Update ReadProcMaps() to reflect lack of atomicity when reading /proc/self/maps.

base/debug/proc_maps_linux.h

Index: base/debug/proc_maps_linux.h
diff --git a/base/debug/proc_maps_linux.h b/base/debug/proc_maps_linux.h
index d055712444c8c43df70114af7758987f349fe3a4..9fbd47898d378407aab5cb6f7b20e96050018883 100644
--- a/base/debug/proc_maps_linux.h
+++ b/base/debug/proc_maps_linux.h
@@ -43,6 +43,40 @@ struct MappedMemoryRegion {
 
 // Reads the data from /proc/self/maps and stores the result in |proc_maps|.
 // Returns true if successful, false otherwise.
+//
+// There is *NO* guarantee that the resulting contents will be free of
+// duplicates or even contain valid entries by time the method returns.
+//
+//
+// THE GORY DETAILS
+//
+// Did you know it's next-to-impossible to atomically read the whole contents
+// of /proc/<pid>/maps? You would think that if we passed in a large-enough
+// buffer to read() that It Should Just Work(tm), but sadly that's not the case.
+//
+// Linux's procfs uses seq_file [1] for handling iteration, text formatting,
+// and dealing with resulting data that is larger than the size of a page. That
+// last bit is especially important because it means that seq_file will never
+// return more than the size of a page in a single call to read().
+//
+// Unfortunately for a program like Chrome the size of /proc/self/maps is
+// larger than the size of page so we're forced to call read() multiple times.
+// If the virtual memory table changed in any way between calls to read() (e.g.,
+// a different thread calling mprotect()), it can make seq_file generate
+// duplicate entries or skip entries.
+//
+// Even if seq_file was changed to keep flushing the contents of its page-sized
+// buffer to the usermode buffer inside a single call to read(), it has to
+// release its lock on the virtual memory table to handle page faults while
+// copying data to usermode. This puts us in the same situation where the table
+// can change while we're copying data.
+//
+// Alternatives such as fork()-and-suspend-the-parent-while-child-reads were
+// attempted, but they present more subtle problems than it's worth. Depending
+// on your use case your best bet may be to read /proc/<pid>/maps prior to
+// starting other threads.
+//
+// [1] http://kernelnewbies.org/Documents/SeqFileHowTo
 BASE_EXPORT bool ReadProcMaps(std::string* proc_maps);
 
 // Parses /proc/<pid>/maps input data and stores in |regions|. Returns true