Patchwork [v3] Add support for the readnever concept

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Submitter Sergio Durigan Junior
Date Nov. 29, 2017, 1:21 a.m.
Message ID <20171129012139.17325-1-sergiodj@redhat.com>
Download mbox | patch
Permalink /patch/24587/
State New
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Comments

Sergio Durigan Junior - Nov. 29, 2017, 1:21 a.m.
Changes from v2:

- Fixed a few nits pointed by Eli in the docs.

- Implemented Pedro's suggestion and moved the logic of readnever out
  of the "*_build_psymtabs" and into the elf_symfile_read function.

- Handle the propagation of the OBJF_READNEVER flag to the separate
  debuginfo file.

- Reword commit title, fix a few typos in the commit message.


The purpose of this concept is to turn the load of debugging
information off, either globally (via the '--readnever' option), or
objfile-specific.  The implementation proposed here is an extension of
the patch distributed with Fedora GDB; looking at the Fedora patch
itself and the history, one can see some reasons why it was never
resubmitted:

  - The patch appears to have been introduced as a workaround, at
    least initially;
  - The patch is far from perfect, as it simply shunts the load of
    DWARF debugging information, without really worrying about the
    other debug format.
  - Who really does non-symbolic debugging anyways?

One use of this feature is when a user simply wants to do the
following sequence: attach, dump core, detach.  Loading the debugging
information in this case is an unnecessary cause of delay.

This patch expands the version shipped with Fedora GDB in order to
make the feature available for all the debuginfo backends, not only
for DWARF.  It also implements a per-objfile flag which can be
activated by using the "-readnever" command when using the
'add-symbol-file' or 'symbol-file' commands.

While implementing the code for the 'symbol-file' command, I noticed a
bug in 'symbol_file_command': GDB adds the symbol file before
finishing parsing all the options, which means that the position of an
option in the command impacts whether it will be considered or not.  I
changed the code there in order to only add the symbol file after all
options have been parsed.

It's also worth mentioning that this patch tests whether GDB correctly
fails to initialize if both '--readnow' and '--readnever' options are
passed.  We didn't have any infrastructure to test that, so I added a
new version of 'gdb_spawn', called 'gdb_spawn_ignore_error', which can
be used to start a GDB that will fail without compromising the entire
testcase.

Tested on the BuildBot.

gdb/ChangeLog:

2017-11-28  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0: Mention new '--readnever'
	feature.
	* dwarf2read.c: Include "top.h".
	(dwarf2_has_info): Return 0 if READNEVER_SYMBOL_FILES is set.
	* elfread.c: Include "top.h".
	(elf_symfile_read): Do not map over sections with
	'elf_locate_sections' if readnever is on.
	* main.c (validate_readnow_readnever): New function.
	(captured_main_1): Add support for --readnever.
	(print_gdb_help): Document --readnever.
	* objfile-flags.h (enum objfile_flag) <OBJF_READNEVER>: New
	flag.
	* symfile.c (readnever_symbol_files): New global.
	(symbol_file_add_with_addrs): Set 'OBJF_READNEVER' when
	'READNEVER_SYMBOL_FILES' is set.
	(validate_readnow_readnever): New function.
	(symbol_file_command): Handle '-readnever' option.  Move call
	to 'symbol_file_add_main_1' out of the argument parsing loop.
	Call 'validate_readnow_readnever'.
	(add_symbol_file_command): Handle '-readnever' option.
	Document it.  Call 'validate_readnow_readnever'.
	* top.h (readnever_symbol_files): New extern global.

gdb/doc/ChangeLog:

2017-11-28  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (File Options): Document --readnever.
	(Commands to Specify Files): Likewise, for 'symbol-file'.

gdb/testsuite/ChangeLog:

