On Mon, Jul 14, 2014 at 11:10:50AM +0300, Konstantin Belousov wrote:
> On Mon, Jul 14, 2014 at 04:38:17AM +0000, Mark Johnston wrote:
> > Author: markj
> > Date: Mon Jul 14 04:38:17 2014
> > New Revision: 268600
> > URL: http://svnweb.freebsd.org/changeset/base/268600
> >
> > Log:
> > Invoke the DTrace trap handler before calling trap() on amd64. This
> > matches
> > the upstream implementation and helps ensure that a trap induced by
> > tracing
> > fbt::trap:entry is handled without recursively generating another trap.
> >
> > This makes it possible to run most (but not all) of the DTrace tests under
> > common/safety/ without triggering a kernel panic.
> >
> > Submitted by: Anton Rang <anton.r...@isilon.com> (original version)
> > Phabric: D95
> >
> > Modified:
> > head/sys/amd64/amd64/exception.S
> > head/sys/amd64/amd64/trap.c
> > head/sys/cddl/dev/dtrace/amd64/dtrace_subr.c
> > head/sys/cddl/dev/dtrace/i386/dtrace_subr.c
> > head/sys/cddl/dev/dtrace/mips/dtrace_subr.c
> > head/sys/cddl/dev/dtrace/powerpc/dtrace_subr.c
> > head/sys/i386/i386/trap.c
> > head/sys/mips/mips/trap.c
> > head/sys/powerpc/aim/trap.c
> > head/sys/sys/dtrace_bsd.h
> >
> > Modified: head/sys/amd64/amd64/exception.S
> > ==============================================================================
> > --- head/sys/amd64/amd64/exception.S Mon Jul 14 00:16:49 2014
> > (r268599)
> > +++ head/sys/amd64/amd64/exception.S Mon Jul 14 04:38:17 2014
> > (r268600)
> > @@ -228,7 +228,24 @@ alltraps_pushregs_no_rdi:
> > .type calltrap,@function
> > calltrap:
> > movq %rsp,%rdi
> > +#ifdef KDTRACE_HOOKS
> > + /*
> > + * Give DTrace a chance to vet this trap and skip the call to trap() if
> > + * it turns out that it was caused by a DTrace probe.
> > + */
> > + movq dtrace_trap_func,%rax
> > + testq %rax,%rax
> > + je skiphook
> > + call *%rax
> > + testq %rax,%rax
> > + jne skiptrap
> > + movq %rsp,%rdi
> > +skiphook:
> > +#endif
> > call trap
> Why is this fragment done in asm ? I see it relatively easy to
> either move to the start of trap(), or create a new wrapper around
> current trap() which calls dtrace_trap_func conditionally, and then
> resorts to the old trap.
You're right, it looks like this could be done in C by introducing a
wrapper. I'm not sure how moving the conditional call to
dtrace_trap_func() around within trap() would fix the original problem,
though.
The diff below adds such a wrapper, and modifies DTrace to explicitly
ignore it when creating function boundary probes. Additionally, trap()
is marked __noinline to ensure that it can still be instrumented. I've
named it "_trap" for now for lack of a better name, but that will need
to change.
Thanks,
-Mark
diff --git a/sys/amd64/amd64/exception.S b/sys/amd64/amd64/exception.S
index bb5fd56..3d34724 100644
--- a/sys/amd64/amd64/exception.S
+++ b/sys/amd64/amd64/exception.S
@@ -228,24 +228,7 @@ alltraps_pushregs_no_rdi:
.type calltrap,@function
calltrap:
movq %rsp,%rdi
-#ifdef KDTRACE_HOOKS
- /*
- * Give DTrace a chance to vet this trap and skip the call to trap() if
- * it turns out that it was caused by a DTrace probe.
- */
- movq dtrace_trap_func,%rax
- testq %rax,%rax
- je skiphook
- call *%rax
- testq %rax,%rax
- jne skiptrap
- movq %rsp,%rdi
-skiphook:
-#endif
- call trap
-#ifdef KDTRACE_HOOKS
-skiptrap:
-#endif
+ call _trap
MEXITCOUNT
jmp doreti /* Handle any pending ASTs */
diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
index d9203bc..545174a 100644
--- a/sys/amd64/amd64/trap.c
+++ b/sys/amd64/amd64/trap.c
@@ -97,7 +97,8 @@ PMC_SOFT_DEFINE( , , page_fault, write);
#include <sys/dtrace_bsd.h>
#endif
-extern void trap(struct trapframe *frame);
+extern void _trap(struct trapframe *frame);
+extern void __noinline trap(struct trapframe *frame);
extern void syscall(struct trapframe *frame);
void dblfault_handler(struct trapframe *frame);
@@ -604,6 +605,19 @@ out:
return;
}
+/*
+ * Ensure that we ignore any DTrace-induced faults. This function cannot
+ * be instrumented, so it cannot generate such faults itself.
+ */
+void
+_trap(struct trapframe *frame)
+{
+
+ if (dtrace_trap_func != NULL && (*dtrace_trap_func)(frame))
+ return;
+ trap(frame);
+}
+
static int
trap_pfault(frame, usermode)
struct trapframe *frame;
diff --git a/sys/cddl/dev/fbt/fbt.c b/sys/cddl/dev/fbt/fbt.c
index 7e256e7..0cc2db2 100644
--- a/sys/cddl/dev/fbt/fbt.c
+++ b/sys/cddl/dev/fbt/fbt.c
@@ -232,13 +232,18 @@ fbt_provide_module_function(linker_file_t lf, int symindx,
int size;
u_int8_t *instr, *limit;
- if (strncmp(name, "dtrace_", 7) == 0 &&
- strncmp(name, "dtrace_safe_", 12) != 0) {
+ if ((strncmp(name, "dtrace_", 7) == 0 &&
+ strncmp(name, "dtrace_safe_", 12) != 0) ||
+ strcmp(name, "_trap") == 0) {
/*
* Anything beginning with "dtrace_" may be called
* from probe context unless it explicitly indicates
* that it won't be called from probe context by
* using the prefix "dtrace_safe_".
+ *
+ * Additionally, we avoid instrumenting _trap() to avoid the
+ * possibility of generating a fault in probe context before
+ * DTrace's fault handler is called.
*/
return (0);
}
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