RFR: JDK-8299790: os::print_hex_dump is racy [v4]

[David Holmes]

On Mon, 7 Aug 2023 10:33:27 GMT, Thomas Stuefe <stuefe 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.
>
> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8299790-os-print-hex-dump-should-use-SafeFetch
>  - remove superfluous include
>  - feedback Aleksey; small code reshuffle
>  - remove debug output from gtest
>  - JDK-8299790-os-print-hex-dump-should-use-SafeFetch

Looks okay to me. Thanks

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14895#pullrequestreview-1571185404