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Re: --gc-section leftovers workaround.
- From: Pedro Alves <pedro at codesourcery dot com>
- To: gdb-patches at sourceware dot org
- Date: Wed, 28 Oct 2009 19:29:50 +0000
- Subject: Re: --gc-section leftovers workaround.
- References: <200910281851.01364.pedro@codesourcery.com>
On Wednesday 28 October 2009 18:51:00, Pedro Alves wrote:
> With --gc-sections, gnu ld zeros out the start addresses of
> unreferences FDEs, but leaves them behind. On targets where
> addresses near 0 are valid, GDB can easily confuse these
> leftovers for the valid FDEs, ending up with invalid debug
> info loaded for such low addresses. Since this is a problem
> that has beein in the field for a while, an FSF GDB
> workaround seems appropriate, given that it is a simple
> workaround --- give preference to FDEs that do not
> start at zero when overlaps are detected.
>
> Given that we now sort and bsearch FDEs, GDB is already giving
> such preference as a side effect, but the code that discards
> duplicate FDEs could be discarding the valid FDEs at 0, and
> keep an FDE that should have been garbage collected. This patch
> makes the overlap check explicit so as to avoid that. Tested on
> arm-none-eabi where it fixed the problem, and on
> x86_64-unknown-linux-gnu. Applied.
I managed to post and apply the wrong patch. I've reverted
it already.
Here's the correct patch that I'll (re-)apply in a bit.
--
Pedro Alves
2009-10-28 Pedro Alves <pedro@codesourcery.com>
* dwarf2-frame.c (dwarf2_build_frame_info): Discard --gc-section
leftover FDEs.
---
gdb/dwarf2-frame.c | 64 ++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 49 insertions(+), 15 deletions(-)
Index: src/gdb/dwarf2-frame.c
===================================================================
--- src.orig/gdb/dwarf2-frame.c 2009-10-28 18:59:53.000000000 +0000
+++ src/gdb/dwarf2-frame.c 2009-10-28 19:07:29.000000000 +0000
@@ -2101,7 +2101,9 @@ dwarf2_build_frame_info (struct objfile
if (fde_table.num_entries != 0)
{
struct dwarf2_fde_table *fde_table2;
- int i, j;
+ struct dwarf2_fde *fde_prev = NULL;
+ struct dwarf2_fde *first_non_zero_fde = NULL;
+ int i;
/* Prepare FDE table for lookups. */
qsort (fde_table.entries, fde_table.num_entries,
@@ -2110,22 +2112,54 @@ dwarf2_build_frame_info (struct objfile
/* Copy fde_table to obstack: it is needed at runtime. */
fde_table2 = (struct dwarf2_fde_table *)
obstack_alloc (&objfile->objfile_obstack, sizeof (*fde_table2));
+ fde_table2->num_entries = 0;
- /* Since we'll be doing bsearch, squeeze out identical (except for
- eh_frame_p) fde entries so bsearch result is predictable. */
- for (i = 0, j = 0; j < fde_table.num_entries; ++i)
- {
- const int k = j;
-
- obstack_grow (&objfile->objfile_obstack, &fde_table.entries[j],
- sizeof (fde_table.entries[0]));
- while (++j < fde_table.num_entries
- && (fde_table.entries[k]->initial_location
- == fde_table.entries[j]->initial_location))
- /* Skip. */;
- }
+ /* Check for leftovers from --gc-sections. The GNU linker sets
+ the relevant symbols to zero, but doesn't zero the FDE *end*
+ ranges because there's no relocation there. It's (offset,
+ length), not (start, end). On targets where address zero is
+ just another valid address this can be a problem, since the
+ FDEs appear to be non-empty in the output --- we could pick
+ out the wrong FDE. To work around this, when overlaps are
+ detected, we prefer FDEs that do not start at zero.
+
+ Start by finding the first FDE with non-zero start. Below
+ we'll discard all FDEs that start at zero and overlap this
+ one. */
+ for (i = 0; i < fde_table.num_entries; i++)
+ {
+ struct dwarf2_fde *fde = fde_table.entries[i];
+
+ if (fde->initial_location != 0)
+ {
+ first_non_zero_fde = fde;
+ break;
+ }
+ }
+
+ /* Since we'll be doing bsearch, squeeze out identical (except
+ for eh_frame_p) fde entries so bsearch result is predictable.
+ Also discard leftovers from --gc-sections. */
+ for (i = 0; i < fde_table.num_entries; i++)
+ {
+ struct dwarf2_fde *fde = fde_table.entries[i];
+
+ if (fde->initial_location == 0
+ && first_non_zero_fde != NULL
+ && (first_non_zero_fde->initial_location
+ < fde->initial_location + fde->address_range))
+ continue;
+
+ if (fde_prev != NULL
+ && fde_prev->initial_location == fde->initial_location)
+ continue;
+
+ obstack_grow (&objfile->objfile_obstack, &fde_table.entries[i],
+ sizeof (fde_table.entries[0]));
+ ++fde_table2->num_entries;
+ fde_prev = fde;
+ }
fde_table2->entries = obstack_finish (&objfile->objfile_obstack);
- fde_table2->num_entries = i;
set_objfile_data (objfile, dwarf2_frame_objfile_data, fde_table2);
/* Discard the original fde_table. */