RFR: JDK-8299790: os::print_hex_dump is racy

[Thomas Stuefe]

On Mon, 17 Jul 2023 12:47:28 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Tiny fix for a tiny problem. 
>> 
>> `os::print_hex_dump` uses `os::is_readable_pointer` to check the to-be-printed memory for readability; `os::is_readable_pointer` uses `SafeFetch` to probe the memory for access, which is good, but then, by the time we actually print that information, we reread the memory location again. It may be unreadable now (either because the region had been unmapped or protected by a concurrent thread), and we would crash the VM.
>> 
>> The patch rewrites the function to not use `os::is_readable_pointer`, but to use `SafeFetch` to read from memory directly and then use the result of that read for printing. That requires a bit of bit fiddling, since we only can read word-wise, but the hex-dump could be in units between bytes and qwords.
>> 
>> Tests: manual and GHA-driven gtests on all platforms. The gtests test this function exhaustively.
>
> src/hotspot/share/runtime/os.cpp line 960:
> 
>> 958:     }
>> 959:     uint64_t value = (((uint64_t)i2) << 32) | i;
>> 960:     st->print("%016" FORMAT64_MODIFIER "x", value);
> 
> This implies endianness, it is not? We handle endianness for 64-bit version below.

Sure, for completeness; though I don't think we have a 32-bit big-endian platform.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14895#discussion_r1265734208