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From: Robert P. J. Day on 8 Jul 2010 08:00

trying very hard to make a long story short, documenting how to use
gdb to poke around in a loaded module. the important part of the
module (to demonstrate the visibility of different types of
variables):

static int rpjday_1;
int rpjday_2 = 20;
int rpjday_3 = 30;

EXPORT_SYMBOL(rpjday_3);


once i load that module, i can see the good stuff in /proc/kallsyms:

$ grep rpjday /proc/kallsyms
ffffffffa007c090 r __ksymtab_rpjday_3 [crash_gdb]
ffffffffa007c0a8 r __kstrtab_rpjday_3 [crash_gdb]
ffffffffa007c0a0 r __kcrctab_rpjday_3 [crash_gdb]
ffffffffa007c0b4 d rpjday_2 [crash_gdb]
ffffffffa007c0b8 D rpjday_3 [crash_gdb]
$

i can examine the salient files under
/sys/module/crash_gdb/sections/, like .data

0xffffffffa007c0b4

so here's a couple questions.

if i want to add that module's symbol table info to my running gdb
session, i would use "add-symbol-file" but the first arg *must* be the
address of the text section and this module has *no* .text section.
so what do i use for that value? the address of the .exit.text
section?

and if i try to load:

(gdb) add-symbol-file /tmp/crash_gdb.ko 0xffffffffa007c000 \
-s .data 0xffffffffa007c0b4
add symbol table from file "/tmp/crash_gdb.ko" at
.text_addr = 0xffffffffa007c000
.data_addr = 0xffffffffa007c0b4
(y or n) y
Reading symbols from /tmp/crash_gdb.ko...done.
(gdb)

it certainly *looks* like that worked, and:

(gdb) whatis rpjday_2
type = int
(gdb) whatis rpjday_3
type = int

looks good, but here's where things go wrong:

(gdb) p rpjday_2
Cannot access memory at address 0xffffffffa007c0b4
(gdb) p rpjday_3
Cannot access memory at address 0xffffffffa007c0b8

that sort of thing used to work when i was testing this on a 32-bit
fedora system, but i'm now on 64-but ubuntu and i don't see why those
last couple commands no longer work. the addresses certainly match
what's in /proc/kallsyms, and i don't recall having to do anything
special last time. have i omitted a critical compile-time option for
this? thanks.

rday

--

========================================================================
Robert P. J. Day Waterloo, Ontario, CANADA

Top-notch, inexpensive online Linux/OSS/kernel courses
http://crashcourse.ca

Twitter: http://twitter.com/rpjday
LinkedIn: http://ca.linkedin.com/in/rpjday
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