as a followup to my earlier post, here's something i want to
clarify. there's a difference in how much symbol-based debugging you
can do based on whether you configured your running kernel with
CONFIG_DEBUG_INFO or not, so i wanted to test what the differences
were between two kernels, one configured with, one without.
after i built both kernels, you can see the predictable difference
in size (ignore vmlinux.o, of course):
$ ls -l vmlinux*
-rwxrwxr-x. 1 rpjday rpjday 15649485 2009-07-30 16:51 vmlinux
-rwxrwxr-x. 1 rpjday rpjday 116565773 2009-07-30 16:34 vmlinux.debug.info
-rw-rw-r--. 1 rpjday rpjday 15891639 2009-07-30 16:51 vmlinux.o
$
my basic question with the above is whether, apart from the
additional debug info embedded in the kernel image, were the two
kernels *exactly* the same -- that is, would they have the same
sections, with the same sizes, at the same offsets, so i could bounce
from one to the other with "gdb" to test the differences and not have
to keep rebooting?
as a quick test, i ran
$ objdump --section-headers <kernel image name>
on both, and the only difference was the additional debug sections at
the end of the debug version:
2c2
< vmlinux: file format elf64-x86-64
---
> vmlinux.debug.info: file format elf64-x86-64
89a90,107
> 42 .debug_aranges 0001b040 0000000000000000 0000000000000000 00cace00 2**4
> CONTENTS, READONLY, DEBUGGING
> 43 .debug_pubnames 00051010 0000000000000000 0000000000000000 00cc7e40 2**0
> CONTENTS, READONLY, DEBUGGING
> 44 .debug_info 04c22970 0000000000000000 0000000000000000 00d18e50 2**0
> CONTENTS, READONLY, DEBUGGING
> 45 .debug_abbrev 001e0d1e 0000000000000000 0000000000000000 0593b7c0 2**0
> CONTENTS, READONLY, DEBUGGING
> 46 .debug_line 0044f12b 0000000000000000 0000000000000000 05b1c4de 2**0
> CONTENTS, READONLY, DEBUGGING
> 47 .debug_frame 0013ed00 0000000000000000 0000000000000000 05f6b610 2**3
> CONTENTS, READONLY, DEBUGGING
> 48 .debug_str 0018f17c 0000000000000000 0000000000000000 060aa310 2**0
> CONTENTS, READONLY, DEBUGGING
> 49 .debug_loc 0090b310 0000000000000000 0000000000000000 0623948c 2**0
> CONTENTS, READONLY, DEBUGGING
> 50 .debug_ranges 001a5f10 0000000000000000 0000000000000000 06b447a0 2**4
> CONTENTS, READONLY, DEBUGGING
the fact that all of the prior sections were (apparently) identical
suggests that i can, in fact, swap one kernel image with the other for
"gdb" and everything should continue to work, with the exception of
having symbols or not. is that a safe assumption? it *seems* to be,
i'm just wondering if anyone knows for sure.
rday
p.s. the above suggests that, if those debug sections are the only
difference, rather than build both versions of the kernel, i could
build the DEBUG_INFO version, save that, then strip a copy of it of
all of those debug sections to get the same effect, yes?
--
========================================================================
Robert P. J. Day Waterloo, Ontario, CANADA
Linux Consulting, Training and Annoying Kernel Pedantry.
Web page: http://crashcourse.ca
Twitter: http://twitter.com/rpjday
"Kernel Newbie Corner" column @ linux.com: http://cli.gs/WG6WYX
========================================================================
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