Date: Tue, 9 Oct 2018 22:35:43 +0000 (UTC) From: John Baldwin <jhb@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-11@freebsd.org Subject: svn commit: r339273 - in stable/11/sys: amd64/amd64 i386/i386 Message-ID: <201810092235.w99MZhrG071806@repo.freebsd.org>
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Author: jhb Date: Tue Oct 9 22:35:43 2018 New Revision: 339273 URL: https://svnweb.freebsd.org/changeset/base/339273 Log: MFC 338976: Don't clear DR6 for debug exceptions from userland. This reverts part of r333368. The attempt to clear DR6 was occuring too soon as trapsignal() does not pause to let the debugger notice the SIGTRAP and query DR6. The signal exchange does not occur until much later during ast(). As a result, GDB was no longer recognizing hardware breakpoints and watchpoints on x86. In addition, any userland programs that want to inspect DR6 in a SIGTRAP handler don't have a way to do this if we clear DR6 in the exception handler. Instead of relying on the kernel to clear DR6, debuggers will have to explicitly clear it after a trace trap (which they needed to do on older kernels anyway). Modified: stable/11/sys/amd64/amd64/trap.c stable/11/sys/i386/i386/trap.c Directory Properties: stable/11/ (props changed) Modified: stable/11/sys/amd64/amd64/trap.c ============================================================================== --- stable/11/sys/amd64/amd64/trap.c Tue Oct 9 22:22:15 2018 (r339272) +++ stable/11/sys/amd64/amd64/trap.c Tue Oct 9 22:35:43 2018 (r339273) @@ -656,12 +656,6 @@ trap(struct trapframe *frame) KASSERT((read_rflags() & PSL_I) != 0, ("interrupts disabled")); trapsignal(td, &ksi); - /* - * Clear any pending debug exceptions after allowing a - * debugger to read DR6 while stopped in trapsignal(). - */ - if (type == T_TRCTRAP) - load_dr6(0); userret: userret(td, frame); KASSERT(PCB_USER_FPU(td->td_pcb), Modified: stable/11/sys/i386/i386/trap.c ============================================================================== --- stable/11/sys/i386/i386/trap.c Tue Oct 9 22:22:15 2018 (r339272) +++ stable/11/sys/i386/i386/trap.c Tue Oct 9 22:35:43 2018 (r339273) @@ -758,12 +758,6 @@ kernel_trctrap: KASSERT((read_eflags() & PSL_I) != 0, ("interrupts disabled")); trapsignal(td, &ksi); - /* - * Clear any pending debug exceptions after allowing a - * debugger to read DR6 while stopped in trapsignal(). - */ - if (type == T_TRCTRAP) - load_dr6(0); user: userret(td, frame); KASSERT(PCB_USER_FPU(td->td_pcb),
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