2017-11-28  Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.base/readnever.c, gdb.base/readnever.exp: New files.
	* lib/gdb.exp (default_gdb_spawn): Add 'ignore_error'
	parameter.  Handle case when 'ignore_error' is set.
	(gdb_spawn_ignore_error): New function.
---
 gdb/NEWS                             |  6 +++
 gdb/doc/gdb.texinfo                  | 17 ++++++++
 gdb/dwarf2read.c                     |  4 ++
 gdb/elfread.c                        |  3 +-
 gdb/main.c                           | 37 +++++++++++++++--
 gdb/objfile-flags.h                  |  4 ++
 gdb/symfile.c                        | 33 +++++++++++++---
 gdb/testsuite/gdb.base/readnever.c   | 43 ++++++++++++++++++++
 gdb/testsuite/gdb.base/readnever.exp | 77 ++++++++++++++++++++++++++++++++++++
 gdb/testsuite/lib/gdb.exp            | 18 ++++++---
 gdb/top.h                            |  1 +
 11 files changed, 229 insertions(+), 14 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/readnever.c
 create mode 100644 gdb/testsuite/gdb.base/readnever.exp
Eli Zaretskii - Nov. 29, 2017, 3:39 a.m.
> From: Sergio Durigan Junior <sergiodj@redhat.com>
> Cc: Joel Brobecker <brobecker@adacore.com>,
> 	Yao Qi <qiyaoltc@gmail.com>,
> 	Pedro Alves <palves@redhat.com>,
> 	Eli Zaretskii <eliz@gnu.org>,
> 	Sergio Durigan Junior <sergiodj@redhat.com>
> Date: Tue, 28 Nov 2017 20:21:39 -0500
> 
> Changes from v2:
> 
> - Fixed a few nits pointed by Eli in the docs.
> 
> - Implemented Pedro's suggestion and moved the logic of readnever out
>   of the "*_build_psymtabs" and into the elf_symfile_read function.
> 
> - Handle the propagation of the OBJF_READNEVER flag to the separate
>   debuginfo file.
> 
> - Reword commit title, fix a few typos in the commit message.

OK for the docs.

Thanks.
Pedro Alves - Nov. 29, 2017, 12:24 p.m.
On 11/29/2017 01:21 AM, Sergio Durigan Junior wrote:
> Changes from v2:
> 
> - Fixed a few nits pointed by Eli in the docs.
> 
> - Implemented Pedro's suggestion and moved the logic of readnever out
>   of the "*_build_psymtabs" and into the elf_symfile_read function.

See my comments here:
  https://sourceware.org/ml/gdb-patches/2017-11/msg00766.html

> While implementing the code for the 'symbol-file' command, I noticed a
> bug in 'symbol_file_command': GDB adds the symbol file before
> finishing parsing all the options, which means that the position of an
> option in the command impacts whether it will be considered or not.  I
> changed the code there in order to only add the symbol file after all
> options have been parsed.

Is this the same as:

 https://sourceware.org/ml/gdb-patches/2017-11/msg00656.html

?

Sounds like something that would be better split off to
a precursor patch.

> +++ b/gdb/testsuite/gdb.base/readnever.c
> @@ -0,0 +1,43 @@
> +/* Copyright 2016-2017 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/>.  */
> +
> +#include <stdio.h>

This include isn't necessary.

> +
> +static void
> +fun_three (int a, char b, void *c)
> +{
> +  /* Do nothing.  */
> +}
> +
> +static void
> +fun_two (unsigned int p, const char *y)
> +{
> +  fun_three ((int) p, '1', (void *) y);
> +}
> +
> +static void
> +fun_one (int *x)
> +{
> +  fun_two (10, (const char *) x);
> +}
> +
> +int
> +main (void)
> +{
> +  int a = 10;
> +
> +  fun_one (&a);
> +  return 0;
> +}

> +# Test invalid combination of flags.
> +save_vars { GDBFLAGS } {
> +    append GDBFLAGS " --readnever --readnow"
> +    gdb_exit
> +    gdb_spawn_ignore_error
> +
> +    set test "test readnow and readnever at the same time"
> +    gdb_test_multiple "" $test {
> +	eof {
> +	    pass $test
> +	}
> +    }

I had suggested to match GDB's error output before the eof,
so I'm surprised to see only eof expected?

> +}
> +

>  # Default gdb_spawn procedure.
>  
> -proc default_gdb_spawn { } {
> +proc default_gdb_spawn { { ignore_error 0 } } {
>      global use_gdb_stub
>      global GDB
>      global INTERNAL_GDBFLAGS GDBFLAGS
> @@ -1610,11 +1610,12 @@ proc default_gdb_spawn { } {
>  	}
>      }
>      set res [remote_spawn host "$GDB $INTERNAL_GDBFLAGS $GDBFLAGS [host_info gdb_opts]"]
> -    if { $res < 0 || $res == "" } {
> -	perror "Spawning $GDB failed."
> -	return 1
> +    if { !$ignore_error } {
> +	if { $res < 0 || $res == "" } {
> +	    perror "Spawning $GDB failed."
> +	    return 1
> +	}
>      }

Hmm, I think I'm now confused on why this is needed in the first
place?  GDB-the-executable should be starting successfully, 
print some error output, and exit.  What error do you see
here, exactly?

