[PATCH v5] Add support for the readnever concept

Sergio Durigan Junior
2017-12-01T22:16:09Z

Changes from v4:

- Cleaned-up the commit message and ChangeLogs.

- Simplified condition when checking for readnever. Removed "top.h"
from files that didn't need it.

- Added xref to "-readnever" option (on symbol-file command) pointing
to "--readnever".

- Merged READNOW_HELP and READNEVER_HELP.

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.

It's also worth mentioning that this patch tests whether GDB correctly
fails to initialize if both '--readnow' and '--readnever' options are
passed.

Tested on the BuildBot.

gdb/ChangeLog:

2017-12-01 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.
* coffread.c (coff_symfile_read): Do not map over sections with
'coff_locate_sections' if readnever is on.
* dwarf2read.c (dwarf2_has_info): Return 0 if
readnever is on.
* elfread.c (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.
Call 'validate_readnow_readnever'.
(add_symbol_file_command): Handle '-readnever' option.
Call 'validate_readnow_readnever'.
(_initialize_symfile): Document new '-readnever' option for
both 'symbol-file' and 'add-symbol-file' commands.
* top.h (readnever_symbol_files): New extern global.
* xcoffread.c (xcoff_initial_scan): Do not read debug
information if readnever is on.

gdb/doc/ChangeLog:

2017-12-01 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' and
'add-symbol-file'.

gdb/testsuite/ChangeLog:

2017-12-01 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.
---
gdb/NEWS | 6 +++
gdb/coffread.c | 3 +-
gdb/doc/gdb.texinfo | 21 ++++++++-
gdb/dwarf2read.c | 3 ++
gdb/elfread.c | 3 +-
gdb/main.c | 37 ++++++++++++++--
gdb/objfile-flags.h | 4 ++
gdb/symfile.c | 40 ++++++++++++++---
gdb/testsuite/gdb.base/readnever.c | 41 ++++++++++++++++++
gdb/testsuite/gdb.base/readnever.exp | 83 ++++++++++++++++++++++++++++++++++++
gdb/top.h | 1 +
gdb/xcoffread.c | 48 +++++++++++----------
12 files changed, 255 insertions(+), 35 deletions(-)
create mode 100644 gdb/testsuite/gdb.base/readnever.c
create mode 100644 gdb/testsuite/gdb.base/readnever.exp

diff --git a/gdb/NEWS b/gdb/NEWS
index 06df52819b..ff9a819918 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/coffread.c b/gdb/coffread.c
index 62b6d7945a..98f6eec820 100644
--- a/gdb/coffread.c
+++ b/gdb/coffread.c
@@ -699,7 +699,8 @@ coff_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
}
}

- bfd_map_over_sections (abfd, coff_locate_sections, (void *) info);
+ if (!(objfile->flags & OBJF_READNEVER))
+ bfd_map_over_sections (abfd, coff_locate_sections, (void *) info);

if (info->stabsects)
{
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 4f552225aa..005fed89c5 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -1037,6 +1037,16 @@ 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
+@anchor{--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
@@ -18576,6 +18586,15 @@ 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.
+@xref{--readnever}.
+
@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
@@ -18604,7 +18623,7 @@ the program is running. To do this, use the @code{kill} command
@kindex add-symbol-file
@cindex dynamic linking
@item add-symbol-file @var{filename} @var{address}
-@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{]}
+@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{|} -readnever @r{]}
@itemx add-symbol-file @var{filename} @var{address} -s @var{section} @var{address} @dots{}
The @code{add-symbol-file} command reads additional symbol table
information from the file @var{filename}. You would use this command
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 2572179a75..78c663c49b 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -2319,6 +2319,9 @@ int
dwarf2_has_info (struct objfile *objfile,
const struct dwarf2_debug_sections *names)
{
+ if (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..b806ca3b40 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 (!(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 1c9f836eb3..8f45d45d85 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 +1132,12 @@ symbol_file_add_with_addrs (bfd *abfd, const char *name,
flags |= OBJF_READNOW;
add_flags &= ~SYMFILE_NO_READ;
}
+ else if (readnever_symbol_files
+ || (parent != NULL && (parent->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 +1601,16 @@ 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:
@@ -1641,6 +1658,8 @@ 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, "--") == 0)
stop_processing_options = true;
else
@@ -1650,6 +1669,8 @@ symbol_file_command (const char *args, int from_tty)
if (name == NULL)
error (_("no symbol file name was specified"));

+ validate_readnow_readnever (flags);
+
symbol_file_add_main_1 (name, add_flags, flags);
}
}
@@ -2236,6 +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)
{
if (argv[argcnt + 1] == NULL)
@@ -2257,6 +2280,8 @@ add_symbol_file_command (const char *args, int from_tty)
if (filename == NULL)
error (_("You must provide a filename to be loaded."));

