[plcrashreporter] push by landon.j.fuller@gmail.com - Explain what vm_remap() is used for. on 2013-04-04 23:16 GMT

[codesite...@google.com]

Revision: 2884e03342ad
Author: Landon Fuller <lan...@plausible.coop>
Date: Thu Apr 4 16:16:20 2013
Log: Explain what vm_remap() is used for.

http://code.google.com/p/plcrashreporter/source/detail?r=2884e03342ad

Modified:
/Source/PLCrashAsync.h

=======================================
--- /Source/PLCrashAsync.h Thu Apr 4 16:03:12 2013
+++ /Source/PLCrashAsync.h Thu Apr 4 16:16:20 2013
@@ -41,12 +41,19 @@

/**
* @internal
- * If defined, the target architecture has a broken vm_remap() or
mach_vm_remap() implementation that results in a
- * kernel panic. This appears to be the case on iOS 6.0 through 6.1.2,
possibly fixed in 6.1.3. Note that no stable
- * release of PLCrashReporter shipped with the vm_remap() code.
+ *
+ * PLCrashReporter uses vm_remap() to atomically validate that a target
memory range is valid, to ensure that it will
+ * remain valid throughought the report generation process, and to
transparently support out-of-process execution by
+ * remapping pages from the target task. This replaced previous use of
vm_read_overwrite(), which required a syscall
+ * and a copy for every memory read; the cost of which was unsuitable for
any sufficiently large read operations, such
+ * as reading __LINKEDIT and the Objective-C metadata.
+ *
+ * If this macro is defined, the target architecture has a broken
vm_remap() or mach_vm_remap() implementation that
+ * results in a kernel panic. This appears to be the case on iOS 6.0
through 6.1.2, possibly fixed in 6.1.3. Note that
+ * no stable release of PLCrashReporter shipped with the vm_remap() code.
*
- * Investigation seems to show an over-release of the target vm_map and
backing vm_object, leading to NULL dereference,
- * invalid memory references, and in some cases, deadlocks that result in
watchdog timeouts.
+ * Investigation of the failure seems to show an over-release of the
target vm_map and backing vm_object, leading to
+ * NULL dereference, invalid memory references, and in some cases,
deadlocks that result in watchdog timeouts.
*
* In one example case, the crash occurs in update_first_free_ll() as a
NULL dereference of the vm_map_entry_t parameter.
* Analysis of the limited reports shows that this is called via
vm_map_store_update_first_free(). No backtrace is