> -
>      set gdb_spawn_id $res
>      return 0
>  }

Thanks,
Pedro Alves
Sergio Durigan Junior - Nov. 29, 2017, 6:42 p.m.
On Wednesday, November 29 2017, Pedro Alves wrote:

> On 11/29/2017 01:21 AM, Sergio Durigan Junior wrote:
>> Changes from v2:
>> 
>> - Fixed a few nits pointed by Eli in the docs.
>> 
>> - Implemented Pedro's suggestion and moved the logic of readnever out
>>   of the "*_build_psymtabs" and into the elf_symfile_read function.
>
> See my comments here:
>   https://sourceware.org/ml/gdb-patches/2017-11/msg00766.html

I'll address them, thanks.

>> While implementing the code for the 'symbol-file' command, I noticed a
>> bug in 'symbol_file_command': GDB adds the symbol file before
>> finishing parsing all the options, which means that the position of an
>> option in the command impacts whether it will be considered or not.  I
>> changed the code there in order to only add the symbol file after all
>> options have been parsed.
>
> Is this the same as:
>
>  https://sourceware.org/ml/gdb-patches/2017-11/msg00656.html
>
> ?

No, it seems like a different issue, even though it's in the same part
of the code.

> Sounds like something that would be better split off to
> a precursor patch.

Sure, I can do that.

>> +++ b/gdb/testsuite/gdb.base/readnever.c
>> @@ -0,0 +1,43 @@
>> +/* Copyright 2016-2017 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/>.  */
>> +
>> +#include <stdio.h>
>
> This include isn't necessary.

Removed.

>> +
>> +static void
>> +fun_three (int a, char b, void *c)
>> +{
>> +  /* Do nothing.  */
>> +}
>> +
>> +static void
>> +fun_two (unsigned int p, const char *y)
>> +{
>> +  fun_three ((int) p, '1', (void *) y);
>> +}
>> +
>> +static void
>> +fun_one (int *x)
>> +{
>> +  fun_two (10, (const char *) x);
>> +}
>> +
>> +int
>> +main (void)
>> +{
>> +  int a = 10;
>> +
>> +  fun_one (&a);
>> +  return 0;
>> +}
>
>> +# Test invalid combination of flags.
>> +save_vars { GDBFLAGS } {
>> +    append GDBFLAGS " --readnever --readnow"
>> +    gdb_exit
>> +    gdb_spawn_ignore_error
>> +
>> +    set test "test readnow and readnever at the same time"
>> +    gdb_test_multiple "" $test {
>> +	eof {
>> +	    pass $test
>> +	}
>> +    }
>
> I had suggested to match GDB's error output before the eof,
> so I'm surprised to see only eof expected?

Wow, I spent a lot of time trying to make this work before and didn't
succeed, but now I think I managed to do it.  See below.

>> +}
>> +
>
>>  # Default gdb_spawn procedure.
>>  
>> -proc default_gdb_spawn { } {
>> +proc default_gdb_spawn { { ignore_error 0 } } {
>>      global use_gdb_stub
>>      global GDB
>>      global INTERNAL_GDBFLAGS GDBFLAGS
>> @@ -1610,11 +1610,12 @@ proc default_gdb_spawn { } {
>>  	}
>>      }
>>      set res [remote_spawn host "$GDB $INTERNAL_GDBFLAGS $GDBFLAGS [host_info gdb_opts]"]
>> -    if { $res < 0 || $res == "" } {
>> -	perror "Spawning $GDB failed."
>> -	return 1
>> +    if { !$ignore_error } {
>> +	if { $res < 0 || $res == "" } {
>> +	    perror "Spawning $GDB failed."
>> +	    return 1
>> +	}
>>      }
>
> Hmm, I think I'm now confused on why this is needed in the first
> place?  GDB-the-executable should be starting successfully, 
> print some error output, and exit.  What error do you see
> here, exactly?

