From 6d78332e7748754d4a77a31e692fb8760c8c12ff Mon Sep 17 00:00:00 2001 From: John Baldwin Date: Fri, 21 Dec 2018 10:18:11 -0800 Subject: [PATCH] Workaround a FreeBSD kernel bug resulting in spurious SIGTRAP events. The ptrace command PT_LWPINFO to request detailed information about a stopped thread can return stale signal information from an earlier stop. Events which are reporting an intercepted signal will always report the correct information, but signal stops for some other events such as system call enter/exit events might include stale siginfo from an earlier signal. In particular, if a thread reports a system call entry or exit event after previously reporting a single-step or breakpoint event via SIGTRAP, fbsd_handle_debug_trap believed the system call event was the previous event and claimed it resulting in a spurious SIGTRAP event. True breakpoint and single-step events will never report another event in the pl_flags member of struct ptrace_lwpinfo. Use this to detect stale siginfo by requiring pl_flags to have only the PL_FLAG_SI flag and no other flags before treating a SIGTRAP as a single-step or breakpoint trap. gdb/ChangeLog: * fbsd-nat.c (fbsd_handle_debug_trap): Require pl.pl_flags to equal PL_FLAG_SI. (fbsd_nat_target::stopped_by_sw_breakpoint): Likewise. --- gdb/ChangeLog | 6 ++++++ gdb/fbsd-nat.c | 12 +++++++++--- 2 files changed, 15 insertions(+), 3 deletions(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index db7a745a9c..4fcd2ebf4f 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,9 @@ +2018-12-21 John Baldwin + + * fbsd-nat.c (fbsd_handle_debug_trap): Require pl.pl_flags to + equal PL_FLAG_SI. + (fbsd_nat_target::stopped_by_sw_breakpoint): Likewise. + 2018-12-21 Paul Marechal PR gdb/23974 diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c index 6ec273b1cf..e6b6894919 100644 --- a/gdb/fbsd-nat.c +++ b/gdb/fbsd-nat.c @@ -1238,8 +1238,14 @@ fbsd_handle_debug_trap (ptid_t ptid, const struct ptrace_lwpinfo &pl) { /* Ignore traps without valid siginfo or for signals other than - SIGTRAP. */ - if (! (pl.pl_flags & PL_FLAG_SI) || pl.pl_siginfo.si_signo != SIGTRAP) + SIGTRAP. + + FreeBSD kernels prior to r341800 can return stale siginfo for at + least some events, but those events can be identified by + additional flags set in pl_flags. True breakpoint and + single-step traps should not have other flags set in + pl_flags. */ + if (pl.pl_flags != PL_FLAG_SI || pl.pl_siginfo.si_signo != SIGTRAP) return false; /* Trace traps are either a single step or a hardware watchpoint or @@ -1517,7 +1523,7 @@ fbsd_nat_target::stopped_by_sw_breakpoint () sizeof pl) == -1) return false; - return ((pl.pl_flags & PL_FLAG_SI) + return (pl.pl_flags == PL_FLAG_SI && pl.pl_siginfo.si_signo == SIGTRAP && pl.pl_siginfo.si_code == TRAP_BRKPT); }