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TL;DR: When stepping over a breakpoint with displaced stepping, the core must be notified of all signals, otherwise the displaced step fixup code confuses a breakpoint trap in the signal handler for the expected trap indicating the displaced instruction was single-stepped normally/successfully. Detailed version: Running sigstep.exp with displaced stepping on, against my x86 software single-step branch, I got: FAIL: gdb.base/sigstep.exp: step on breakpoint, to handler: performing step FAIL: gdb.base/sigstep.exp: next on breakpoint, to handler: performing next FAIL: gdb.base/sigstep.exp: continue on breakpoint, to handler: performing continue Turning on debug logs, we see: (gdb) step infrun: clear_proceed_status_thread (process 32147) infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT) infrun: resume (step=1, signal=GDB_SIGNAL_0), trap_expected=1, current thread [process 32147] at 0x400842 displaced: stepping process 32147 now displaced: saved 0x400622: 49 89 d1 5e 48 89 e2 48 83 e4 f0 50 54 49 c7 c0 displaced: %rip-relative addressing used. displaced: using temp reg 2, old value 0x3615eafd37, new value 0x40084c displaced: copy 0x400842->0x400622: c7 81 1c 08 20 00 00 00 00 00 displaced: displaced pc to 0x400622 displaced: run 0x400622: c7 81 1c 08 LLR: Preparing to resume process 32147, 0, inferior_ptid process 32147 LLR: PTRACE_CONT process 32147, 0 (resume event thread) linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 32147, No child processes LLW: waitpid 32147 received Alarm clock (stopped) LLW: PTRACE_CONT process 32147, Alarm clock (preempt 'handle') LNW: waitpid(-1, ...) returned 0, No child processes LLW: exit (ignore) sigchld infrun: target_wait (-1.0.0, status) = infrun: -1.0.0 [process -1], infrun: status->kind = ignore infrun: TARGET_WAITKIND_IGNORE infrun: prepare_to_wait linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 32147, No child processes LLW: waitpid 32147 received Trace/breakpoint trap (stopped) CSBB: process 32147 stopped by software breakpoint LNW: waitpid(-1, ...) returned 0, No child processes LLW: trap ptid is process 32147. LLW: exit infrun: target_wait (-1.0.0, status) = infrun: 32147.32147.0 [process 32147], infrun: status->kind = stopped, signal = GDB_SIGNAL_TRAP infrun: TARGET_WAITKIND_STOPPED displaced: restored process 32147 0x400622 displaced: fixup (0x400842, 0x400622), insn = 0xc7 0x81 ... displaced: restoring reg 2 to 0x3615eafd37 displaced: relocated %rip from 0x400717 to 0x400937 infrun: stop_pc = 0x400937 infrun: delayed software breakpoint trap, ignoring infrun: no line number info infrun: stop_waiting 0x0000000000400937 in __dso_handle () 1: x/i $pc => 0x400937: and %ah,0xa0d64(%rip) # 0x4a16a1 (gdb) FAIL: gdb.base/sigstep.exp: displaced=on: step on breakpoint, to handler: performing step What should have happened is that the breakpoint hit in the signal handler should have been presented to the user. But note that "preempt 'handle'" -- what happened instead is that displaced_step_fixup confused the breakpoint in the signal handler for the expected SIGTRAP indicating the displaced instruction was single-stepped normally/successfully. This should be affecting all software single-step targets in the same way. The fix is to make sure the core sees all signals when displaced stepping, just like we already must see all signals when doing an stepping over a breakpoint in-line. We now get: infrun: target_wait (-1.0.0, status) = infrun: 570.570.0 [process 570], infrun: status->kind = stopped, signal = GDB_SIGNAL_ALRM infrun: TARGET_WAITKIND_STOPPED displaced: restored process 570 0x400622 infrun: stop_pc = 0x400842 infrun: random signal (GDB_SIGNAL_ALRM) infrun: signal arrived while stepping over breakpoint infrun: inserting step-resume breakpoint at 0x400842 infrun: resume (step=0, signal=GDB_SIGNAL_ALRM), trap_expected=0, current thread [process 570] at 0x400842 LLR: Preparing to resume process 570, Alarm clock, inferior_ptid process 570 LLR: PTRACE_CONT process 570, Alarm clock (resume event thread) infrun: prepare_to_wait linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 0, No child processes LLW: exit (ignore) infrun: target_wait (-1.0.0, status) = infrun: -1.0.0 [process -1], infrun: status->kind = ignore sigchld infrun: TARGET_WAITKIND_IGNORE infrun: prepare_to_wait linux_nat_wait: [process -1], [TARGET_WNOHANG] LLW: enter LNW: waitpid(-1, ...) returned 570, No child processes LLW: waitpid 570 received Trace/breakpoint trap (stopped) CSBB: process 570 stopped by software breakpoint LNW: waitpid(-1, ...) returned 0, No child processes LLW: trap ptid is process 570. LLW: exit infrun: target_wait (-1.0.0, status) = infrun: 570.570.0 [process 570], infrun: status->kind = stopped, signal = GDB_SIGNAL_TRAP infrun: TARGET_WAITKIND_STOPPED infrun: stop_pc = 0x400717 infrun: BPSTAT_WHAT_STOP_NOISY infrun: stop_waiting Breakpoint 3, handler (sig=14) at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/sigstep.c:35 35 done = 1; Hardware single-step targets already behave this way, because the Linux backends (both native and gdbserver) always report signals to the core if the thread was single-stepping. As mentioned in the new comment in do_target_resume, we can't fix this by instead making the displaced_step_fixup phase skip fixing up the PC if the single step stopped somewhere we didn't expect. Here's what the backtrace would look like if we did that: Breakpoint 3, handler (sig=14) at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/sigstep.c:35 35 done = 1; 1: x/i $pc => 0x400717 <handler+7>: movl $0x1,0x200943(%rip) # 0x601064 <done> (gdb) bt #0 handler (sig=14) at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/sigstep.c:35 #1 <signal handler called> #2 0x0000000000400622 in _start () (gdb) FAIL: gdb.base/sigstep.exp: displaced=on: step on breakpoint, to handler: backtrace gdb/ChangeLog: 2015-04-10 Pedro Alves <palves@redhat.com> * infrun.c (displaced_step_in_progress): New function. (do_target_resume): Advise target to report all signals if displaced stepping. gdb/testsuite/ChangeLog: 2015-04-10 Pedro Alves <palves@redhat.com> * gdb.base/sigstep.exp (breakpoint_to_handler) (breakpoint_to_handler_entry): New parameter 'displaced'. Use it. Test "backtrace" in handler. (breakpoint_over_handler): New parameter 'displaced'. Use it. (top level): Add new "displaced" test axis to breakpoint_to_handler, breakpoint_to_handler_entry and breakpoint_over_handler. |
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.