Here's what I see when I try to use gdb_start:

  spawn /home/sergio/work/src/git/binutils-gdb/readnever/build-64/gdb/testsuite/../../gdb/gdb -nw -nx -data-directory /home/sergio/work/src/git/binutils-gdb/readnever/build-64/gdb/testsuite/../data-directory --readnever --readnow
  /home/sergio/work/src/git/binutils-gdb/readnever/build-64/gdb/testsuite/../../gdb/gdb: '--readnow' and '--readnever' cannot be specified simultaneously
  ERROR: : spawn id exp8 not open
      while executing
  "expect {
  -i exp8 -timeout 10 
          -re "$gdb_prompt $" { 
              verbose "Setting height to 0." 2
          }
          timeout {
              warning "Couldn't set the height to 0"
  ..."
      ("uplevel" body line 1)
      invoked from within
  "uplevel $body" NONE : spawn id exp8 not open
  WARNING: Couldn't set the height to 0
  ERROR: : spawn id exp8 not open
      while executing
  "expect {
  -i exp8 -timeout 10 
          -re "$gdb_prompt $" {
              verbose "Setting width to 0." 2
          }
          timeout {
              warning "Couldn't set the width to 0."
          }..."
      ("uplevel" body line 1)
      invoked from within
  "uplevel $body" NONE : spawn id exp8 not open
  WARNING: Couldn't set the width to 0.
  ERROR: : spawn id exp8 not open
      while executing
  "expect {
  -i exp8 -timeout 10 
          -re ".*A problem internal to GDB has been detected" {
              fail "$message (GDB internal error)"
              gdb_internal_error..."
      ("uplevel" body line 1)
      invoked from within
  "uplevel $body" NONE : spawn id exp8 not open
  UNRESOLVED: gdb.base/readnever.exp: test readnow and readnever at the same time

It doesn't work because gdb_start does more than spawn GDB; it also sets
height/width and other things.  So I tried invoking gdb_spawn directly,
and it worked fine!  Thanks for poking me a bit more.

I'll send a v4 with the necessary adjustments.

Thanks,

Patch

diff --git a/gdb/NEWS b/gdb/NEWS
index 754ce103bd..22502df149 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,12 @@ 
 
 *** Changes since GDB 8.0
 
+* New "--readnever" command line option instructs GDB to not read each
+  symbol file's symbolic debug information.  This makes startup faster
+  but at the expense of not being able to perform symbolic debugging.
+  This option is intended for use cases where symbolic debugging will
+  not be used, e.g., when you only need to dump the debuggee's core.
+
 * GDB now uses the GNU MPFR library, if available, to emulate target
   floating-point arithmetic during expression evaluation when the target
   uses different floating-point formats than the host.  At least version
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 675f6e7bc8..2aca978d0f 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -1037,6 +1037,15 @@  Read each symbol file's entire symbol table immediately, rather than
 the default, which is to read it incrementally as it is needed.
 This makes startup slower, but makes future operations faster.
 
+@item --readnever
+@cindex @code{--readnever}, command-line option
+Do not read each symbol file's symbolic debug information.  This makes
+startup faster but at the expense of not being able to perform
+symbolic debugging.  DWARF unwind information is also not read,
+meaning backtraces may become incomplete or inaccurate.  One use of
+this is when a user simply wants to do the following sequence: attach,
+dump core, detach.  Loading the debugging information in this case is
+an unnecessary cause of delay.
 @end table
 
 @node Mode Options
@@ -18492,6 +18501,14 @@  tables by using the @samp{-readnow} option with any of the commands that
 load symbol table information, if you want to be sure @value{GDBN} has the
 entire symbol table available.
 
+@cindex @code{-readnever}, option for symbol-file command
+@cindex never read symbols
+@cindex symbols, never read
+@item symbol-file @r{[} -readnever @r{]} @var{filename}
+@itemx file @r{[} -readnever @r{]} @var{filename}
+You can instruct @value{GDBN} to never read the symbolic information
+contained in @var{filename} by using the @samp{-readnever} option.
+
 @c FIXME: for now no mention of directories, since this seems to be in
 @c flux.  13mar1992 status is that in theory GDB would look either in
 @c current dir or in same dir as myprog; but issues like competing
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 334d8c2e05..0975d2bbe2 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -82,6 +82,7 @@ 
 #include <unordered_set>
 #include <unordered_map>
 #include "selftest.h"
+#include "top.h"
 
