Date: Thu, 27 Sep 2018 17:33:59 +0000 (UTC) From: John Baldwin <jhb@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r338976 - in head/sys: amd64/amd64 i386/i386 Message-ID: <201809271733.w8RHXx2G043044@repo.freebsd.org>
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Author: jhb Date: Thu Sep 27 17:33:59 2018 New Revision: 338976 URL: https://svnweb.freebsd.org/changeset/base/338976 Log: 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). Reviewed by: kib Approved by: re (delphij) MFC after: 3 days Differential Revision: https://reviews.freebsd.org/D17319 Modified: head/sys/amd64/amd64/trap.c head/sys/i386/i386/trap.c Modified: head/sys/amd64/amd64/trap.c ============================================================================== --- head/sys/amd64/amd64/trap.c Thu Sep 27 17:22:40 2018 (r338975) +++ head/sys/amd64/amd64/trap.c Thu Sep 27 17:33:59 2018 (r338976) @@ -659,12 +659,6 @@ 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: head/sys/i386/i386/trap.c ============================================================================== --- head/sys/i386/i386/trap.c Thu Sep 27 17:22:40 2018 (r338975) +++ head/sys/i386/i386/trap.c Thu Sep 27 17:33:59 2018 (r338976) @@ -762,12 +762,6 @@ 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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