbf607000 0
dummy_set_value
Recording minsym: mst_text 0xbf607010 0
dummy_get_value
without CONFIG_ARM_MODULE_PLTS
------------------------------
Reading symbols from vmlinux...
(gdb) add-symbol-file ~/sandbox/mod_dummy/dummy.ko -s .text 0xbf607000
-s .bss 0xbf6091c0
add symbol table from file "/home/av935118/sandbox/mod_dummy/dummy.ko" at
.text_addr = 0xbf607000
.bss_addr = 0xbf6091c0
(y or n) y
Reading symbols from /home/av935118/sandbox/mod_dummy/dummy.ko...
using ABFD: name=.text idx=2 addr=0 lower=0
addrs[0]: name=.text idx=2 addr=bf607000
using ABFD: name=.bss idx=17 addr=0 lower=bf607000
addrs[1]: name=.bss idx=17 addr=bf6091c0
Reading minimal symbols of objfile
/home/av935118/sandbox/mod_dummy/dummy.ko ...
Recording minsym: mst_file_data 0x141 12
__UNIQUE_ID_depends126
Recording minsym: mst_file_data 0x14a 12
__UNIQUE_ID_name125
Recording minsym: mst_file_data 0x155 12
__UNIQUE_ID_vermagic124
Recording minsym: mst_file_data 0x24 1 _note_15
Recording minsym: mst_file_data 0x3c 1 _note_14
Recording minsym: mst_file_data 0xa4 5 .LC0
Recording minsym: mst_file_text 0x54 3
dummy_exit
Recording minsym: mst_file_data 0xb0 5 .LC1
Recording minsym: mst_file_data 0xc4 5 .LC2
Recording minsym: mst_file_text 0x60 4
dummy_init
Recording minsym: mst_file_bss 0xbf6091c0 17 .LANCHOR0
Recording minsym: mst_file_bss *0xbf6091c0* 17
dummy_value
Recording minsym: mst_file_data 0x184 14
__UNIQUE_ID___addressable_cleanup_module125
Recording minsym: mst_file_data 0x188 15
__UNIQUE_ID___addressable_init_module124
Recording minsym: mst_file_data 0xfc 12
__UNIQUE_ID_license127
Recording minsym: mst_file_data 0x10a 12
__UNIQUE_ID_author126
Recording minsym: mst_data 0x1c0 16
__this_module
Recording minsym: mst_text 0x54 3
cleanup_module
Recording minsym: mst_text 0x60 4
init_module
Recording minsym: mst_text 0xbf607000 2
dummy_set_value
Recording minsym: mst_text 0xbf607010 2
dummy_get_value
-------------- next part --------------
#include <linux/module.h>
static int dummy_value;
void dummy_set_value(int v)
{
dummy_value = v;
}
int dummy_get_value(void)
{
return dummy_value;
}
static int __init dummy_init(void)
{
pr_err("initializing..\n");
dummy_set_value(0xdeadbeef);
pr_err(" module_addr = %px\n", THIS_MODULE->mem[MOD_TEXT].base);
return 0;
}
static void __exit dummy_exit(void)
{
pr_err("leaving\n");
}
module_init(dummy_init);
module_exit(dummy_exit);
MODULE_AUTHOR("Arend van Spriel <arend.vanspriel at broadcom.com>");
MODULE_LICENSE("GPLv2");
-------------- next part --------------
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