Fix regression: cannot start with LD_PRELOAD=libSegFault.so (PR gdb/18653#c7)

At https://sourceware.org/bugzilla/show_bug.cgi?id=18653#c7, Andrew
reports that the fix for PR gdb/18653 made GDB useless if you preload
libSegFault.so, because GDB internal-errors on startup:

 $ LD_PRELOAD=libSegFault.so gdb
 src/gdb/common/signals-state-save-restore.c:64: internal-error: unexpected signal handler
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Aborted (core dumped)
 $

The internal error comes from the code saving the signal dispositions
inherited from gdb's parent:

 (top-gdb) bt
 #0  0x000000000056b001 in internal_error(char const*, int, char const*, ...) (file=0xaf5f38 "src/gdb/common/signals-state-save-restore.c", line=64, fmt=0xaf5f18 "unexpected signal handler") at src/gdb/common/errors.c:54
 #1  0x00000000005752c9 in save_original_signals_state() () at src/gdb/common/signals-state-save-restore.c:64
 #2  0x00000000007425de in captured_main_1(captured_main_args*) (context=0x7fffffffd860)
     at src/gdb/main.c:509
 #3  0x0000000000743622 in captured_main(void*) (data=0x7fffffffd860) at src/gdb/main.c:1145
 During symbol reading, cannot get low and high bounds for subprogram DIE at 24065.
 #4  0x00000000007436f9 in gdb_main(captured_main_args*) (args=0x7fffffffd860) at src/gdb/main.c:1171
 #5  0x0000000000413acd in main(int, char**) (argc=1, argv=0x7fffffffd968) at src/gdb/gdb.c:32

This commit downgrades the internal error to a warning.  You'll get
instead:

~~~
 $ LD_PRELOAD=libSegFault.so gdb
 warning: Found custom handler for signal 11 (Segmentation fault) preinstalled.
 Some signal dispositions inherited from the environment (SIG_DFL/SIG_IGN)
 won't be propagated to spawned programs.
 GNU gdb (GDB) 8.0.50.20171213-git
 Copyright (C) 2017 Free Software Foundation, Inc.
 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
 This is free software: you are free to change and redistribute it.
 There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
 and "show warranty" for details.
 This GDB was configured as "x86_64-pc-linux-gnu".
 Type "show configuration" for configuration details.
 For bug reporting instructions, please see:
 <http://www.gnu.org/software/gdb/bugs/>.
 Find the GDB manual and other documentation resources online at:
 <http://www.gnu.org/software/gdb/documentation/>.
 For help, type "help".
 Type "apropos word" to search for commands related to "word"...
 (gdb)
~~~

This also moves the location where save_original_signals_state is
called a bit further below (to after option processing), so that "-q"
disables the warning:

~~~
 $ LD_PRELOAD=libSegFault.so gdb -q
 (gdb)
~~~

New testcase included.

gdb/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* common/signals-state-save-restore.c
	(save_original_signals_state): New parameter 'quiet'.  Warn if we
	find a custom handler preinstalled, instead of internal erroring.
	But only warn if !quiet.
	* common/signals-state-save-restore.h
	(save_original_signals_state): New parameter 'quiet'.
	* main.c (captured_main_1): Move save_original_signals_state call
	after option handling, and pass QUIET.

gdb/gdbserver/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* server.c (captured_main): Pass quiet=false to
	save_original_signals_state.

gdb/testsuite/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* gdb.base/libsegfault.exp: New.

1 files changed, 84 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/libsegfault.exp b/gdb/testsuite/gdb.base/libsegfault.exp
new file mode 100644
index 0000000000..e1de253c43
--- /dev/null
+++ b/gdb/testsuite/gdb.base/libsegfault.exp
@@ -0,0 +1,84 @@
+# Copyright 2017-2018 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 that GDB tolerates being started with libSegFault.so preloaded
+# with LD_PRELOAD, and that GDB warns about a custom SIGSEGV custom
+# handler.  See PR gdb/18653
+# <https://sourceware.org/bugzilla/show_bug.cgi?id=18653#c7>.
+
+# We cannot expect remote hosts to see environment variables set on
+# the local machine.
+if { [is_remote host] } {
+    unsupported "can't set environment variables on remote host"
+    return -1
+}
+
+# Spawn GDB with LIB preloaded with LD_PRELOAD.  CMDLINE_OPTS are
+# command line options passed to GDB.
+
+proc gdb_spawn_with_ld_preload {lib cmdline_opts} {
+    global env
+
+    save_vars { env(LD_PRELOAD) } {
+	if { ![info exists env(LD_PRELOAD) ]
+	     || $env(LD_PRELOAD) == "" } {
+	    set env(LD_PRELOAD) "$lib"
+	} else {
+	    append env(LD_PRELOAD) ":$lib"
+	}
+
+	gdb_spawn_with_cmdline_opts $cmdline_opts
+    }
+}
+
+proc test_libsegfault {} {
+    global gdb_prompt
+
+    set libsegfault "libSegFault.so"
+
+    # When started normally, if libSegFault.so is preloaded, GDB
+    # should warn about not being able to propagate the signal
+    # disposition of SIGSEGV.
+    gdb_exit
+    gdb_spawn_with_ld_preload $libsegfault ""
+
+    set test "gdb emits custom handler warning"
+    gdb_test_multiple "" $test {
+	-re "cannot be preloaded.*\r\n$gdb_prompt $" {
+	    # Glibc 2.22 outputs:
+	    # ERROR: ld.so: object 'libSegFault.so' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
+	    untested "cannot preload libSegFault.so"
+	    return
+	}
+	-re "Found custom handler.*won't be propagated.*\r\n$gdb_prompt $" {
+	    pass $test
+	}
+    }
+
+    # "-q" should disable the warning, though.
+    gdb_exit
+    gdb_spawn_with_ld_preload $libsegfault "-q"
+
+    set test "quiet suppresses custom handler warning"
+    gdb_test_multiple "" $test {
+	-re "^$gdb_prompt $" {
+	    pass $test
+	}
+    }
+}
+
+test_libsegfault