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Don't clear DR6 for debug exceptions from userland.

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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
This commit is contained in:
John Baldwin 2018-09-27 17:33:59 +00:00
parent 5bbde333cd
commit 83382d027f
2 changed files with 0 additions and 12 deletions
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6
sys/amd64/amd64/trap.c
View File

@ -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),

6
sys/i386/i386/trap.c
View File

@ -762,12 +762,6 @@ trap(struct trapframe *frame)
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),