From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 26876 invoked by alias); 14 Mar 2011 14:20:27 -0000 Received: (qmail 26857 invoked by uid 22791); 14 Mar 2011 14:20:24 -0000 X-SWARE-Spam-Status: No, hits=0.8 required=5.0 tests=AWL,BAYES_00,RCVD_IN_DNSWL_NONE X-Spam-Check-By: sourceware.org Received: from lon1-post-1.mail.demon.net (HELO lon1-post-1.mail.demon.net) (195.173.77.148) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Mon, 14 Mar 2011 14:20:20 +0000 Received: from [80.177.246.162] (helo=hestia.halldom.com) by lon1-post-1.mail.demon.net with esmtp (Exim 4.69) id 1Pz8d7-0005A8-YO for gdb@sourceware.org; Mon, 14 Mar 2011 14:20:17 +0000 Received: from hyperion.halldom.com ([80.177.246.170] helo=HYPERION) by hestia.halldom.com with esmtpsa (TLSv1:AES128-SHA:128) (Exim 4.71) (envelope-from ) id 1Pz8d6-0007jW-Qz for gdb@sourceware.org; Mon, 14 Mar 2011 14:20:16 +0000 From: "Chris Hall" To: Subject: Non-stop mode disfunctional ? Date: Mon, 14 Mar 2011 14:20:00 -0000 Message-ID: <014b01cbe252$f0f736c0$d2e5a440$@highwayman.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-IsSubscribed: yes Mailing-List: contact gdb-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sourceware.org X-SW-Source: 2011-03/txt/msg00096.txt.bz2 Sorry to follow up my own posting... ... I'm hoping to discover if the fact that non-stop doesn't work is (a) because I'm not using it correctly, or (b) a well known problem I should know about, or (c) an actual bug, or (d) something else ? Thanks, Chris > -----Original Message----- > From: gdb-owner@sourceware.org [mailto:gdb-owner@sourceware.org] On > Behalf Of Chris Hall > Sent: 07 March 2011 11:01 > To: gdb@sourceware.org > Subject: SIGSEGV on exit from subroutines -- problem with non-stop ? > > Hi, > > I am using gdb 7.2-14.fc14 to work on a large multi-threaded > application, in C, x86-64. > > I have .gdbinit, per the book: > > set target async 1 > set pagination off > set non-stop on > > When I step using 's' or 'n', as it leaves some subroutines I keep > getting SIGSEGV, such as: > > Program received signal SIGSEGV, Segmentation fault. > signal_set (signo=Cannot access memory at address > 0xffffffffffffff5c) > at ... > > When I 'disass' the current instruction is a leaveq. Examining the > registers I observe that rbp is zero, which is clearly nonsense. > > I found one instance which was repeatable, which happened to be > before > any threads were started: if I 'ni' through a particular function, > it > gets to the leaveq, and gets stuck there. Each time I do ni, the > rsp > and the rbp are updated by the repeated leaveq, until it goes bang. > > So... I began to think this isn't something complicated to do with > multiple threads... so here is a test: > > <<--test.c----------------------------------------------- > #include > #include > > static void > target(const char* message) { > printf("%s ...BANG!\n", message) ; > } > > int main(int argc, char* argv[]) { > > target("Light the blue touch paper") ; > > return 0 ; > } > ------------------------------------------------------->> > > Compiled by gcc 4.5.1 "-g -O0". > > If I do "gdb test", stepping by "n": > > <<------------------------------------------------------- > (gdb) show non-stop > Controlling the inferior in non-stop mode is on. > (gdb) b target > Breakpoint 1 at 0x4004d0: file test.c, line 6. > (gdb) run > Starting program: ...........test > > Breakpoint 1, target (message=0x400615 "Light the blue touch paper") > at test.c:6 > 6 printf("%s ...BANG!\n", message) ; > (gdb) n > Light the blue touch paper ...BANG! > 7 } > (gdb) n > > Program received signal SIGSEGV, Segmentation fault. > target (message=Cannot access memory at address 0xfffffffffffffff8 > ) at test.c:7 > 7 } > (gdb) info reg > .... > rbp 0x0 0x0 > rsp 0x7fffffffe248 0x7fffffffe248 > .... > rip 0x4004e9 0x4004e9 > .... > ------------------------------------------------------->> > > Or, stepping by 'ni': > > <<------------------------------------------------------- > (gdb) show non-stop > Controlling the inferior in non-stop mode is on. > (gdb) b target > Breakpoint 1 at 0x4004d0: file test.c, line 6. > (gdb) disass target > Dump of assembler code for function target: > 0x00000000004004c4 <+0>: push %rbp > 0x00000000004004c5 <+1>: mov %rsp,%rbp > 0x00000000004004c8 <+4>: sub $0x10,%rsp > 0x00000000004004cc <+8>: mov %rdi,-0x8(%rbp) > 0x00000000004004d0 <+12>: mov $0x400608,%eax > 0x00000000004004d5 <+17>: mov -0x8(%rbp),%rdx > 0x00000000004004d9 <+21>: mov %rdx,%rsi > 0x00000000004004dc <+24>: mov %rax,%rdi > 0x00000000004004df <+27>: mov $0x0,%eax > 0x00000000004004e4 <+32>: callq 0x4003b8 > 0x00000000004004e9 <+37>: leaveq > 0x00000000004004ea <+38>: retq > End of assembler dump. > (gdb) disp/i $pc > (gdb) run > Starting program: .......test > > Breakpoint 1, target (message=0x400615 "Light the blue touch paper") > at test.c:6 > 6 printf("%s ...BANG!\n", message) ; > ..... > 1: x/i $pc > => 0x4004e4 : callq 0x4003b8 > (gdb) ni > Light the blue touch paper ...BANG! > 7 } > 1: x/i $pc > => 0x4004e9 : leaveq > (gdb) ni > target (message=0x100000000
) at > test.c:7 > 7 } > 1: x/i $pc > => 0x4004e9 : leaveq > (gdb) ni > Cannot access memory at address 0x8 > (gdb) ni > The program is not being run. > ------------------------------------------------------->> > > I note that if I turn off the "non-stop" option, it works. So this > is > something to do with debugging multi-threaded ! > > I note also that if I change the target to: > > static int > target(const char* message) { > printf("%s ...BANG!\n", message) ; > return 0 ; > } > > the problem goes away... so one extra instruction between the callq > and the leaveq makes a difference: > > 0x00000000004004dc <+24>: mov %rax,%rdi > 0x00000000004004df <+27>: mov $0x0,%eax > 0x00000000004004e4 <+32>: callq 0x4003b8 > 0x00000000004004e9 <+37>: mov $0x0,%eax > 0x00000000004004ee <+42>: leaveq > 0x00000000004004ef <+43>: retq > > This goes some way to explaining why it appeared to be a sporadic > problem. > > Is this me, or is this a bug ? It used to work :-( > > Thanks, > > Chris