Re: [PATCH] [PR gdb/21695] Fix lost line info for symbol at addr zero

[Omair Javaid <omair dot javaid at linaro dot org>]

On 28 May 2018 at 15:49, Omair Javaid <omair.javaid@linaro.org> wrote:
> On 15 May 2018 at 19:04, Omair Javaid <omair.javaid@linaro.org> wrote:
>> This patch fixes a unique condition where GDB fails to provide line
>> information of symbol at address zero when code is compiled with text
>> address zero but loaded at an offset > 0.
>>
>> For example lets compile following code snippet:
>>
>> int main() {
>>   return 0;
>> }
>>
>> gcc -O0 -g3 -nostdlib -emain -Wl,-Ttext=0x00 -o file.out file.c
>>
>> Start gdb and run:
>>
>> add-symbol-file file.out 0xffff0000
>> info line main
>>
>> GDB will return error saying no line info is available for the symbol.
>>
>> This is a direct consequence of the fix for PR 12528 where GDB tries to ignore
>> line table for a function which has been garbage collected by the linker.
>>
>> As the garbage collected symbols are sent to address zero GDB assumes a symbol
>> actually placed at address zero as garbage collected.
>>
>> This was fixed with an additional check address < lowpc. But when symbol is
>> loaded at an offset lowpc becomes lowpc + offset while no offset is added to
>> address rather final symbol address is calculated based on baseaddr and address
>> added together. So in case where symbols are loaded at an offset the condition
>> address < lowpc will always return true.
>>
>> This patch fixes this by comparing address against a non offset lowpc.
>> This patch also adds a GDB test case to replicate this behavior.
>>
>> gdb:
>>
>> 2018-05-15  Omair Javaid  <omair.javaid@linaro.org>
>>
>>         PR gdb/21695
>>         * dwarf2read.c (lnp_state_machine::check_line_address): Update declaration.
>>         (dwarf_decode_lines_1): Adjust.
>>
>> gdb/testsuite:
>>
>> 2018-05-15  Omair Javaid  <omair.javaid@linaro.org>
>>
>>         PR gdb/21695
>>         * gdb.base/infoline-reloc-main-from-zero.exp: New test.
>>         * gdb.base/infoline-reloc-main-from-zero.c: New file.
>> ---
>>  gdb/dwarf2read.c                                   |  8 ++--
>>  .../gdb.base/infoline-reloc-main-from-zero.c       | 24 ++++++++++
>>  .../gdb.base/infoline-reloc-main-from-zero.exp     | 51 ++++++++++++++++++++++
>>  3 files changed, 79 insertions(+), 4 deletions(-)
>>  create mode 100644 gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.c
>>  create mode 100644 gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.exp
>>
>> diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
>> index 575d316..b171bb2 100644
>> --- a/gdb/dwarf2read.c
>> +++ b/gdb/dwarf2read.c
>> @@ -20411,7 +20411,7 @@ public:
>>       sequence.  */
>>    void check_line_address (struct dwarf2_cu *cu,
>>                            const gdb_byte *line_ptr,
>> -                          CORE_ADDR lowpc, CORE_ADDR address);
>> +                          CORE_ADDR lowpc_minus_base, CORE_ADDR address);
>>
>>    void handle_set_discriminator (unsigned int discriminator)
>>    {
>> @@ -20755,14 +20755,14 @@ lnp_state_machine::lnp_state_machine (gdbarch *arch, line_header *lh,
>>  void
>>  lnp_state_machine::check_line_address (struct dwarf2_cu *cu,
>>                                        const gdb_byte *line_ptr,
>> -                                      CORE_ADDR lowpc, CORE_ADDR address)
>> +                                      CORE_ADDR lowpc_minus_base, CORE_ADDR address)
>>  {
>>    /* If address < lowpc then it's not a usable value, it's outside the
>>       pc range of the CU.  However, we restrict the test to only address
>>       values of zero to preserve GDB's previous behaviour which is to
>>       handle the specific case of a function being GC'd by the linker.  */
>>
>> -  if (address == 0 && address < lowpc)
>> +  if (address == 0 && address < lowpc_minus_base)
>>      {
>>        /* This line table is for a function which has been
>>          GCd by the linker.  Ignore it.  PR gdb/12528 */
>> @@ -20857,7 +20857,7 @@ dwarf_decode_lines_1 (struct line_header *lh, struct dwarf2_cu *cu,
>>                     line_ptr += bytes_read;
>>
>>                     state_machine.check_line_address (cu, line_ptr,
>> -                                                     lowpc, address);
>> +                                                     lowpc - baseaddr, address);
>>                     state_machine.handle_set_address (baseaddr, address);
>>                   }
>>                   break;
>> diff --git a/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.c b/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.c
>> new file mode 100644
>> index 0000000..8902051
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.c
>> @@ -0,0 +1,24 @@
>> +/* This testcase is part of GDB, the GNU debugger.
>> +
>> +   Copyright 2011-2018 Free Software Foundation, Inc.
>> +
>> +   This program is free software; you can redistribute it and/or modify
>> +   it under the terms of the GNU General Public License as published by
>> +   the Free Software Foundation; either version 3 of the License, or
>> +   (at your option) any later version.
>> +
>> +   This program is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +   GNU General Public License for more details.
>> +
>> +   You should have received a copy of the GNU General Public License
>> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
>> +
>> +// Test case for PR gdb/21695
>> +
>> +int
>> +main ()
>> +{
>> +  return 0;
>> +}
>> diff --git a/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.exp b/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.exp
>> new file mode 100644
>> index 0000000..68564688
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.base/infoline-reloc-main-from-zero.exp
>> @@ -0,0 +1,51 @@
>> +# Copyright 2011-2018 Free Software Foundation, Inc.
>> +
>> +# This program is free software; you can redistribute it and/or modify
>> +# it under the terms of the GNU General Public License as published by
>> +# the Free Software Foundation; either version 3 of the License, or
>> +# (at your option) any later version.
>> +#
>> +# This program is distributed in the hope that it will be useful,
>> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +# GNU General Public License for more details.
>> +#
>> +# You should have received a copy of the GNU General Public License
>> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
>> +
>> +# This file is part of the gdb testsuite
>> +
>> +#
>> +# test running programs
>> +#
>> +
>> +standard_testfile .c
>> +
>> +if [get_compiler_info] {
>> +    return -1
>> +}
>> +
>> +# Build executable with stripped startup code and text section starting at zero
>> +
>> +set opts {debug "additional_flags=-nostdlib -emain -Wl,-Ttext=0x00"}
>> +
>> +if {[build_executable $testfile.exp $testfile $srcfile $opts] == -1} {
>> +    untested "failed to compile"
>> +    return -1
>> +}
>> +
>> +gdb_exit
>> +gdb_start
>> +gdb_reinitialize_dir $srcdir/$subdir
>> +
>> +# Load symbols at an offset 0xffff0000 using add-symbol-file
>> +
>> +gdb_test "add-symbol-file [standard_output_file ${testfile}] 0xffff000" \
>> +    "Reading symbols from .*" \
>> +    "add-symbol-file" \
>> +    "add symbol table from file \".*\" at.*\\(y or n\\) " "y"
>> +
>> +# Check if we are able to read offset adjusted line information of main
>> +
>> +gdb_test "info line main" \
>> +       "Line.*starts at address.*and ends at.*"
>> --
>> 2.7.4
>>
>
>
> Ping!

Ping!