[Gc] Allocating Executable Memory

Noah Lavine noah.b.lavine at gmail.com
Fri Jul 23 23:16:29 PDT 2010


Ooops! Here it is.

2010/7/24 Ivan Maidanski <ivmai at mail.ru>:
> Hello.
>
> You forget to attach the patch.
>
> Fri, 23 Jul 2010 20:32:47 -0400 Noah Lavine <noah.b.lavine at gmail.com>:
>
>> Hello,
>>
>> Here is a second version of the patch. I believe it fixes almost
>> everything you said. However, I have a question about changing the
>> behavior on non-unix platforms: the code suggests that these platforms
>> currently don't obey the NO_EXECUTE_PERMISSION configure option, and
>> just always return executable memory. I can change that, but it seems
>> like a separate issue from this change - would you like it as a
>> separate patch?
>>
>> A ChangeLog entry could look like this:
>>
>> 2010-07-23 Noah Lavine <noah.b.lavine at gmail.com>
>>         * configure.ac: change documentation for NO_EXECUTE_PERMISSION
>> to match following change.
>>         * os_dep.c: add functions GC_set_executable_pages(int),
>> GC_get_executable_pages() to set and check whether the GC allocator
>> functions will return executable memory. Make the
>> NO_EXECUTE_PERMISSION configuration option determine the default
>> choice.
>>         * gc.h: add prototypes and documentation comments for
>> GC_set_executable_pages(int) and GC_get_executable_pages().
>>
>> There's also a larger issue which I wanted to ask about because I
>> don't understand the internals of libgc very well: my original idea
>> was that the executable option would take effect immediately after
>> being changed. However, I realized that for instance
>> GC_set_pages_executable(1) could be called at a time when the GC has
>> partially-allocated non-executable pages that it is giving out. Would
>> it be better to try to change the protection on those pages, save them
>> and allocate new executable pages, or just change the documentation on
>> this option so it can only be called just after the library is
>> initialized?
>>
>> Thanks
>> Noah
>>
>> 2010/7/23 Ivan Maidanski <ivmai at mail.ru>:
>> >
>> > The patch is incomplete. gc.h should be affected too. Also supply better doc comments (in gc.h). Update NO_EXECUTE_PERMISSION documentation. Send me Changelog entries please. Also, PROT_EXEC is not defined for every platform (e.g. Win32 which supports executable code in the allocated memory) - we shouldn't define a static variable which is always a const. GC_get_pages_executable() should better actually return 1/0 instead of exec_flag.
>> >
>> > Fri, 23 Jul 2010 15:21:34 -0400 Noah Lavine <noah.b.lavine at gmail.com>:
>> >
>> >> Thanks a lot!
>> >>
>> >> Here is a patch against the latest CVS which implements this. On my
>> >> computer libgc passes all tests with it applied.
>> >>
>> >> Noah
>> >>
>> >> On Fri, Jul 23, 2010 at 3:10 PM, Boehm, Hans <hans.boehm at hp.com> wrote:
>> >> > I don't mind this either. ?We've generally been moving static configuration options to runtime wherever the performance impact was minor. ?We didn't get around to this one yet.
>> >> >
>> >> > Hans
>> >> >
>> >> >> -----Original Message-----
>> >> >> From: gc-bounces at linux.hpl.hp.com [mailto:gc-bounces at linux.hpl.hp.com]
>> >> >> On Behalf Of Ivan Maidanski
>> >> >> Sent: Friday, July 23, 2010 11:40 AM
>> >> >> To: Noah Lavine
>> >> >> Cc: gc at linux.hpl.hp.com
>> >> >> Subject: Re: [Gc] Allocating Executable Memory
>> >> >>
>> >> >> Hello!
>> >> >>
>> >> >> I like the idea.
>> >> >> I don't mind adding this feature in this release (probably Hans
>> >> >> wouldn't neither since the added coded is small and easily verifiable).
>> >> >>
>> >> >> The API should be:
>> >> >> - GC_set_pages_executable(int) // non-zero means executable
>> >> >> - GC_get_pages_executable() // returns non-zero if it is allowed to
>> >> >> execute code in allocated memory
>> >> >>
>> >> >> NO_EXECUTE_PERMISSION controls the initial value only.
>> >> >>
>> >> >> If you'd like to add it, I expect you'll provide the patch against the
>> >> >> current CVS.
>> >> >>
>> >> >>
>> >> >> Fri, 23 Jul 2010 13:28:17 -0400 Noah Lavine <noah.b.lavine at gmail.com>:
>> >> >>
>> >> >> > Hello GC Developers,
>> >> >> >
>> >> >> > I am writing with a feature request for the next version of your
>> >> >> library.
>> >> >> >
>> >> >> > I am working on adding JIT compilation support to GNU Guile (a Scheme
>> >> >> > implementation), which uses this library for garbage collection. In
>> >> >> > order to make GC work, we'll need to allocate executable memory.
>> >> >> > However, I have discovered that allocating executable memory is a
>> >> >> > build-time configuration option (in version 7.1, it appears to be set
>> >> >> > in configure.ac, at lines 401 and 495). This gives Guile something of
>> >> >> > a problem - currently we use whatever libgc is on a user's computer.
>> >> >> > However, the only way to ensure that libgc will allocate executable
>> >> >> > memory would be to build our own version of it, which would add space
>> >> >> > to our executable and be redundant. We could also hack together our
>> >> >> > own memory allocator only for executable memory, but we like using
>> >> >> > your library. :-)
>> >> >> >
>> >> >> > Therefore, I'd like to request that the next version of the library
>> >> >> > have allocating executable memory as a runtime configuration option
>> >> >> > that can be set by programs. I don't think it would affect
>> >> >> performance
>> >> >> > very much - in fact, I can find only two uses of it in the entire
>> >> >> > program (calls to mmap and mprotect in os_dep.c).
>> >> >> >
>> >> >> > I also have a question - is there some way to make an older version
>> >> >> of
>> >> >> > libgc manage executable memory anyway? For instance, if I mmap some
>> >> >> > pages of executable memory, will libgc scan those for pointers even
>> >> >> > though it didn't allocate them? Are there other ways to work around
>> >> >> > this?
>> >> >>
>> >> >> See GC_add_roots.
>> >> >>
>> >> >> >
>> >> >> > Thank you very much
>> >> >> > Noah Lavine
>> >> >> > _______________________________________________
>> >> >> > Gc mailing list
>> >> >> > Gc at linux.hpl.hp.com
>> >> >> > http://www.hpl.hp.com/hosted/linux/mail-archives/gc/
>> >> >>
>> >> >> _______________________________________________
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>> >> >
>> >>
>> >>
>> >> ATTACHMENT: application/x-patch add_executable_option.patch_______________________________________________
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>> >
>> >
>>
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>
>
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