On Aug 19 07:04, Ryan Johnson wrote: > On 19/08/2013 6:49 AM, Ryan Johnson wrote: > >One thing I don't understand, though: shouldn't a stack overflow > >normally manifest as a seg fault when trying to access the invalid > >addresses, rather than silent memory corruption?
That would be helpful. > >However, /proc/pid/maps for emacs shows: > >>00010000-00020000 rw-s 00000000 0000:0000 0 > >>[win heap 1 default shared] > >>00020000-00030000 rw-s 00000000 0000:0000 0 [win heap 2 default shared] > >>00030000-001E4000 ===p 00000000 0000:0000 0 [stack (tid 4896)] > >>001E4000-001E6000 rw-g 001B4000 0000:0000 0 [stack (tid 4896)] > >>001E6000-00230000 rw-p 001B6000 0000:0000 0 [stack (tid 4896)] > >GDB reports that thread 4896 is the main thread... so I guess > >Windows doesn't reserve a red zone around its stack, but instead > >chooses to place the main thread stack right next to the > >fully-mapped global shared heap to maximize the potential for Fun? Right. I have no idea what the two shared mem regions preceeding the stack are good for, though. > Some googling turns up > http://comments.gmane.org/gmane.comp.java.openjdk.hotspot.runtime.devel/7706 > >Windows only uses reserved but only partially committed memory for its > >stacks. In order to detect when to > >commit more stack, it installs a one-shot guard page (btw the same type of > >guard page that is used for the > >hotspot yellow and red zone) right at the edge of the currently commited > >stack zone. When a thread accesses > >this guard page an exception is thrown which Windows catches internally, > >commits more stack and > >re-establishes the one-shot guard page at the new edge of the commited zone. > >When Windows detects such an > >exception inside the _last 4 pages_ of a stack (I couldn't find any > >documentation for that on MSDN, I found > >this value from manually testing on several Windows machines with 4k stack > >pages) it throws a STACK_OVERFLOW_EXCEPTION. > So maybe emacs just had the incredibly bad luck to alloca() a large > buffer right at end-of-stack and then somehow managed to skip over > the 4 guard pages when accessing it? That's unlikely since alloca is designed to probe the stack in 4K steps. And STATUS_STACK_OVERFLOW is translated to a SEGV by Cygwin's exception handler. Corinna -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat
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