+ 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
@@ -3882,26 +3907,29 @@ _initialize_symfile (void)

observer_attach_free_objfile (symfile_free_objfile);

-#define READNOW_HELP \
+#define READNOW_READNEVER_HELP \
"The '-readnow' option will cause GDB to read the entire symbol file\n\
immediately. This makes the command slower, but may make future operations\n\
-faster."
+faster.\n\
+The '-readnever' option will prevent GDB from reading the symbol file's\n\
+symbolic debug information."

c = add_cmd ("symbol-file", class_files, symbol_file_command, _("\
Load symbol table from executable file FILE.\n\
-Usage: symbol-file [-readnow] FILE\n\
+Usage: symbol-file [-readnow | -readnever] FILE\n\
The `file' command can also load symbol tables, as well as setting the file\n\
-to execute.\n" READNOW_HELP), &cmdlist);
+to execute.\n" READNOW_READNEVER_HELP), &cmdlist);
set_cmd_completer (c, filename_completer);

c = add_cmd ("add-symbol-file", class_files, add_symbol_file_command, _("\
Load symbols from FILE, assuming FILE has been dynamically loaded.\n\
-Usage: add-symbol-file FILE ADDR [-readnow | -s SECT-NAME SECT-ADDR]...\n\
+Usage: add-symbol-file FILE ADDR [-readnow | -readnever | \
+-s SECT-NAME SECT-ADDR]...\n\
ADDR is the starting address of the file's text.\n\
Each '-s' argument provides a section name and address, and\n\
should be specified if the data and bss segments are not contiguous\n\
with the text. SECT-NAME is a section name to be loaded at SECT-ADDR.\n"
-READNOW_HELP),
+READNOW_READNEVER_HELP),
&cmdlist);
set_cmd_completer (c, filename_completer);

diff --git a/gdb/testsuite/gdb.base/readnever.c b/gdb/testsuite/gdb.base/readnever.c
new file mode 100644
index 0000000000..6489e891de
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.c
@@ -0,0 +1,41 @@
+/* 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/>. */
+
+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..151f12a6bb
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.exp
@@ -0,0 +1,83 @@
+# 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
+
+ set test "test readnow and readnever at the same time"
+ gdb_test_multiple "" $test {
+ "'--readnow' and '--readnever' cannot be specified simultaneously" {
+ pass $test
+ set test "expect eof after failure"
+ 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/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 *);
diff --git a/gdb/xcoffread.c b/gdb/xcoffread.c
index 46a43bc9ae..b1c634c93b 100644
--- a/gdb/xcoffread.c
+++ b/gdb/xcoffread.c
@@ -2975,31 +2975,33 @@ xcoff_initial_scan (struct objfile *objfile, symfile_add_flags symfile_flags)
/* Read the string table. */
init_stringtab (abfd, stringtab_offset, objfile);

- /* Read the .debug section, if present. */
- {
- struct bfd_section *secp;
- bfd_size_type length;
- bfd_byte *debugsec = NULL;
+ /* Read the .debug section, if present and if we're not ignoring
+ it. */
+ if (!(objfile->flags & OBJF_READNEVER))
+ {
+ struct bfd_section *secp;
+ bfd_size_type length;
+ bfd_byte *debugsec = NULL;

- secp = bfd_get_section_by_name (abfd, ".debug");
- if (secp)
- {
- length = bfd_section_size (abfd, secp);
- if (length)
- {
- debugsec
- = (bfd_byte *) obstack_alloc (&objfile->objfile_obstack,
- length);
+ secp = bfd_get_section_by_name (abfd, ".debug");
+ if (secp)
+ {
+ length = bfd_section_size (abfd, secp);
+ if (length)
+ {
+ debugsec
+ = (bfd_byte *) obstack_alloc (&objfile->objfile_obstack,
+ length);

- if (!bfd_get_full_section_contents (abfd, secp, &debugsec))
- {
- error (_("Error reading .debug section of `%s': %s"),
- name, bfd_errmsg (bfd_get_error ()));
- }
- }
- }
- info->debugsec = (char *) debugsec;
- }
+ if (!bfd_get_full_section_contents (abfd, secp, &debugsec))
+ {
+ error (_("Error reading .debug section of `%s': %s"),
+ name, bfd_errmsg (bfd_get_error ()));
+ }
+ }
+ }
+ info->debugsec = (char *) debugsec;
+ }
}

/* Read the symbols. We keep them in core because we will want to
--
2.13.3