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[Bug win32/18027] New: dwarf2 debug info after rebasing DLLs unusable
- From: "corinna at vinschen dot de" <sourceware-bugzilla at sourceware dot org>
- To: gdb-prs at sourceware dot org
- Date: Wed, 25 Feb 2015 15:54:26 +0000
- Subject: [Bug win32/18027] New: dwarf2 debug info after rebasing DLLs unusable
- Auto-submitted: auto-generated
https://sourceware.org/bugzilla/show_bug.cgi?id=18027
Bug ID: 18027
Summary: dwarf2 debug info after rebasing DLLs unusable
Product: gdb
Version: HEAD
Status: NEW
Severity: normal
Priority: P2
Component: win32
Assignee: unassigned at sourceware dot org
Reporter: corinna at vinschen dot de
Hi,
I reported the analog problem to binutils, see
https://sourceware.org/bugzilla/show_bug.cgi?id=18025
but GDB is affected as well.
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(1) package which comes with a DLL called
cygmagic-1.dll. The output of objdump -h 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. 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.
However, when splitting off the debug info into an external *.dbg
file and stripping the debug info from the original DLL, then
debugging works fine even after rebsing the original DLL. So apparently
GDB knows how to handle this situation, but it doesn't take it into
account when the debug info is still present in the DLL itself.
Can this be fixed, please?
Ultimately I think the right thing to do would be to change the
Dwarf2 debug info to 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.
I mentioned this in the binutls bugreport
https://sourceware.org/bugzilla/show_bug.cgi?id=18025 as well.
Thanks,
Corinna
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