[Bug win32/18027] New: dwarf2 debug info after rebasing DLLs unusable

["corinna at vinschen dot de" <sourceware-bugzilla at sourceware dot org>]

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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