 /* When == 1, print basic high level tracing messages.
    When > 1, be more verbose.
@@ -2319,6 +2320,9 @@  int
 dwarf2_has_info (struct objfile *objfile,
                  const struct dwarf2_debug_sections *names)
 {
+  if (readnever_symbol_files || (objfile->flags & OBJF_READNEVER))
+    return 0;
+
   dwarf2_per_objfile = ((struct dwarf2_per_objfile *)
 			objfile_data (objfile, dwarf2_objfile_data_key));
   if (!dwarf2_per_objfile)
diff --git a/gdb/elfread.c b/gdb/elfread.c
index f2483025ba..e8b4e2a759 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -1171,7 +1171,8 @@  elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
   struct elfinfo ei;
 
   memset ((char *) &ei, 0, sizeof (ei));
-  bfd_map_over_sections (abfd, elf_locate_sections, (void *) & ei);
+  if (!readnever_symbol_files && !(objfile->flags & OBJF_READNEVER))
+    bfd_map_over_sections (abfd, elf_locate_sections, (void *) & ei);
 
   elf_read_minimal_symbols (objfile, symfile_flags, &ei);
 
diff --git a/gdb/main.c b/gdb/main.c
index 61168faf50..8f04da107a 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -402,6 +402,19 @@  symbol_file_add_main_adapter (const char *arg, int from_tty)
   symbol_file_add_main (arg, add_flags);
 }
 
+/* Perform validation of the '--readnow' and '--readnever' flags.  */
+
+static void
+validate_readnow_readnever ()
+{
+  if (readnever_symbol_files && readnow_symbol_files)
+    {
+      error (_("%s: '--readnow' and '--readnever' cannot be "
+	       "specified simultaneously"),
+	     gdb_program_name);
+    }
+}
+
 /* Type of this option.  */
 enum cmdarg_kind
 {
@@ -579,14 +592,17 @@  captured_main_1 (struct captured_main_args *context)
       OPT_NOWINDOWS,
       OPT_WINDOWS,
       OPT_IX,
-      OPT_IEX
+      OPT_IEX,
+      OPT_READNOW,
+      OPT_READNEVER
     };
     static struct option long_options[] =
     {
       {"tui", no_argument, 0, OPT_TUI},
       {"dbx", no_argument, &dbx_commands, 1},
-      {"readnow", no_argument, &readnow_symbol_files, 1},
-      {"r", no_argument, &readnow_symbol_files, 1},
+      {"readnow", no_argument, NULL, OPT_READNOW},
+      {"readnever", no_argument, NULL, OPT_READNEVER},
+      {"r", no_argument, NULL, OPT_READNOW},
       {"quiet", no_argument, &quiet, 1},
       {"q", no_argument, &quiet, 1},
       {"silent", no_argument, &quiet, 1},
@@ -809,6 +825,20 @@  captured_main_1 (struct captured_main_args *context)
 	    }
 	    break;
 
+	  case OPT_READNOW:
+	    {
+	      readnow_symbol_files = 1;
+	      validate_readnow_readnever ();
+	    }
+	    break;
+
+	  case OPT_READNEVER:
+	    {
+	      readnever_symbol_files = 1;
+	      validate_readnow_readnever ();
+	    }
+	    break;
+
 	  case '?':
 	    error (_("Use `%s --help' for a complete list of options."),
 		   gdb_program_name);
@@ -1183,6 +1213,7 @@  Selection of debuggee and its files:\n\n\
   --se=FILE          Use FILE as symbol file and executable file.\n\
   --symbols=SYMFILE  Read symbols from SYMFILE.\n\
   --readnow          Fully read symbol files on first access.\n\
+  --readnever        Do not read symbol files.\n\
   --write            Set writing into executable and core files.\n\n\
 "), stream);
   fputs_unfiltered (_("\
diff --git a/gdb/objfile-flags.h b/gdb/objfile-flags.h
index 43ba8aaefd..f2a2ccfa8c 100644
--- a/gdb/objfile-flags.h
+++ b/gdb/objfile-flags.h
@@ -64,6 +64,10 @@  enum objfile_flag
        unrelated to filesystem names.  It can be for example
        "<image in memory>".  */
     OBJF_NOT_FILENAME = 1 << 6,
+
+    /* User requested that we do not read this objfile's symbolic
+       information.  */
+    OBJF_READNEVER = 1 << 7,
   };
 
