https://sourceware.org/bugzilla/show_bug.cgi?id=18025
Bug ID: 18025
Summary: dwarf2 debug info after rebasing DLLs unusable
Product: binutils
Version: 2.26 (HEAD)
Status: NEW
Severity: normal
Priority: P2
Component: binutils
Assignee: unassigned at sourceware dot org
Reporter: corinna at vinschen dot de
Target: cygwin, i686 and x86_64
Hi,
we're encountering a problem evaluating Dwarf2 debug info in DLLs after
rebasing the DLL. Rebasing, that is, moving the image base address of a
DLL and adjusting the relocation information, is an essential part of
DLL handling in the Cygwin distro, required for smooth operation of
the fork emulation.
Consider a DLL built with debug info, unstripped. As an example, I'm
using the latest file-5.22-1 package which comes with a DLL called
cygmagic-1.dll. The output of objdump -h on the built DLL looks like this:
$ objdump -h cygmagic-1.dll.
cygmagic-1.dll: file format pei-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00013618 00000004d9221000 00000004d9221000 00000600 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE, DATA
1 .data 00000068 00000004d9235000 00000004d9235000 00013e00 2**5
CONTENTS, ALLOC, LOAD, DATA
2 .rdata 00005258 00000004d9236000 00000004d9236000 00014000 2**6
CONTENTS, ALLOC, LOAD, READONLY, DATA
[...]
10 .debug_aranges 00000510 00000004d9243000 00000004d9243000 0001c400 2**4
CONTENTS, READONLY, DEBUGGING
11 .debug_info 0002b4ab 00000004d9244000 00000004d9244000 0001ca00 2**0
CONTENTS, READONLY, DEBUGGING
12 .debug_abbrev 00003d2b 00000004d9270000 00000004d9270000 00048000 2**0
CONTENTS, READONLY, DEBUGGING
13 .debug_line 00006046 00000004d9274000 00000004d9274000 0004be00 2**0
CONTENTS, READONLY, DEBUGGING
14 .debug_frame 00002b68 00000004d927b000 00000004d927b000 00052000 2**3
CONTENTS, READONLY, DEBUGGING
15 .debug_str 00000302 00000004d927e000 00000004d927e000 00054c00 2**0
CONTENTS, READONLY, DEBUGGING
16 .debug_loc 000259a2 00000004d927f000 00000004d927f000 00055000 2**0
CONTENTS, READONLY, DEBUGGING
17 .debug_ranges 00003230 00000004d92a5000 00000004d92a5000 0007aa00 2**0
CONTENTS, READONLY, DEBUGGING
Notice the VMA addresses. The DLL is based at 0x4d9220000. This DLL
works and evaluating the debug info works nicely. This is the `nm -l'
output before rebasing:
$ nm -l cygmagic-1.dll | grep usr/src/debug
[...]
00000004d922bd80 T file_fmttime /usr/src/debug/file-5.22-1/src/print.c:232
00000004d922c520 T file_fsmagic /usr/src/debug/file-5.22-1/src/fsmagic.c:104
00000004d922d3f0 T file_getbuffer
/usr/src/debug/file-5.22-1/src/funcs.c
:321
00000004d922b120 T file_is_tar /usr/src/debug/file-5.22-1/src/is_tar.c:64
00000004d922a1d0 T file_looks_utf8
/usr/src/debug/file-5.22-1/src/encodin
g.c:295
[...]
So it shows the sources and line numbers for the symbols as expected:
$ nm -l cygmagic-1.dll | grep usr/src/debug | wc -l
198
And here's what happens after rebase to some arbitrary address:
$ rebase -b 0x300000000 cygmagic-1.dll
$ nm -l cygmagic-1.dll | grep usr/src/debug | wc -l
0
nm lost all connection between the symbols and their sources.
Checking with GDB: In GDB you can set a breakpoint on this function and
it's loaded to the same address. When breaking, GDB shows the function,
its arguments, and the source line:
Breakpoint 1, file_fsmagic (ms=ms@entry=0x6000394f0,
fn=fn@entry=0x23cb75 "./file", sb=sb@entry=0x23c8f0)
at /usr/src/debug/file-5.22-1/src/fsmagic.c:104
104 {
After rebasing to, e.g., 0x300000000 as above, it looks like this:
$ rebase -b 0x300000000 cygmagic-1.dll
$ nm cygmagic-1.dll | grep file_fsmagic
000000030000c520 T file_fsmagic
(gdb) r ./file
Starting program: /usr/bin/file ./file
[New Thread 2696.0xecc]
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x4d922c520
So, apparently the debug info uses absolute addresses, rather than
image base relative or section relative addresses. After rebasing,
the info points to invalid addresses.
Shouldn't binutils be aware of the effect of rebasing, and make sure
that the existing debug info is correctly evaluated even after rebase?
Even better, shouldn't the dwarf2 debug info be defined image base
relative rather than using absolute addressing, to make sure it
still contains valid information even after rebasing a DLL? At least
on PE/COFF targets?
Thanks,
Corinna
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