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Update comments in IsExceptionNonfatalResource() given 10.12 source

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With reference to 10.12 source, commentary regarding RESOURCE_TYPE_IO
can be authoritative.

Cursory examination of 10.12 source reveals that RESOURCE_TYPE_MEMORY
can now be fatal, although deeper examination reveals that this is
impossible on macOS. State this authoritatively as well.

BUG=crashpad:124

Change-Id: I52124c68fe017015983ab46e54006ba97ecd0142
Reviewed-on: https://chromium-review.googlesource.com/434297
Reviewed-by: Robert Sesek <rsesek@chromium.org>
This commit is contained in:
Mark Mentovai 2017-01-27 18:17:05 -05:00
parent 1e4be91918
commit 3e5ae2dc87
1 changed files with 20 additions and 3 deletions
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23
util/mach/exception_types.cc
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@ -309,15 +309,32 @@ bool IsExceptionNonfatalResource(exception_type_t exception,
if (resource_type == RESOURCE_TYPE_MEMORY &&
resource_flavor == FLAVOR_HIGH_WATERMARK) {
// These exceptions are never fatal. See 10.10
// These exceptions were never fatal prior to 10.12. See 10.10
// xnu-2782.1.97/osfmk/kern/task.c
// THIS_PROCESS_CROSSED_HIGH_WATERMARK__SENDING_EXC_RESOURCE().
//
// A superficial examination of 10.12 shows that these exceptions may be
// fatal, as determined by the P_MEMSTAT_FATAL_MEMLIMIT bit of the
// kernel-internal struct proc::p_memstat_state. See 10.12.3
// xnu-3789.41.3/osfmk/kern/task.c task_footprint_exceeded(). This bit is
// not exposed to user space, which makes it difficult to determine whether
// the kernel considers a given instance of this exception fatal. However, a
// close read reveals that it is only possible for this bit to become set
// when xnu-3789.41.3/bsd/kern/kern_memorystatus.c
// memorystatus_cmd_set_memlimit_properties() is called, which is only
// possible when the kernel is built with CONFIG_JETSAM set, or if the
// kern.memorystatus_highwater_enabled sysctl is used, which is only
// possible when the kernel is built with DEVELOPMENT or DEBUG set. Although
// CONFIG_JETSAM is used on iOS, it is not used on macOS. DEVELOPMENT and
// DEBUG are also not set for production kernels. It therefore remains
// impossible for these exceptions to be fatal, even on 10.12.
return true;
}
if (resource_type == RESOURCE_TYPE_IO) {
// These exceptions don’t ever appear to be fatal. See
// https://crashpad.chromium.org/bug/124.
// These exceptions are never fatal. See 10.12.3
// xnu-3789.41.3/osfmk/kern/task.c
// SENDING_NOTIFICATION__THIS_PROCESS_IS_CAUSING_TOO_MUCH_IO().
return true;
}