8361433: [Big Endian] assert(verify_guards()) failed: Expected valid memory guards after 8357601

[TheRealMDoerr]

Big Endian fix for JDK-8357601.

Note: The first commit is only a Big Endian fix. The code looks still wrong for platforms which don't support unaligned accesses. Also see UseUnalignedAccesses.
The second commit changes to check os::is_readable_range().

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Issue

• JDK-8361433: [Big Endian] assert(verify_guards()) failed: Expected valid memory guards after 8357601 (Bug - P2)

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[mlbridge]

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• 01: Full - Incremental (2f467b3a)
• 00: Full (ced69858)

[tstuefe]

Your fix is fine. Pre-existing: I wonder about the SafeFetch. A byte pointer traverses byte-wise over the memory. We use it to load a 32-bit value via SafeFetch32. 3/4 of those loads will be unaligned and also redundant. (I am surprised the unaligned access works on all platforms).

The guards themselves can be located at unaligned addresses I think, so we cannot just use int loads. Therefore I would have just used the old code without safefetch but preceded it with a safefetch check:

u_char* c = (u_char*) _guard;
+ if (!os::is_readable_pointer(align_up(c, BytesPerInt))) {
+   return false;
+ }
   u_char* end = c + GUARD_SIZE;
   while (c < end) {
     if (*c != badResourceValue) {
       return false;
     }
     c++;
   }

I think that would be pragmatic since the guard is only 16 byte and the chance of a guard straddling a page boundary is very small. But if one wanted to be more careful, one could use os::is_readable_range() instead.

@dholmes-ora does this make sense?

[TheRealMDoerr]

Thanks, Thomas! I've changed to use os::is_readable_range(). Note that alignment is already handled inside of that function. Please take a look!

[dholmes-ora]

@TheRealMDoerr , @tstuefe let me do a backout of JDK-8357601 in the morning. I need that fix to be complete and safe so it can be backported in a simple manner. Thanks.

I will look into @tstuefe suggested alternative.

[dholmes-ora]

The original change has been backed out. Sorry for the inconvenience, and thanks for the suggested fix.

[TheRealMDoerr]

Will be handled by JDK-8361447.