RFR (S): 8012015: Use PROT_NONE when reserving memory

[David Holmes]

Hi Mikael,

On 19/04/2013 8:58 AM, Mikael Vidstedt wrote:
>
> Please review the below patch which changes the access rights when
> reserving memory on Linux and BSD from using read+write to none, which
> matches what's done on Solaris. Full background below.

This seems quite reasonable.

> Webrev:
> http://cr.openjdk.java.net/~mikael/webrevs/8012015/webrev.00/webrev/
> Bug: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8012015
>
> Passes JPRT testing, and I also verified manually that CDS (still) works
> on my Linux workstation.
>
> I'm also taking suggestion on how to implement a regression test for
> this.

Add it to the whitebox testing code perhaps?

David
-----

One alternative would be to parse /proc/self/maps on Linux, find
> the corresponding range and verify that the protection flags are
> correct, do almost the same thing for OSX but using /proc/PID/task/vmmap
> instead etc, but that obvious is a lot of platform dependent scaffolding
> for a regression test. An alternative I'm leaning towards would be to
> just read and/or write to the page and assert that a SIGSEGV was raised.
> Other suggestions?
>
>
> Background (copied from the bug report for your convenience):
>
> Memory is reserved on the *nix platforms using mmap and passing in the
> MAP_NORESERVE. Before the memory can actually used it needs to be
> committed, and this is done by calling mmap without the MAP_NORESERVE
> flag. The commit call also specifies the requested access/protection
> bits for the address range.
>
> Currently Linux and BSD/OSX the protection used when reserving memory is
> PROT_READ|PROT_WRITE. This is done in the anon_mmap in the respective
> os_*.cpp files. This means that the memory range is actually readable
> and writable, but because the MAP_NORESERVE flag has been specified
> there is no guarantee that a read/write will succeed - if the system is
> low on memory and out of swap space for example the read/write may raise
> a signal.
>
> This is not normally a problem - before the memory is used it is
> typically committed. However, for subtle bugs where wild pointers are
> used etc it would be preferable to get a SEGSEGV and catch the bug early
> rather than have the use of the wild pointer silently succeed.
>
> In the Solaris implementation of anon_mmap there is a comment about
> exactly this:
>
>    // Map uncommitted pages PROT_NONE so we fail early if we touch an
>    // uncommitted page. Otherwise, the read/write might succeed if we
>    // have enough swap space to back the physical page.
>
>
> Also, on both Linux and BSD/OSX the respective pd_uncommit_memory
> functions both restore the memory to PROT_NONE, so newly reserved memory
> currently gets PROT_READ|PROT_WRITE, but memory which gets uncommitted
> gets PROT_NONE which does not appear to be very symmetrical.
>
> Cheers,
> Mikael