 DEF_ENUM_FLAGS_TYPE (enum objfile_flag, objfile_flags);
diff --git a/gdb/symfile.c b/gdb/symfile.c
index feb50f8b79..1be4a54024 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -81,6 +81,7 @@  static void clear_symtab_users_cleanup (void *ignore);
 
 /* Global variables owned by this file.  */
 int readnow_symbol_files;	/* Read full symbols immediately.  */
+int readnever_symbol_files;	/* Never read full symbols.  */
 
 /* Functions this file defines.  */
 
@@ -1131,6 +1132,11 @@  symbol_file_add_with_addrs (bfd *abfd, const char *name,
       flags |= OBJF_READNOW;
       add_flags &= ~SYMFILE_NO_READ;
     }
+  else if (readnever_symbol_files || (objfile->flags & OBJF_READNEVER))
+    {
+      flags |= OBJF_READNEVER;
+      add_flags |= SYMFILE_NO_READ;
+    }
 
   /* Give user a chance to burp if we'd be
      interactively wiping out any existing symbols.  */
@@ -1594,6 +1600,16 @@  find_separate_debug_file_by_debuglink (struct objfile *objfile)
   return debugfile;
 }
 
+/* Make sure that OBJF_{READNOW,READNEVER} are not set
+   simultaneously.  */
+
+static void
+validate_readnow_readnever (objfile_flags flags)
+{
+  if ((flags & OBJF_READNOW) && (flags & OBJF_READNEVER))
+    error (_("-readnow and -readnever cannot be used simultaneously"));
+}
+
 /* This is the symbol-file command.  Read the file, analyze its
    symbols, and add a struct symtab to a symtab list.  The syntax of
    the command is rather bizarre:
@@ -1631,17 +1647,20 @@  symbol_file_command (const char *args, int from_tty)
 	{
 	  if (strcmp (arg, "-readnow") == 0)
 	    flags |= OBJF_READNOW;
+	  else if (strcmp (arg, "-readnever") == 0)
+	    flags |= OBJF_READNEVER;
 	  else if (*arg == '-')
 	    error (_("unknown option `%s'"), arg);
 	  else
-	    {
-	      symbol_file_add_main_1 (arg, add_flags, flags);
-	      name = arg;
-	    }
+	    name = arg;
 	}
 
       if (name == NULL)
 	error (_("no symbol file name was specified"));
+
+      validate_readnow_readnever (flags);
+
+      symbol_file_add_main_1 (name, add_flags, flags);
     }
 }
 
@@ -2238,6 +2257,8 @@  add_symbol_file_command (const char *args, int from_tty)
 	    }
 	  else if (strcmp (arg, "-readnow") == 0)
 	    flags |= OBJF_READNOW;
+	  else if (strcmp (arg, "-readnever") == 0)
+	    flags |= OBJF_READNEVER;
 	  else if (strcmp (arg, "-s") == 0)
 	    {
 	      expecting_sec_name = 1;
@@ -2245,10 +2266,12 @@  add_symbol_file_command (const char *args, int from_tty)
 	    }
 	  else
 	    error (_("USAGE: add-symbol-file <filename> <textaddress>"
-		     " [-readnow] [-s <secname> <addr>]*"));
+		     " [-readnow|-readnever] [-s <secname> <addr>]*"));
 	}
     }
 
