I was testing current gdb on a program compiled with Two Level Linetables http://wiki.dwarfstd.org/index.php?title=TwoLevelLineTables and found gdb went into an infinite loop of SIGFPE->handle_sigfpe->SIGFPE ->handle_sigfpe->... The bug for bad line table header version handling is bug 17890. This bug is for the bad SIGFPE handling. How is this supposed to work? Repro: Add a divide-by-zero to gdb after the event loop has been set up, put a breakpoint on handle_sigfpe, run, continue, continue, continue, ... diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c index 715b090..a4bc9ff 100644 --- a/gdb/dwarf2read.c +++ b/gdb/dwarf2read.c @@ -17140,6 +17140,9 @@ dwarf_decode_line_header (unsigned int offset, struct dwarf2_cu *cu) struct dwarf2_section_info *section; bfd *abfd; + i = 0; + i = 3/i; + section = get_debug_line_section (cu); dwarf2_read_section (dwarf2_per_objfile->objfile, section); if (section->buffer == NULL) Program received signal SIGFPE, Arithmetic exception. 0x00000000007c688c in dwarf_decode_line_header (offset=0, cu=0x12b5610) at ../../binutils-gdb/gdb/dwarf2read.c:17144 17144 i = 3/i; (top-gdb) c Continuing. Breakpoint 3, handle_sigfpe (sig=8) at ../../binutils-gdb/gdb/event-top.c:1006 1006 mark_async_signal_handler (sigfpe_token); (top-gdb) Continuing. Program received signal SIGFPE, Arithmetic exception. 0x00000000007c688c in dwarf_decode_line_header (offset=0, cu=0x12b5610) at ../../binutils-gdb/gdb/dwarf2read.c:17144 17144 i = 3/i; (top-gdb) Continuing. Breakpoint 3, handle_sigfpe (sig=8) at ../../binutils-gdb/gdb/event-top.c:1006 1006 mark_async_signal_handler (sigfpe_token); (top-gdb) ...
The master branch has been updated by Andrew Burgess <aburgess@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=fb550a919a88bf4e3950dd7bcdf72f0a18d94206 commit fb550a919a88bf4e3950dd7bcdf72f0a18d94206 Author: Andrew Burgess <andrew.burgess@embecosm.com> Date: Thu Jun 10 10:44:43 2021 +0100 gdb: terminate upon receipt of SIGFPE GDB's SIGFPE handling is broken, this is PR gdb/16505 and PR gdb/17891. We currently try to use an async event token to process SIGFPE. So, when a SIGFPE arrives the signal handler calls mark_async_signal_handler then returns, effectively ignoring the signal (for now). The intention is that later the event loop will see that the async token associated with SIGFPE has been marked and will call the async handler, which just throws an error. The problem is that SIGFPE is not safe to ignore. Ignoring a SIGFPE (unless it is generated artificially, e.g. by raise()) is undefined behaviour, after ignoring the signal on many targets we return to the instruction that caused the SIGFPE to be raised, which immediately causes another SIGFPE to be raised, we get stuck in an infinite loop. The behaviour is certainly true on x86-64. To view this behaviour I simply added some dummy code to GDB that performed an integer divide by zero, compiled this on x86-64 GNU/Linux, ran GDB and saw GDB hang. In this commit, I propose to remove all special handling of SIGFPE and instead just let GDB make use of the default SIGFPE action, that is, to terminate the process. The only user visible change here should be: - If a user sends a SIGFPE to GDB using something like kill, previously GDB would just print an error and remain alive, now GDB will terminate. This is inline with what happens if the user sends GDB a SIGSEGV from kill though, so I don't see this as an issue. - If a bug in GDB causes a real SIGFPE, previously the users GDB session would hang. Now the GDB session will terminate. Again, this is inline with what happens if GDB receives a SIGSEGV due to an internal bug. In bug gdb/16505 there is mention that it would be nice if GDB did more than just terminate when receiving a fatal signal. I haven't done that in this commit, but later commits will move in that direction. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16505 Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17891
(In reply to Sourceware Commits from comment #1) > The master branch has been updated by Andrew Burgess > <aburgess@sourceware.org>: > > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git; > h=fb550a919a88bf4e3950dd7bcdf72f0a18d94206 > > commit fb550a919a88bf4e3950dd7bcdf72f0a18d94206 > Author: Andrew Burgess <andrew.burgess@embecosm.com> > Date: Thu Jun 10 10:44:43 2021 +0100 > > gdb: terminate upon receipt of SIGFPE > > GDB's SIGFPE handling is broken, this is PR gdb/16505 and > PR gdb/17891. Can this be closed now?