[PATCH] Fix GDB internal error by using text (instead of data) section offset

Kevin Buettner via Gdb-patches
2022-01-25T23:05:31Z

Fedora Rawhide is now using gcc-12.0. As part of updating to the
gcc-12.0 package set, Rawhide is also now using a version of libgcc_s
which lacks a .data section. This causes gdb to fail in the following
fashion while debugging a program (such as gdb) which uses libgcc_s:

(top-gdb) run
Starting program: rawhide-master/bld/gdb/gdb
...
objfiles.h:467: internal-error: sect_index_data not initialized
A problem internal to GDB has been detected,
further debugging may prove unreliable.
...

I snipped the backtrace from the above output. Instead, here's a
portion of a backtrace obtained using GDB's backtrace command.
(Obviously, in order to obtain it, I used a GDB which has been patched
with this commit.)

#0 internal_error (
file=0xc6a508 "gdb/objfiles.h", line=467,
fmt=0xc6a4e8 "sect_index_data not initialized")
at gdbsupport/errors.cc:51
#1 0x00000000005f9651 in objfile::data_section_offset (this=0x4fa48f0)
at gdb/objfiles.h:467
#2 0x000000000097c5f8 in relocate_address (address=0x17244, objfile=0x4fa48f0)
at gdb/stap-probe.c:1333
#3 0x000000000097c630 in stap_probe::get_relocated_address (this=0xa1a17a0,
objfile=0x4fa48f0)
at gdb/stap-probe.c:1341
#4 0x00000000004d7025 in create_exception_master_breakpoint_probe (
objfile=0x4fa48f0)
at gdb/breakpoint.c:3505
#5 0x00000000004d7426 in create_exception_master_breakpoint ()
at gdb/breakpoint.c:3575
#6 0x00000000004efcc1 in breakpoint_re_set ()
at gdb/breakpoint.c:13407
#7 0x0000000000956998 in solib_add (pattern=0x0, from_tty=0, readsyms=1)
at gdb/solib.c:1001
#8 0x00000000009576a8 in handle_solib_event ()
at gdb/solib.c:1269
...

The function 'relocate_address' in gdb/stap-probe.c attempts to do
its "relocation" by using objfile->data_section_offset(). That
method, data_section_offset() is defined as follows in objfiles.h:

CORE_ADDR data_section_offset () const
{
return section_offsets[SECT_OFF_DATA (this)];
}

The internal error occurs when the SECT_OFF_DATA macro finds that the
'sect_index_data' field is -1:

#define SECT_OFF_DATA(objfile) \
((objfile->sect_index_data == -1) \
? (internal_error (__FILE__, __LINE__, \
_("sect_index_data not initialized")), -1) \
: objfile->sect_index_data)

The obvious solution is to use some other section offset instead - as
I recall, on Linux, the section offsets (for those sections which
exist) will all be the same. SECT_OFF_TEXT / text_section_offset
seems like a logical choice, so that's what I've used. Actually, in
this context, I think that text_section_offset is a better choice even
setting aside the current difficulty. (The breakpoint related code
which calls it is dealing with code addresses, not data addresses.
Therefore it's more likely to be correct even on OSes for which
section offsets can differ.)

Searching the sources turned up one other use of data_section_offset,
in gdb/dtrace-probe.c, so I've updated that code as well. (I'd
guess that one was copied from the other.)

So, what happens if there's no .text section? If that were to
occur, GDB would be in real trouble elsewhere since a search
for "text_section_offset" reveals 55 uses of this method, 27
of which are in DWARF related code.

Summary:

* gdb/dtrace-probe.c (dtrace_probe::get_relocated_address):
Use method text_section_offset instead of data_section_offset.
* gdb/stap-probe.c (relocate_address): Likewise.
---
gdb/dtrace-probe.c | 2 +-
gdb/stap-probe.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/gdb/dtrace-probe.c b/gdb/dtrace-probe.c
index ea4999ccebe..9aeefb6060c 100644
--- a/gdb/dtrace-probe.c
+++ b/gdb/dtrace-probe.c
@@ -684,7 +684,7 @@ dtrace_probe::is_enabled () const
CORE_ADDR
dtrace_probe::get_relocated_address (struct objfile *objfile)
{
- return this->get_address () + objfile->data_section_offset ();
+ return this->get_address () + objfile->text_section_offset ();
}

/* Implementation of the get_argument_count method. */
diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
index 2310fdec86d..c4b24df7564 100644
--- a/gdb/stap-probe.c
+++ b/gdb/stap-probe.c
@@ -1330,7 +1330,7 @@ stap_probe::parse_arguments (struct gdbarch *gdbarch)
static CORE_ADDR
relocate_address (CORE_ADDR address, struct objfile *objfile)
{
- return address + objfile->data_section_offset ();
+ return address + objfile->text_section_offset ();
}

/* Implementation of the get_relocated_address method. */
--
2.34.1