+  validate_readnow_readnever (flags);
+
   /* This command takes at least two arguments.  The first one is a
      filename, and the second is the address where this file has been
      loaded.  Abort now if this address hasn't been provided by the
diff --git a/gdb/testsuite/gdb.base/readnever.c b/gdb/testsuite/gdb.base/readnever.c
new file mode 100644
index 0000000000..7b2fe1cae4
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.c
@@ -0,0 +1,43 @@ 
+/* Copyright 2016-2017 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/>.  */
+
+#include <stdio.h>
+
+static void
+fun_three (int a, char b, void *c)
+{
+  /* Do nothing.  */
+}
+
+static void
+fun_two (unsigned int p, const char *y)
+{
+  fun_three ((int) p, '1', (void *) y);
+}
+
+static void
+fun_one (int *x)
+{
+  fun_two (10, (const char *) x);
+}
+
+int
+main (void)
+{
+  int a = 10;
+
+  fun_one (&a);
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/readnever.exp b/gdb/testsuite/gdb.base/readnever.exp
new file mode 100644
index 0000000000..a6d48db9b6
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.exp
@@ -0,0 +1,77 @@ 
+# Copyright 2016-2017 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/>.
+
+standard_testfile .c
+
+if { [build_executable "failed to build" $testfile $srcfile { debug }] == -1 } {
+    untested "Couldn't compile ${srcfile}"
+    return -1
+}
+
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " --readnever"
+    clean_restart ${binfile}
+}
+
+if ![runto_main] then {
+    perror "couldn't run to breakpoint"
+    continue
+}
+
+gdb_test "break fun_three" \
+         "Breakpoint $decimal at $hex"
+
+gdb_test "continue" \
+         "Breakpoint $decimal, $hex in fun_three \\(\\)"
+
+gdb_test "backtrace" \
+         [multi_line "#0  $hex in fun_three \\(\\)" \
+                     "#1  $hex in fun_two \\(\\)" \
+                     "#2  $hex in fun_one \\(\\)" \
+                     "#3  $hex in main \\(\\)" ]
+
+gdb_test_no_output "maint info symtabs" \
+    "maint info symtabs no output for --readnever"
+gdb_test_no_output "maint info psymtabs" \
+    "maint info psymtabs no output for --readnever"
+
+# Test invalid combination of flags.
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " --readnever --readnow"
+    gdb_exit
+    gdb_spawn_ignore_error
+
+    set test "test readnow and readnever at the same time"
+    gdb_test_multiple "" $test {
+	eof {
+	    pass $test
+	}
+    }
+}
+
+
+# Test symbol-file's -readnever option.
+
+# Restart GDB without the --readnever option.
+gdb_exit
+gdb_start
+gdb_test "symbol-file ${binfile}0.o -readnever" \
+    "Reading symbols from ${binfile}0\.o\.\.\.\\\(no debugging symbols found\\\)\.\.\.done\." \
+    "use symbol-file -readnever"
+
+gdb_test_no_output "maint info symtabs" \
+    "maint info symtabs no output for symbol-file -readnever"
+gdb_test_no_output "maint info psymtabs" \
+    "maint info psymtabs no output for symbol-file -readnever"
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 8d6972ab1f..6a6326344a 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -1580,7 +1580,7 @@  proc gdb_file_cmd { arg } {
 
 # Default gdb_spawn procedure.
 
-proc default_gdb_spawn { } {
+proc default_gdb_spawn { { ignore_error 0 } } {
     global use_gdb_stub
     global GDB
     global INTERNAL_GDBFLAGS GDBFLAGS
@@ -1610,11 +1610,12 @@  proc default_gdb_spawn { } {
 	}
     }
     set res [remote_spawn host "$GDB $INTERNAL_GDBFLAGS $GDBFLAGS [host_info gdb_opts]"]
-    if { $res < 0 || $res == "" } {
-	perror "Spawning $GDB failed."
-	return 1
+    if { !$ignore_error } {
+	if { $res < 0 || $res == "" } {
+	    perror "Spawning $GDB failed."
+	    return 1
+	}
     }
-
     set gdb_spawn_id $res
     return 0
 }
@@ -4077,6 +4078,13 @@  proc gdb_spawn { } {
     default_gdb_spawn
 }
 
+# Spawn the gdb process, but ignore errors.  This is useful if you
+# want GDB to fail, e.g. when testing invalid command line options.
+
+proc gdb_spawn_ignore_error { } {
+    default_gdb_spawn 1
+}
+
 # Spawn GDB with CMDLINE_FLAGS appended to the GDBFLAGS global.
 
 proc gdb_spawn_with_cmdline_opts { cmdline_flags } {
diff --git a/gdb/top.h b/gdb/top.h
index 26fe87842f..a1df64f383 100644
--- a/gdb/top.h
+++ b/gdb/top.h
@@ -267,6 +267,7 @@  extern int gdb_in_secondary_prompt_p (struct ui *ui);
 
 /* From random places.  */
 extern int readnow_symbol_files;
+extern int readnever_symbol_files;
 
 /* Perform _initialize initialization.  */
 extern void gdb_init (char *);